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%\received{}
%\accepted{}
%\journalid{}{}
%\articleid{}{}
\slugcomment{Submitted to The Astrophysical Journal}
\lefthead{Ferruit et al.}
\righthead{The extended emission line region of the Seyfert galaxy Mkn~573}


\begin{document}

\title{The extended emission line region of the Seyfert galaxy Markarian 573
\footnote{Based on observations with the NASA/ESA {\it Hubble
Space telescope} obtained at the Space Telescope Science Institute, which
is operated by the Association of Universities for Research in Astronomy,
Inc., under NASA contract NAS 5-26555, and on observations collected at the
Canada-France-Hawaii Telescope, which is operated by CNRS of France, NRC of Canada and
the University of Hawaii.}}

\author{Pierre Ferruit and Andrew S. Wilson\altaffilmark{2}}
\affil{Astronomy Department, University of Maryland, College Park,
MD 20742, USA}
\authoremail{pierre@astro.umd.edu, wilson@astro.umd.edu}
\altaffiltext{2}{Adjunct Astronomer, Space Telescope Science Institute}

\author{Heino Falcke}
\affil{Max-Planck-Institut f\"ur Radioastronomie, Auf dem H\"ugel 69,
D-53121 Bonn, Germany}
\authoremail {hfalcke@mpifr-bonn.mpg.de}

\author{Chris Simpson}
\affil{Subaru Telescope, National Astronomical Observatory of Japan, 650
N. A`Oh\={o}k\={u} Place, Hilo, HI 96720, USA}
\authoremail {simpson@naoj.org}

\author{Emmanuel P\'econtal}
\affil{Centre de Recherche Astronomique de Lyon - Observatoire de Lyon, 9 av.
Charles Andr\'e, F-69561 Saint-Genis-Laval Cedex, France;} 
\authoremail{pecontal@obs.univ-lyon1.fr}

\author{Florence Durret\altaffilmark{3}}
\affil{Institut d'Astrophysique de Paris, CNRS, Universit\'e Pierre et Marie Curie,
98bis Bd Arago, F-75014 Paris, France} 
\authoremail{durret@iap.fr}
\altaffiltext{3}{DAEC, Observatoire de Paris, Universit\'e Paris VII, CNRS (UA 173),
F92195, Meudon Cedex, France}

\begin{abstract}
   We present the results of observations of the extended emission-line region of the
   Seyfert 2 galaxy Mrk~573, obtained using the WFPC2 camera on board HST and the TIGER
   integral field spectrograph on the Canada France Hawaii Telescope. The WFPC2
   observations consist of a set of narrow-band linear ramp filter images in the
   \OIIIwb\ and \Ha\ + \NIIwb\ lines, as well as continuum and other emission-line
   images obtained from the HST archive. The TIGER 3D (x,y,$\lambda$) data cubes cover
   the wavelength regions 5000/400~\AA\ (central wavelength/range of wavelength),
   including \Hb\ and  \OIIIww, and 6750/400~\AA\ including \Ha, \NIIww\ and \SIIwwb.
   We describe and use a new deconvolution technique in which the spatial resolution
   of the TIGER data cubes is improved by using the higher resolution HST images in
   the same emission-lines, in combination with a standard Richardson-Lucy
   deconvolution. This `guided' deconvolution allows us to obtain the \OIII\ gaseous
   kinematics with unprecedented spatial resolution of $\sim$0\farcsec35. 
   
   The HST images outline the detailed complex structure of the central $\sim$3~kpc of
   this galaxy in the \OIII\ and \Ha+\NII\ lines, with strings of knots and a system
   of arcs straddling the nucleus. The \OIII/(\Ha+\NII) line ratio map shows that,
   taken together, all these individual features define a double wedge form with
   sharp edges, consistent with an ionization cone. The `linear' radio structure is
   closely associated with the strings of emission-line knots, with the northwest
   radio lobe lying inside one of the arcs. Strong velocity perturbations are found in
   the vicinity of all of the radio components, witnessing the interaction between the
   radio ejecta and the ambient material.
   
   The spectral and kinematical properties of the different components of the
   emission-line region of Mrk~573 are discussed in terms of various models of the
   interaction between the ejecta and the ambient gas. The emission-line knots,
   associated with the radio knots and the velocity perturbations, probably trace the
   deflection of the radio jet by clouds. The inner arcs, 1\farcsec8 (570~pc) and
   2\farcsec3 (720~pc) from the nucleus, may represent radiative bow shocks driven by
   the radio jets. The similar structure of each of these arcs in the different
   emission lines, and the absence of any strong kinematic perturbation at the arcs
   themselves, indicate that they are not `photoionizing shocks', in which the gas is
   ionized by radiation from the radiative shock itself. Instead, the arcs are
   photoionized by an external source of radiation. The outer arcs, 2\farcsec9
   (0.9~kpc) and 3\farcsec6 (1.1~kpc) from the nucleus, mark the transition between
   the narrow line and extended narrow line regions of Mrk~573. As in the case of the
   inner arcs, photoionizing shocks are ruled out on the basis of their optical
   emission-line properties. We use the measured variation of the ionization parameter
   and gas density to derive the flux of ionizing photons as a function of distance
   from the nucleus. We find that, unless the ionizing photons flux of the compact
   central source has decreased by a factor ten over the last 4000 years, a model in
   which all the ionizing photons originate in the central source is excluded.
   Instead, we speculate that, if the central ionizing source has not varied, fast
   shocks, possibly associated with the jet/cloud interactions, may provide the
   required spatially extended source of ionizing photons.
\end{abstract}

\keywords{galaxies: active --- galaxies: individual (Mrk 573) --- galaxies : kinematics and dynamics ---
galaxies: jets --- galaxies: nuclei --- galaxies: Seyfert }

\section{Introduction}
   Study of the narrow line region (NLR) and extended narrow line region (ENLR) of
   Seyfert galaxies, in particular their relationship to the ionizing nuclear
   radiation, is a powerful tool to probe the inner structure of active galaxy nuclei
   (AGN). The existence of `ionization cones' in a number of Seyfert galaxies (e.g.
   Wilson \& Tsvetanov \cite{Wilson94}) and arguments that the ionized gas in Seyfert
   2 galaxies is illuminated by more ionizing photons than are radiated toward Earth
   (e.g. Kinney et al. \cite{Kinney91}), have strongly supported the idea that the
   nuclear ionizing radiation is collimated, as expected in the so-called `unified
   model' for AGN (see reviews by Antonucci \cite{Antonucci93}; Osterbrock
   \cite{Osterbrock93}; Urry \& Padovani \cite{Urry95}).
   
   However, even if this simple picture holds for the ENLR (see e.g. Unger et al.
   \cite{Unger87}), the close morphological (Haniff, Wilson \& Ward
   \cite{Haniff88}; Capetti et al. \cite{Capetti96}; Falcke et al.
   \cite{Falcke96}; Falcke, Wilson \& Simpson \cite{Falcke98}) and
   kinematical (e.g. Whittle et al. \cite{Whittle88}; P\'econtal et al.
   \cite{Pecontal97}) association between the NLR gas and the radio-emitting material
   suggests a more complicated picture, in which interaction between the radio
   ejecta and the ambient gas plays a significant role in the energy budget of the
   NLR, as first proposed by Wilson \& Willis (\cite{Wilson80}). The isolated Seyfert
   2 galaxy Mrk~573 stands as a good example of this dichotomy between the NLR and the
   ENLR, and of the complexities resulting from the presence of both collimated
   ionizing radiation and an interaction between the radio ejecta and ambient gas.
   
   In narrow-band emission-line imaging, Mrk~573 exhibits a structure suggestive
   of ambient gas illuminated by a bi-conical distribution of ionizing radiation. Such
   images reveal an extended $\sim$13\arcsec\ ($\sim$4.1~kpc) emission-line region
   elongated along PA = 110\degr-130\degr\ (Haniff et al. \cite{Haniff88},
   \cite{Haniff91}; Tsvetanov \& Walsh \cite{Tsvetanov92}) with arc-like structures
   (Pogge \& De Robertis \cite{Pogge95}; Capetti et al. \cite{Capetti96}; Falcke et
   al. \cite{Falcke98}) contained within two wedge-shaped regions, apparently the
   projection of a bi-cone (Haniff et al. \cite{Haniff91}; Pogge \& De Robertis
   \cite{Pogge95}; Falcke et al. \cite{Falcke98}). Evidence for scattered nuclear
   continuum light is also found on both sides of the nucleus, within this bi-conical
   structure (Pogge \& De Robertis \cite{Pogge93}; Kotilainen \& Ward
   \cite{Kotilainen97}).
   
   However, the gaseous kinematics reveal clear departures from this
   simple scenario. Long-slit spectroscopy of the emission-line region (Unger et al.
   \cite{Unger87}; Whittle et al. \cite{Whittle88}; Durret \& Warin \cite{Durret90};
   Tsvetanov \& Walsh \cite{Tsvetanov92}) shows that, while  the ENLR gas
   ($>$~4\arcsec\ [1.3~kpc] from the nucleus) follows a smooth velocity field and
   displays very narrow emission-lines ($<$~45~\kms, Unger et al. \cite{Unger87}), the
   NLR gas ($<$~4\arcsec\ [1.3~kpc] from the nucleus) displays very disturbed kinematics,
   including much broader line profiles (100--350~\kms, Whittle et al.
   \cite{Whittle88}). Comparison with radio continuum maps (Ulvestad \&
   Wilson \cite{Ulvestad84}; Falcke et al. \cite{Falcke98}) shows that these disturbed
   areas are located in the vicinity of the northwest and southeast radio lobes
   (Whittle et al. \cite{Whittle88}; Tsvetanov \& Walsh \cite{Tsvetanov92}),
   suggesting that in Mrk~573, the nuclear radio ejecta perturb the gaseous
   kinematics of the NLR. Further evidence for an interaction between the radio ejecta
   and the ionized gas comes from a comparison between the highest spatial resolution
   emission-line images (Pogge \& De Robertis \cite{Pogge95}; Schmitt \& Kinney
   \cite{Schmitt96}; Capetti et al. \cite{Capetti96}; Falcke et al. \cite{Falcke98})
   and the VLA radio maps, which shows emission-line structures accompanying each of
   the radio features, and emission-line arcs suggestive of bow shocks.
   
   Last, the extensive spectroscopic study of Mrk~573 by Tsvetanov \& Walsh
   (\cite{Tsvetanov92}) shows that the ionization parameter of the gas is
   roughly constant within 2\arcsec\ (630~pc) of the nucleus and then increases with
   nuclear distance further out. Tsvetanov \& Walsh (\cite{Tsvetanov92}) attributed
   this increase to a rapid decline of gas density with increasing distance from the
   nucleus. However, using emission-line ratio maps, Capetti et al. (\cite{Capetti96})
   have argued that the density decrease is not fast enough and that local sources of
   ionizing radiation (e.g. hot shocked gas) are required to explain the variation of
   ionization parameter with nuclear distance. One possible explanation is that the
   outflow traced by the radio ejecta is sufficiently powerful to generate ionizing
   radiation which is comparable to or exceeds that from the nucleus (see also
   Bicknell et al. \cite{Bicknell98}).
   
   The optical studies of Mrk~573 to date have thus involved imaging at the resolutions
   achieved by ground-based telescopes and HST, and spectroscopy at ground-based
   resolutions. However, given the compact nature of the nebulosity, a detailed
   understanding of the physical processes associated with the interaction between
   radio ejecta and ionized gas requires the diagnostic power of emission-line
   spectroscopy combined with HST-like spatial resolution. In the present paper, we
   report such a study. Our observations comprise HST/WFPC2 imaging of the ionized gas
   in Mrk~573 in \OIIIwb\ and \Ha+\NII\ (some of these observations were
   briefly described in Falcke et al. \cite{Falcke98}), and high-spatial
   resolution 3D(x,y,$\lambda$) TIGER spectroscopy of Mrk~573 in the \Hb\
   + \OIII\ and \Ha\ + \NII\ + \SII\ wavelength domains. We discuss the morphological,
   kinematical and spectral properties of the various components of the emission-line
   region and their relationship with the radio ejecta.

   Throughout this paper, we will use a heliocentric systemic velocity of 5150 $\pm$
   11~\kms\ (stellar Ca\,{\sc ii} triplet, Nelson \& Whittle \cite{Nelson95}) for
   Mrk~573. This gives a velocity relative to the cosmic microwave background of
   4858~\kms\ (--292~\kms\ correction, NED\footnote{The NASA/IPAC Extragalactic
   Database (NED) is operated by the Jet Propulsion Laboratory, California Institute
   of Technology, under contract with the National Aeronautics and Space
   Administration.} on-line velocity calculator), yielding a distance and a linear
   scale of 65~Mpc and 315~pc per arcsec, respectively (H$_{\mbox{\small o}}$ = 75
   \kms\ Mpc$^{-1}$; q$_{\mbox{\small o}}$ = 0).

\section{Observations and Data Reduction}
\subsection{HST imaging}
   \label{SectionHST}
   \subsubsection{Observations}
      \label{SectionHSTObservations}
      These observations of the emission-line region of Mrk~573 have been acquired with
      the Wide Field and Planetary Camera 2 (WFPC2) on board HST, using the
      linear-ramp filters (LRF) and the Wide Field chips (0\farcsec0996 pixel$^{-1}$).
      Details of the observations can be found in Falcke et al. (\cite{Falcke98}).
      The central wavelengths (and FWHM bandpasses) of the LRF were 6679 \AA\ (86~\AA)
      and 5092 \AA\ (66~\AA) for the \Ha+\NIIwb\ and \OIII\
      on-band images, respectively.
      
      Using the efficiency curve of the LRF configuration, and assuming a typical
      \NIIwb/\Ha\ ratio of 0.8 for Mrk~573 (Tsvetanov \& Walsh \cite{Tsvetanov92}), the
      \NIIwb\ line contributes $\sim$40\,\%\ of the flux in the \Ha+\NIIwb\ image.
      However, as the central wavelength of the observations varies by as much as 24
      \AA\ along the major axis of the emission-line region (due to the
      position-dependent transmission of the LRF), the relative contributions of the
      \Ha\ and \NIIwb\ lines to the flux in the image will change with position. The emission-line image is modified by this instrumental effect, as well as by changes in the intrinsic \NIIwb/\Ha\ ratio over the emission-line region (Tsvetanov \& Walsh \cite{Tsvetanov92}), and wavelength shifts of the lines due to velocity gradients (e.g. Whittle et al. \cite{Whittle88}).
      
      In addition to the LRF images, we also used archived broad-band images (F569W,
      F675W, F814W, and F606W), taken on the Planetary Camera (PC) at 0\farcs0455
      pixel$^{-1}$. Observational details for the datasets from the first three
      filters are given in Capetti et al.~(\cite{Capetti96}), and for the last one in
      Malkan, Gorjian, \& Tam (\cite{Malkan98}).
   
   \subsubsection{Data-reduction}
      \label{SectionHSTReduction}
      Details of the data-reduction of the LRF images can be found in Falcke et al.
      (\cite{Falcke98}). The broad-band filter images were processed and flat-fielded
      with the STScI pipeline and the flux calibration was done using the header
      information provided. Only the F569W image was split and could have
      cosmic-rays removed using the IRAF/STSDAS task CRREJ. Hence,
      cosmic-rays rejection for the other images was done -- less effectively -- using
      the IRAF task COSMICRAYS. Alignment of the F569W, F675W and F814W images was not
      necessary since they were all taken at the same position on the PC.
      
      The F569W and F675W filters contain a number of emission-lines, such as
      \Hb+\OIII\ and \OI+\Ha+\NII+\SII, respectively. Because of the large equivalent
      width of these lines, the emission-line structure of Mrk~573 can be readily seen
      in the broad-band images. The F814W image was used to subtract the continuum
      from these images. However, since the filters are so broad and the spectral
      shape of the continuum changes over the image, a satisfactory continuum
      subtraction cannot be achieved at all points, resulting in negative fluxes
      in some areas. To avoid this, we scaled down the F814W image by 30\% until the
      negative fluxes were removed. Unfortunately, this means that in other areas we
      will not have subtracted enough of the continuum. Therefore, these images which
      are better sampled than the LRF images because they were taken on the PC, will
      only be used to obtain qualitative, rather than quantitative, structural
      information.
      
   \subsubsection{Alignment of the LRF \OIII\ and \Ha+\NIIwb\ images}
      \label{SectionHSTAlignment}
      Since the point-spread function (PSF) is undersampled by the WFC, sub-pixel
      accuracy in the alignment between two LRF emission-line images is necessary for
      the construction of an excitation map, especially if those images contain sharp,
      unresolved features as seen in Mrk~573. Using the relative astrometry provided
      in the image headers to align the \OIII\ and \Ha+\NII\ images, we find that the
      central emission-line peak in the \OIII\ image of Mrk~573 is offset by roughly
      half a pixel from the \Ha+\NII\ peak. A similar offset is found for two
      off-nuclear isolated blobs. For this alignment, the excitation map, obtained by
      clipping both the \OIII\ and \Ha+\NII\ images at 3 times the r.m.s. noise and
      dividing the former by the latter, displays unusually high \OIII-to-(\Ha+\NIIwb)
      ratios in extremely narrow structures, which are almost certainly artefacts
      resulting from image misalignment. This strongly suggests that the apparent
      offset between the \OIII\ and \Ha+\NII\ peaks is not real (in particular, it is
      not due to reddening) and should be corrected.
      
      To check that the shift-vector derived from a comparison of the positions of
      individual features in the \OIII\ and \Ha+\NII\ images was correct, we have
      used the following procedure which provided us with a second estimate of the
      shift-vector. We first calculated an array of excitation maps in which the
      \Ha+\NII\ image was shifted by various fractions of a pixel with respect to the
      position derived from the relative HST astrometry. For each of these excitation
      maps, we then calculated the sum, $\Sigma$, of all the ratios \OIII/(\Ha+\NII) which are greater than 2. For sharp structures in the image, even a slight error in the
      alignment between \OIII\ and \Ha+\NII\ maps increases $\Sigma$ by
      artificially creating areas of apparent high excitation. Hence, we selected the shift-vector
      for which $\Sigma$ had a minimum value. This method yielded essentially the same shift
      as determined from comparing the positions of individual features. It is
      noteable that, because the emission-line structure of Mrk~573 is dominated by
      thin arcs, most of which are elongated roughly north-south, the shift-vector was
      much better determined in the east-west direction ($\pm0.15$ pixel) than in the
      north-south direction ($\pm 0.3$ pixel). The fact that these two different
      methods yield the same shift-vector gives us confidence that the offset was
      indeed a consequence of uncertainties in the internal astrometry of HST,
      resulting from the telescope moves between the different LRF exposures.

\subsection{TIGER 3D spectroscopy}
   \label{SectionTiger}
   \subsubsection{Observations and data-reduction}
      \label{SectionTigerReduction}
      These 3D (x,y,$\lambda$) observations of Mrk~573 have been obtained at the Canada France Hawaii
      Telescope on 1993 November 19 and 20, using the TIGER integral field
      spectrograph. A detailed description of the instrument, which uses a compact
      array of more than 300 circular micro-lenses, can be found in Bacon et al.
      (\cite{Bacon95}). Two overlapping fields, one centered east and the other west
      of the nucleus, have each been observed in two different spectral
      configurations (Table~\ref{TableTigerLog}), one centered at 5000~\AA\ (B
      configuration) and the other at 6750~\AA\ (R configuration). With a total field
      of view of 11\arcsec$\times$6\arcsec\ centered on the nucleus, we cover the
      whole radio structure (Ulvestad \& Wilson \cite{Ulvestad84}; Falcke et al.
      \cite{Falcke98}) and the inner bright structures of the emission-line region
      (Pogge \& De~Robertis \cite{Pogge95}; Capetti et al. \cite{Capetti96}). The
      spatial sampling was 0\farcsec39 per lens, with a seeing limited resolution of
      $\sim$0\farcsec9-1\farcsec0.
      
      The data-reduction has been performed using the TIGER software (Rousset
      \cite{Rousset92}) under the ESO-MIDAS environment. Residuals from the
      wavelength calibration relation were smaller than 0.1\,\AA\
      in both spectral configurations, much smaller than the 1.8\,\AA\ size of the
      spectral pixel. The absolute wavelength calibration of the R configuration has
      been checked using sky lines and found to be accurate to within 10-20 \kms.
      Comparison of the \Ha\ (R configuration) and \Hb\ (B configuration)
      centroid velocities at various locations in our field of view has shown a
      --50~\kms\ systematic shift of the B configuration relative to the R
      configuration. In view of the accuracy of the R configuration wavelengths noted
      above, we have assumed this shift is an instrumental effect in the B
      configuration data and have corrected for it, by adding 50\,\kms\ to the \Hb\
      and \OIII\ velocities. In the following, all the cited velocities are
      heliocentric (a correction of --17\,\kms\ having been applied for the motion of
      the earth).
      
      The spectra were flux calibrated by means of observations of the standard stars
      HD\,19445 and Hiltner\,600. Comparison of their spectra shows that differences
      in the flux calibration are $<$~6\,\%. Integrating our data over a
      2\farcsec7$\times$4\farcsec0 aperture centered on the nucleus, we derive
      F$_{\mbox{\scriptsize H}\beta}$ = 5.9 $\times$ \ten{-14}\,\ergscm\ and
      F$_{\mbox{\scriptsize H}\alpha+\mbox{\scriptsize [N{\sc
      ii}]}\,\lambda\lambda\,6548,6583}$ = 5.2 $\times$ \ten{-13}\,\ergscm, which are
      14\%\ and 10\%\ higher than the fluxes from Koski (\cite{Koski78}),
      respectively. This is consistent with the differences of up to 20\% reported by
      Capetti et al. (\cite{Capetti96}) and Pogge \& De~Robertis (\cite{Pogge95}) when
      comparing their observations with Koski's fluxes. Comparison
      of the TIGER data with our set of HST emission-line images (see
      Sect.~\ref{SectionHST}) shows differences in flux of 15\,\% (\OIII) and 11\,\%
      (\Ha+\NII).
      
      As Mrk~573 filled our field of view, preventing us from deriving a sky
      spectrum free of contamination by the galaxy, sky subtraction of the R
      configuration spectra has been performed using the following procedure. We first
      constructed a spatially averaged spectrum which minimizes the contribution of
      the galaxy by summing together spectra in regions where the galaxy is faint. A
      Gaussian fit to the sky and galaxy lines present in this high signal to noise
      ratio spectrum was then performed (the wavelengths of the sky emission-lines
      were taken from Osterbrock et al. \cite{Osterbrock96}). Last, the resulting synthetic sky
      spectra (one for each data cube of the R configuration) derived from the fit
      have been subtracted from each spectrum of the R1 and R2  data cubes.
      Figure~\ref{FigureSky} shows one of the two synthetic sky spectra and outlines
      the weakness of sky lines compared to the noise in our data.
      
      The two overlapping fields in each spectral configuration have been combined
      using the continuum peak of the galaxy as a reference. The isophotes
      in the region of overlap were carefully examined to check the
      alignment. The final data cubes consist of 570 (B configuration) and 540 (R
      configuration) flux calibrated spectra.

   \subsubsection{Analysis of the spectral data}
      \label{SectionTigerAnalysis}
      The continuum in both the R and B configuration spectra was fitted with a
      straight line across line-free wavelength segments and removed. Contamination by
      stellar absorption is a potential problem for only the \Hb\ emission-line,
      which has equivalent widths (EW) ranging from 15 to 50~\AA\ in the regions of
      interest, whereas the \Hb\ absorption line has a typical EW of 4.5-6.5~\AA, as
      estimated from the E7 and E8 stellar templates of Bica et al. (in Leitherer et
      al. \cite{Leitherer96}), which provide a correct match (especially the template
      E7) to the shape of the continuum in the 3000--7000~\AA\ nuclear spectrum of
      Mrk~573 obtained by Storchi-Bergmann et al. (\cite{Storchi96}, see their
      Fig.~3, upper panel). For \Ha, the typical EW of the absorption-line estimated
      in the same way is 2-2.5~\AA, while the EW of the emission line ranges between
      30 and 230~\AA\
      over the emission-line region.
      
      The basic spectral parameters of the emission-lines (intensity, centroid
      velocity and FWHM) were derived from single component Gaussian fitting of the
      lines using the FIT/SPEC software (Rousset \cite{Rousset92}). Lines from
      doublets (\OIIIww, \NIIww\ and \SIIwwb) were forced to have the same width and
      velocity, as well as fixed or bounded ratios when relevant (\OIIIwb/\OIIIwa\ =
      2.88; \NIIwb/\NIIwa\ = 2.94; 0.35 $<$ \SIIwc/\SIIwd\ $<$ 1.5). In the
      following, all the FWHM given in \AA\ are `as measured', i.e. without any
      correction for instrumental resolution, while the FWHM given in \kms\ have
      been corrected for instrumental resolution by subtraction of 3.6\,\AA\
      in quadrature.
      
   \subsubsection{Deconvolution of the TIGER data cubes}
      \label{SectionTigerDeconvolution}
      The PSF of our TIGER observations, modelled as the sum of two co-aligned
      Gaussian functions of different widths and intensities, has been estimated
      using the HST emission-line images. In this computation, we have used a
      non-linear least-square algorithm, comparing the emission-line image derived
      from the TIGER data cube integrated over the HST filter bandpass with the HST
      emission-line image convolved by various estimates of the PSF. Note that this
      process also provided and allowed correction of any shift and/or rotation of the
      TIGER data cube relative to the HST image. The PSF parameters (FWHM core/FWHM
      halo/ratio of halo to core intensity peaks) are found to be
      0\farcsec95/1\farcsec9/0.22 and 1\farcsec05/2\farcsec3/0.07 for the R and B
      configurations, respectively.
      
      To improve the spatial resolution of the ground-based 3D data cubes, we
      have deconvolved our continuum subtracted TIGER data cubes, resampled onto
      square pixels, using a new method developed in a collaboration between the
      University of Maryland, the CRAL - Observatoire de Lyon and L.B. Lucy (Ferruit
      et al. 1999, in preparation). This method is based on a Richardson-Lucy
      algorithm (Richardson \cite{Richardson72}; Lucy \cite{Lucy74}) modified to use
      the high angular resolution HST emission-line images as a  guide for the
      deconvolution process, following suggestions by Lucy (1997, private
      communication). Basically, this modified Richardson-Lucy algorithm will favor
      solutions in which the HST emission-line image is recovered when the deconvolved
      ground-based data are integrated over the range of wavelength included in the
      HST image (the integration being weighted by the throughput of the HST/LRF at
      each wavelength).
      
      This `guided deconvolution' method has been tested on only a few
      astrophysical examples and not yet through extensive application to artificial
      data cubes. Such a complete study of the method will be published in a
      forthcoming paper (Ferruit et al. 1999, in preparation). We have therefore
      performed a deconvolution of each of the \OIIIwb\ and \Ha+\NII\ data cubes of
      Mrk~573 in three different ways. The first method was a pure Richardson-Lucy
      deconvolution, each velocity slice of the data cube being deconvolved
      independently using the known PSF and a classical Richardson-Lucy algorithm. The
      second and third methods were a `weakly guided' deconvolution in which the
      guiding term (which forces the deconvolved TIGER image to correspond to the HST
      image) is weighted by a coefficient $\alpha$ = 0.1, strongly reducing its
      impact on the deconvolution process, and a `strongly guided' deconvolution with
      full implementation of the guiding term ($\alpha$ = 1).
      
      Figure~\ref{FigureComparison} shows the original data and the results of these
      three deconvolutions for the \OIIIwb\ line of Mrk~573. We display the \OIII\ line
      intensity, centroid velocity and FWHM maps derived from single Gaussian fitting
      of the \OIIIwb\ line in each spectrum of the original TIGER data (top row), as
      well as of each spectrum of the three deconvolved data cubes (second, third and
      fourth rows). As expected, an excellent match to the \OIII\ HST image (see
      Fig.~\ref{FigureHST1}) is obtained using the `strongly guided' method
      (Fig.~\ref{FigureComparison}, bottom row), which also seems to achieve the best
      spatial resolution in the velocity and FWHM maps (e.g. the strongly blueshifted
      feature $\simeq$~1\farcsec2 northwest of the nucleus is split into two knots).
      The two other methods (pure Richardson-Lucy, second row,  and `weakly guided',
      third row) give results similar to those of the `strongly guided' method, but
      with lower spatial resolution. We emphasize that all the features discussed in
      this paper are present in all three deconvolved data cubes. Some features, like
      the blueshifted gas $\simeq$~1\farcsec2 northwest of the nucleus are apparent in
      the original TIGER data cube before deconvolution (Fig.~\ref{FigureComparison},
      top row), as well as in lower spatial resolution spectrographic data from the
      literature (Whittle et al. \cite{Whittle88}; Tsvetanov \& Walsh
      \cite{Tsvetanov92}).
      
      To avoid the possibility of introducing spurious features due to possible
      ambiguities in the guided method, while still taking advantage of the additional
      constraint provided by the HST image, we have decided to base our discussion on
      the results of the `weakly guided' method, but using 300 iterations instead of
      the 150 used in the test. This method also has the advantage of
      minimizing the effects of changes in the LRF bandpass with position on the
      WFC, which may lead to unreliable results in the guided images. Therefore, in
      the following, the \OIII, \NII\ or \Ha\ deconvolved data will refer to the data
      cubes obtained with 300 iterations using the `weakly guided' method.
      Applying to the deconvolved \OIII\ data cube the same method than used for the original (not deconvolved) data cubes
      (see above), we find that, for this cube, the spatial PSF after deconvolution
      has a Gaussian profile, with a FWHM of $\sim$0\farcsec35.
      
      HST images are not available in the \Hb\ and \SIIwwb\ lines, so no guided
      deconvolution was possible and the data cubes have been deconvolved using a pure
      Richardson-Lucy method (150 iterations). The PSF was taken from the neighbouring
      \OIII\ (for the \Hb\ cube) and \Ha+\NII\ (for the \SII\ cube) data cubes.
      
\section{Results}
   \subsection{HST imaging}
      \label{SectionResultsImaging}
      Figures~\ref{FigureHST1} (top panels) and \ref{FigureHST2} show the HST
      emission-line images. The emission-line region of Mrk~573 is seen to
      consist of a bright, elongated central peak and two pairs of arcs which
      are connected by knotty features.  In the following, we will label the arcs as
      SE2-arc, SE1-arc, NW1-arc, and NW2-arc, from the southeast end to the northwest
      end of the emission-line region (see Fig.~\ref{FigureLabel}). The strings of
      knots will be labelled similarly, as SE2-knots, SE1-knots, and NW1-knots
      (Fig.~\ref{FigureLabel}). Overall, the same structures were seen in the Faint
      Object Camera images of Capetti et al. (\cite{Capetti96}). Note that,
      however, in the higher spatial resolution \OIII\
      images of Capetti et al. (\cite{Capetti96}, Fig.~1a), the elongation to the
      southeast of the central peak is spatially resolved into a distinct, off-nuclear
      component which we will consider hereafter as part of SE1-knots. 
      
      In the PC emission-line images (Fig.~\ref{FigureHST2}), two narrow absorption
      features are present immediately north and northwest of the nucleus, running
      roughly perpendicular to the major axis of the emission-line region. The inner
      two arcs (SE1-arc \& NW1-arc) are very narrow and apparently are not resolved
      transversely. A third pair of arcs (labelled SE3-arc and NW3-arc) is present
      9\farcsec4 from the nucleus (Durret \& Warin \cite{Durret90}; Pogge \& De
      Robertis \cite{Pogge95}), but are outside our field of view. Note that SE3-arc
      and NW3-arc coincide with the beginning of small \Ha\
      spiral arms (Pogge \& De Robertis \cite{Pogge95}).
      
      Figure~\ref{FigureHST1} (bottom left panel) shows an excitation map in which
      dark areas correspond to high \OIII-to-(\Ha+\NIIwb) ratios as derived from the
      LRF images. This map (see also Fig.~11 in Falcke et al. \cite{Falcke98}) shows
      very clearly the double wedge form expected if the gas is photoionized by a
      beamed nuclear ionizing continuum. As one can see from
      Table~\ref{TableHSTFluxes} and Fig.~\ref{FigureHST1}, the various emission-line
      features display quite different excitation levels. While the
      \OIII-to-(\Ha+\NII) ratio is relatively low in the innermost two arcs (SE1-arc
      and NW1-arc), it is higher in the next two arcs (SE2-arc and NW2-arc). The
      strings of knots display \OIII-to-(\Ha+\NII) ratios inbetween those of the inner
      and outer arcs. There is a strong north-south gradient in the
      \OIII-to-(\Ha+\NII) ratio along NW2-arc.
      
      In Table~\ref{TableHSTFluxes}, we have listed the main components visible in the
      emission-line images and their properties. We outlined each feature manually and
      determined the radial offsets of their midpoints from the continuum peaks
      (assumed to coincide), their \OIII\ fluxes, their relative contribution to the
      total \OIII\ flux, and the range of \OIII-to-(\Ha+\NII) ratios. For most
      features the encircled area given in Table~\ref{TableHSTFluxes} is best treated
      as an upper limit, as the areas are sensitive to the WF camera resolution.
      
      Figure~\ref{FigureOIII} (top left panel) shows the 2~cm radio continuum
      isophotes (Falcke et al. \cite{Falcke98}) overlaid on the \OIII\ image. The peak
      of the central flat-spectrum radio-blob (Falcke et al. \cite{Falcke98}) has been
      aligned with the optical continuum peaks of our images. It is noteable that the
      optical continuum peak need not be the true nucleus of the galaxy due to
      the dust lane close to it (Fig.~\ref{FigureHST2}; see
      also Capetti et al. \cite{Capetti96}). A point-like nuclear source is detected
      in the mid-infrared L$^\prime$ band by Alonso-Herrero et al. (\cite{Alonso98}),
      and probably coincides with the nucleus, but no accurate position is available.
      With our adopted registration, the northwest radio component is almost
      coincident with the NW1-knots, while the southeast radio component coincides
      with the SE1-arc, close to the beginning of the SE2-knots. The curvature of
      the SE1-knots is very similar to the curvature of the elongation to the
      southeast of the nuclear radio component seen in both the 2~cm and 3.5~cm radio
      maps (Falcke et al. \cite{Falcke98}). The SE1-knots also point toward the
      southeast radio component. This strongly suggests that the inner arcs and knots
      are directly related to the radio material.
      
      Finally, we want to remark that despite the very elongated structure of
      the emission-line region and the presence of a wedge-shaped ionization
      structure, there are several axes in the images. The two inner strings of knots
      (SE1-knots and NW1-knots), for example, define directions that seem to be
      inclined with respect to the central axis of the wedge and point rather towards
      the northern tip of the north-west wedge, and the southern end of the south-east
      wedge. There is also a faint spur of \OIII\ emission extending over
      2\arcsec\ from the eastern edge of SE2-arc to the edge of our field of view
      (Fig.~\ref{FigureHST1}). This feature is present in the ground-based
      \OIII\ images of Pogge \& De Robertis (\cite{Pogge95}, their Fig.~4) and links
      SE2-arc to SE3-arc. The fact that it does not appear in our \Ha+\NII\ image
      (Fig.~\ref{FigureHST1}) suggests that it is a region of high \OIII/(\Ha+\NII)
      ratio. A counterpart to this spur, to faint to be detected in the HST images, is
      seen exactly opposite to it (Pogge \& De Robertis \cite{Pogge95}, their Fig.~4),
      extending from the western edge of NW2-arc, where one sees higher
      \OIII/(\Ha+\NII) ratios in our excitation map (see Fig.~\ref{FigureHST1}), to
      the beginning of NW3-arc. These inner knots and outer spurs define an X-shape
      centered approximately on the nucleus.
   
   \subsection{Spectroscopy}
      \subsubsection{\OIIIwb\ line and gaseous kinematics}
	 \label{SectionResultsOIII}
	 Of the spectral lines available in our TIGER observations of Mrk~573,
	 \OIIIwb\ combines the advantages of being strong (i.e. having a good signal
	 to noise even in the deconvolved data cube) and isolated (i.e. yielding less
	 ambiguity for the final data cube\footnote{The flux in the HST narrow-band
	 \OIIIwb\ image used to guide the deconvolution is distributed over a single
	 line instead of the three lines in the \Ha+\NII\ configuration.}).
	 Therefore, the \OIII\ deconvolved data cube will be our primary source of
	 information on the gaseous kinematics.
	 
	 The results of single component Gaussian fitting of the deconvolved \OIII\
	 data cube (`weakly guided' method, 300 iterations) are shown in
	 Fig.~\ref{FigureOIII}. Most of the features present in the HST \OIII\
	 images (top left) can be found in the \OIII\ map reconstructed from the
	 deconvolved data cube (top right), despite the worse spatial
	 resolution of the latter. The middle right and bottom right panels of
	 Fig.~\ref{FigureOIII} display the \OIII\ velocity field and FWHM map,
	 respectively, as derived from the same single component Gaussian fitting of the deconvolved \OIII\
	 data cube. To show the relative locations of features seen in
	 these maps, their contours are shown superimposed on the \OIII\ intensity
	 image in the middle left and bottom left panels.
	 
	 The second pair of arcs (SE2-arc and NW2-arc) displays a smooth velocity
	 pattern consistent with the ENLR velocity field previously observed
	 between 4\arcsec\ and 6\arcsec\ (Unger et al. \cite{Unger87}; Whittle et al.
	 \cite{Whittle88}; Durret \& Warin \cite{Durret90}; Tsvetanov \& Walsh
	 \cite{Tsvetanov92}). If this smooth velocity pattern traces rotational motion
	 of the ENLR gas, as suggested by previous authors (Durret \& Warin
	 \cite{Durret90}; Tsvetanov \& Walsh \cite{Tsvetanov92}), the kinematical
	 major axis PA of the ENLR gas is $\sim$60\degr-90\degr, which is
	 different from the PA of $\sim$30\degr-40\degr\ inferred by Tsvetanov \&
	 Walsh (\cite{Tsvetanov92}).
	 
	 Within a radius of 2\arcsec\ from the nucleus, three regions of strong
	 deviation from the smooth velocity pattern are seen along the emission-line
	 region major axis. The two perturbations southeast of the nucleus are
	 redshifted with respect to the systemic velocity, in an area where
	 blueshifted gas is expected from the extrapolation of the ENLR velocity field
	 into the nuclear regions. These regions will be labelled SE2-red (a weak
	 velocity deviation at $\sim$2\farcsec1 [660 pc] projected radius, see
	 Fig.~\ref{FigureOIIIProfile}) and SE1-red (a strong velocity deviation at
	 $<$0\farcsec8 [250~pc] projected radius, see Fig.~\ref{FigureOIIIProfile}).
	 Northwest of the nucleus, at $\sim$1\farcsec1 [360~pc] projected radius, the
	 \OIII\ gas displays the strongest blueshifts observed in our entire field of view,
	 peaking at --\,320 \kms\ relative to the systemic velocity. This region will
	 be labelled NW1-blue (see Fig.~\ref{FigureOIIIProfile}). While the
	 highly blueshifted gas at NW1-blue has been detected in previous lower
	 spatial resolution obervations (Whittle et al. \cite{Whittle88}; Tsvetanov
	 \& Walsh \cite{Tsvetanov92}), this is the first time that the weaker
	 deviations SE1-red and SE2-red have been clearly detected. The existence of
	 SE1-red was only suggested by the extended red wings observed in the \OIII\
	 line south of the nucleus by Whittle et al. (\cite{Whittle88}).
	 
	 Figure~\ref{FigureOIII} (middle left) shows that SE1-red and NW1-blue are
	 located within the two inner arcs (SE1-arc and NW1-arc), while SE2-red is located on
	 the outer-edge of SE1-arc and is possibly associated with SE2-knots.
	 It is noteable that these three velocity perturbations are not
	 coincident with any high \OIII\ surface brightness features.
	 Comparison with the 3.6~cm radio continuum map of Falcke et al.
	 (\cite{Falcke98}) shows, however, that each of the three velocity features is
	 located close to one of the three strong radio components
	 (Fig.~\ref{FigureOIII}, middle right).
	 
	 The FWHM map (Fig.~\ref{FigureOIII}, bottom right) shows that the outer arcs
	 (SE2-arc and NW2-arc) have narrow line widths, as expected if they are ambient,
	 photoionized  gas. Surprisingly, the FWHM map does not peak at the nucleus,
	 but the regions of highest FWHM form a ring around the nucleus of Mrk~573,
	 inside the two inner arcs (SE1-arc and NW1-arc), at radii similar to that of
	 the north-west radio lobe. These locations of increased FWHM correspond to
	 the locations of the SE1-red and NW1-blue velocity deviations. Last, a small
	 increase in FWHM is also associated with SE2-red.
	 
	 To compare the kinematical and spectral properties of the various components
	 present in these maps, we have selected a set of eight representative
	 locations (four arcs, the nucleus, and three velocity perturbations), the
	 positions of which are listed in Table~\ref{TableSpecSpec}. Spectra averaged
	 over 0\farcsec2 diameter apertures centered on these locations have been
	 extracted from the deconvolved data cube and used to derive the
	 kinematical and spectral characteristics of  each of these components.
	 Figure~\ref{FigureOIIIProfile} shows the \OIIIwb\ velocity profile for each
	 component as well as their location on the \OIII\ intensity and velocity
	 maps. A strong red (blue) wing, extending up to 700~\kms (--800~\kms), is
	 seen in the spectrum of SE1-red (NW1-blue), consistent with the extended
	 wings observed by Whittle et al. (\cite{Whittle88}) in their high spectral
	 resolution \OIII\ profiles. Results of single component Gaussian fitting of
	 these spectra are displayed in Fig.~\ref{FigureOIIISpec} and listed in
	 Table~\ref{TableSpecSpec}.
	 
	 The \OIII\ velocity at the nucleus is 5140~\kms, which is indistinguishable
	 of the stellar systemic velocity of 5150~\kms (Nelson \& Whittle
	 \cite{Nelson95}). Comparison of the continuum and \OIII\ maps reconstructed
	 from our original (not deconvolved) B configuration observation, shows a
	 0\farcsec2 shift (along PA $\sim$ 160\degr) of the \OIII\ peak relative to
	 the continuum peak. Such a shift was reported in Whittle et al.
	 (\cite{Whittle88}) and results from the bright inner \OIII\ knots to the
	 southeast of the nucleus (SE1-knots, see Fig.~\ref{FigureLabel}).
	 
	 As SE2-red, SE1-red and NW1-blue lie nearly along PA = 125\degr, we can use the
	 velocity gradient of 16 \kms\ arcsec$^{-1}$ derived by Durret \& Warin
	 (\cite{Durret90}) along this PA as a reference to estimate the deviations of
	 the velocities of these three features from the ambient velocity field. Using
	 the locations and centroid velocities listed in Table~\ref{TableSpecSpec}, we
	 find deviations of the velocity centroids of +60 \kms\ (SE2-red), +140 \kms\
	 (SE1-red) and --460 \kms\ (NW1-blue). Application of the same procedure
	 to SE2-arc, which is likely part of the ENLR and should therefore
	 show little deviation from the ambient velocity field, yields a deviation of
	 -80~\kms. This might indicate that the velocity gradient derived from Durret
	 \& Warin's (\cite{Durret90}) data is too low because of their relatively low
	 spatial resolution. 
	 
      \subsubsection{\Hb\ emission-line}
	 Figure~\ref{FigureHb} shows the results of single component Gaussian fitting
	 to the original (not deconvolved) TIGER \Hb\ data cube. Due to the lower
	 signal to noise of this weaker line (compare the \Hb\ intensity map of
	 Fig.~\ref{FigureHb} with the corresponding \OIII\ map displayed in
	 Fig.~\ref{FigureComparison}, top left panel), we have not been able to derive
	 a reliable high spatial resolution map from the deconvolved data cube.
	 Nevertheless, the gross features of the \Hb\ intensity, velocity and FWHM
	 distributions (Fig.~\ref{FigureHb}) agree well with the corresponding
	 distributions for \OIII\ (Fig.~\ref{FigureOIII}). \Hb\ spectra and single
	 component Gaussian fits of the eight regions defined in
	 Fig.~\ref{FigureOIIIProfile} are displayed in Fig.~\ref{FigureHbSpec}, but
	 the low signal to noise ratio of these spectra extracted from the deconvolved
	 \Hb\ data cube makes most of the fitting results listed in
	 Table~\ref{TableSpecSpec} uncertain.
	 
      \subsubsection{\Ha\ and \NIIww\ emission-lines}
	 Figure~\ref{FigureNIIHa} displays the reconstructed maps of the intensity
	 (left), velocity (middle) and FWHM (right) of \Ha\ (top panels) and
	 \NIIwb\ (bottom panels) as derived from single component Gaussian fitting of
	 the deconvolved \Ha+\NII\ data cube (`weakly guided' method, 300
	 iterations). Although somewhat noisier than their \OIII\ equivalents, these
	 maps contain all the features seen in the \OIII\ maps, except the weak
	 SE2-red component. The \Ha+\NII\ spectra at the eight selected locations and
	 their single component Gaussian fits are shown in Fig.~\ref{FigureNIIHaSpec},
	 while numerical values (centroid velocities, FWHMs and \NIIwb/\Ha\ ratios)
	 are listed in Table~\ref{TableSpecSpec}.
	 
	 The derived \Ha\ and \NII\ velocities are in good agreement with the \OIII\
	 velocities for the arcs and the nucleus, but the velocity deviations from
	 the ambient rotation curve at SE2-red, SE1-red and NW1-blue seem smaller for
	 these two lower excitation lines than for \OIII. In both the \Ha\ and \NII\
	 lines, the NW1-blue region is less extended than in \OIII\ and is shifted by
	 a few tenths of an arcsecond to the north compared to its location in \OIII.
	 Unfortunately, one has to be cautious when comparing the results of two
	 different deconvolved data cubes, as it is difficult to disentangle real
	 differences from differences in spatial resolution.
	 
	 As for the \OIII\ line, the second pair of arcs (SE2-arc and NW2-arc) is
	 part of a smooth velocity field and exhibits relatively narrow line widths (see
	 Table~\ref{TableSpecSpec}), suggesting that they mark the beginning of the
	 kinematically less disturbed ENLR. They also differ from the regions closer
	 to the nucleus (i.e. the NLR) in having lower \NIIwb/\Ha\ ratios (0.4-0.5
	 instead of 0.7-1.0 for the inner regions, see Table~\ref{TableSpecSpec}).
	 This is consistent with the results of Tsvetanov \& Walsh
	 (\cite{Tsvetanov92}), who found a decrease of the \NII/\Ha\ ratio with
	 projected radial distance from the nucleus. The \NII\ and \Ha\
	 lines are narrower than the \OIII\
	 lines in the NW1-arc (corrected FWHM of 160, 140 and 230~\kms\ for \NII, \Ha\
	 and \OIII, respectively).
	 
      \subsubsection{\SIIwwb\ emission-lines}
	 The last lines available in our data set are those of the \SIIwwb\
	 doublet, the ratio of which is a density indicator. As for \Hb, the
	 weakness of these lines has prevented us from obtaining accurate high spatial
	 resolution maps using the deconvolved \SII\ data cube. Therefore,
	 Fig.~\ref{FigureSII} shows the results of single component Gaussian fitting
	 to the lines in the original (not deconvolved) \SII\ data cube.
	 
	 The top panels of Fig.~\ref{FigureSII} show the intensity maps in the
	 individual \SIIwc\ (left) and \SIIwd\ (right) lines. As for the \OIwa\
	 emission-line images of Pogge \& De Robertis (\cite{Pogge95}), the two inner
	 arcs (SE1-arc and NW1-arc) are more prominent in these \SII\ images than in
	 the \OIII\ or \Ha\ images, witnessing the peak in \SIIwwb/\Ha\ ratio detected
	 at these locations (Tsvetanov \& Walsh \cite{Tsvetanov92}). The signature of
	 the ambient velocity field is clearly seen in the outer regions of this
	 0\farcsec9-1\farcsec0 resolution velocity map (Fig.~\ref{FigureSII}, middle
	 left). As for \OIII, and assuming that this velocity field traces rotational
	 motions, the inferred kinematical major axis PA is between 60\degr\ and
	 90\degr, which is different from the PA of  30\degr-40\degr\ measured by Tsvetanov
	 \& Walsh (\cite{Tsvetanov92}) at larger distances from the nucleus. Although
	 diluted by the lower spatial resolution, the SE1-red and NW1-blue velocity
	 components are also present, but no trace of SE2-red.
	 
	 The FWHM map (Fig.~\ref{FigureSII}, middle right) shows the contrast between
	 the narrow lines of the second pair of arcs (SE2-arc and NW2-arc) and the
	 broader lines of the inner regions. Even at this spatial resolution, the
	 nucleus is not associated with the regions of maximum FWHM, which in fact
	 straddle it. There is a `finger' of increased FWHM extending southeast from
	 SE1-arc into SE2-arc, which may be related to the SE2-red
	 component.
	 
	 The bottom panels of Fig.~\ref{FigureSII} show the \SIIwc/\SIIwd\ (\SII\
	 ratio hereafter) map, overlaid with isophotes of \SIIwc\ intensity (left)
	 and \SII\ isovelocity contours (right). Lower values of the \SII\ ratio
	 (i.e. darker regions in the figures) correspond to higher gas densities, with
	 high and low density limit ratios of $\sim$0.4-0.5 and $\sim$1.4-1.5,
	 respectively. The SE1-arc and NW1-arc form the boundaries of a region of
	 higher density gas, while the SE2-arc and NW2-arc exhibit \SII\ ratios close
	 to the low density limit.
	 
	 Figure~\ref{FigureSIISpec} displays the \SIIwwb\ spectra extracted from the
	 \SII\ deconvolved data cube (Richardson-Lucy method, 150 iterations),
	 overlaid with their single component Gaussian fits. The derived centroid
	 velocities, FWHM and \SII\ ratios are listed in Table~\ref{TableSpecSpec}.
	 The nuclear spectrum indicates a velocity close to systemic, consistent with
	 the results inferred from the other lines, and a high density of 3600~\cmc.
	 The two inner arcs display intermediate densities -- 850~\cmc\  (SE1-arc) and
	 580~\cmc\ (NW1-arc). Note that, as for the \Ha\ and \NII\ lines, NW1-arc
	 exhibits a small width of $\sim$ 140~\kms\ in the \SII\ lines.
	 
	 The signal to noise achieved in the SE2-red, SE1-red and NW1-blue velocity
	 components is poor (see Fig.~\ref{FigureSIISpec}), preventing us from
	 deriving an accurate estimate of the density in these regions. We can only
	 say that these regions seem to display broader lines than the others, a
	 result in good agreement with what we see for the other lines. Last, the
	 \SII\ spectra of SE2-arc and NW2-arc indicate low densities and narrow line
	 widths (see Table~\ref{TableSpecSpec}), our results being once more limited
	 by the low signal to noise of these spectra.
	 
      \subsubsection{Line ratios and gas density}
	 \label{SectionResultsRatio}
	 Taking advantage of the good spatial resolution and coverage achieved in our
	 TIGER observations even without deconvolution, we have built diagrams similar
	 to those presented by Tsvetanov \& Walsh (\cite{Tsvetanov92}), tracing the
	 evolution of various line ratios as a function of the projected radial
	 distance from the nucleus. We have used the minor axis
	 of the emission-line region in PA = 35\degr\ as a separator, defining a
	 southeast side (negative radii) and a northwest side (positive radii). Figure~\ref{FigureRatio} (top left panel) displays the
	 distribution of \OIIIwb\ intensity as a function of the projected radial
	 distance from the nucleus. In addition to the central peak at the nucleus,
	 four secondary peaks are seen at radii roughly corresponding to those of the
	 emission-line arcs and are labelled accordingly.
	 
	 As mentioned in Sect.~\ref{SectionTigerAnalysis}, contamination of the \Hb\
	 emission-line by stellar absorption is a concern. We
	 have therefore corrected the \Hb\ emission-line fluxes for this
	 absorption by assuming a constant equivalent width of 5.5~\AA\ (an average
	 of the 4.5 and 6.5~\AA\ values estimated in Sect.~\ref{SectionTigerAnalysis})
	 for the stellar \Hb\ absorption line . The \OIII/\Hb\ ratios before
	 (black crosses) and after (blue and red symbols) correction are
	 displayed in Fig.~\ref{FigureRatio} (middle left panel). 
	 Low signal to noise ratios have been rejected by clipping the \Hb\ and
	 \OIIIwb\ peak fluxes (as derived from the single component Gaussian fitting)
	 at 0.4 $\times$ \ten{-15} \ergscmAarcsec\ (i.e. $\sim$13~$\sigma$) before
	 division.
	 
	 Before correction, the \OIIIwb/\Hb\ ratio peaked at the nucleus and decreased
	 steadily with projected radius, with ratios in the 10 to 15 range. After
	 correction, the \OIII/\Hb\ ratio is roughly constant ($\sim$10) within
	 2\arcsec\ of the nucleus, with a slight increase at the location of
	 the nucleus, where it reaches values of $\sim$10--12. Note the
	 relatively large scatter of the points, even close to the nucleus, which is
	 at least partly  due to the combination of the poor signal to noise ratio
	 in \Hb\ and the uncertain correction for contamination by
	 the \Hb\ stellar absorption line.
	 
	 The \NIIwb/\Ha\ (middle right panel) and \SIIwwb/\Ha\ (bottom left
	 panel) line ratios display very similar behaviors, with a minimum at the
	 location of the nucleus, and maxima close to the inner arcs.  The
	 gas density (bottom right panel), as derived from the \SII\ ratio, decreases
	 with increasing radial distance from the nucleus. This largely confirms the
	 results of Tsvetanov \& Walsh (\cite{Tsvetanov92}), but with improved spatial
	 resolution and more extensive spatial coverage. The decrease of the gas
	 density with increasing nuclear distance is steeper to the southeast of the
	 nucleus than to the northwest.

\section{Discussion}
   \subsection{The nucleus}
      \label{SectionDiscussionNucleus}
      The nucleus (see Fig.~\ref{FigureLabel}) exhibits a centroid velocity close to
      systemic and the line profiles from all four sets of ionic species, although
      broad (330-440~\kms, Table~\ref{TableSpecSpec}), are narrower than at SE1-red
      and NW1-blue (see Fig.~\ref{FigureOIII}). The nuclear lines also do not display
      the broad wings seen in these regions.
      
      In the inner 2\arcsec-3\arcsec (630-950~pc), the line ratios built from our data
      cubes (see Fig.~\ref{FigureRatio}) indicate that, in the context of simple
      photoionization models, the ionization parameter of the gas increases toward the
      nucleus (see Sect.~\ref{SectionDiscussionCentralSource} and Fig.~\ref{FigureU}),
      consistent with the results of Tsvetanov \& Walsh (\cite{Tsvetanov92}). The
      nuclear and circumnuclear spectra of Mrk~573 (from Storchi-Bergmann et al.
      \cite{Storchi96}) have also been successfully modelled with a mixture of
      matter-bounded and ionization-bounded (MB-IB) clouds photoionized by the nuclear
      radiation (Binette, Wilson, \& Storchi-Bergmann \cite{Binette96}). These more
      sophisticated photoionization models successfully reproduce the relatively high
      \OIII\ temperature observed in both the nuclear and circumnuclear regions of
      Mrk~573 (13000-17000~K, Tsvetanov \& Walsh \cite{Tsvetanov92}; 12000-13000~K,
      Wilson, Binette, \& Storchi-Bergmann \cite{Wilson97}), as well as its difference
      from the \NII\ temperature ($\sim$9000~K, Wilson et al. \cite{Wilson97}). We
      should, however, bear in mind that the nuclear spectrum of Mrk~573 obtained by
      Storchi-Bergmann et al. (\cite{Storchi96}) is integrated over a
      2\farcsec8$\times$1\farcsec5 region with 1\arcsec\ seeing, and hence includes
      the inner knots and arcs (see relative fluxes of the components in
      Table~\ref{TableHSTFluxes}). The unresolved nucleus (see Fig.~\ref{FigureLabel})
      may contain gas of sufficiently high density that the $^1$D$_2$ levels of
      O$^{+2}$ and N$^{+1}$ suffer collisional de-excitation, rendering temperature
      determinations assuming the gas is in the low density limit unreliable. However,
      this effect is unlikely to be important in the above referenced temperature
      determinations, because of the relatively small contribution of the unresolved
      nucleus to the measured line fluxes (see Table~\ref{TableHSTFluxes}).
      
      From the observed line ratios, we have no way of determining whether the photons
      which ionize the gas in the unresolved nucleus originate in a compact central
      source or in extended photoionizing shocks\footnote{As outlined by Allen,
      Dopita, \& Tsvetanov (\cite{Allen98}), the classical optical line ratios alone
      cannot distinguish between shock+precursor and classical photoionization
      models.}. However, given the absence of a strong kinematical signature of the
      interaction between the radio ejecta and the ambient gas in the nuclear
      spectrum, we favor a scenario in which the energy budget of the nuclear ionized
      gas is dominated by photoionization by the central source.
       
   \subsection{The narrow line region}
      \label{SectionDiscussionNLR}
      We will consider the narrow line region to comprise those parts of the
      emission-line nebulosity with \OIIIwb\ line FWHM (corrected for instrumental
      broadening) $>$~200~\kms. This region extends 2\farcsec5 (790~pc) southeast and
      northwest of the nucleus, and includes the morphological features SE2-knots,
      SE1-arc, SE1-knots, NW1-knots and NW1-arc (see Fig.~\ref{FigureLabel}) and the
      spectroscopic features SE2-red, SE1-red and NW1-blue (see
      Fig.~\ref{FigureOIIIProfile}). In the following, we first discuss the nature of
      the strings of knots and then consider models for the arcs in which the gas is
      ionized either by `external' photons (e.g. produced by the AGN), or by photons
      created `in situ' by shocks in the outflow.
	 
      \subsubsection{The knots -- signatures of jet/ambient medium interactions ?}
	 \label{SectionDiscussionNLRKnots}
	 The small sizes of the strings of knots and the proximity to the nucleus of
	 SE1-knots and NW1-knots, make them more difficult to study than the arcs.
	 They comprise a distribution of bright emission-line knots, possibly linked
	 by low luminosity filaments (Capetti et al. \cite{Capetti96}; see also
	 Fig.~\ref{FigureHST1} and \ref{FigureLabel}). One of the knots of NW1-knots
	 is resolved into an arc-like structure in the high-spatial resolution FOC
	 \OII\ and \OIII\ images of Capetti et al. (\cite{Capetti96}).
	 
	 As emphasized by Capetti et al. (\cite{Capetti96}), the morphology of the
	 inner strings of knots of Mrk~573 is reminiscent of that of the inner regions
	 of NGC~1068. The inner knots are also the locus of strong kinematical
	 disturbances (SE1-red and NW1-blue), similar to (although weaker than)
	 those observed in NGC~1068 (P\'econtal et al. \cite{Pecontal97}; Axon et al.
	 \cite{Axon98}). These kinematic disturbances have been interpreted as the
	 result of interactions between jets and  the interstellar gas (e.g.
	 P\'econtal et al. \cite{Pecontal97}; Steffen et al. \cite{Steffen97b}; Axon
	 et al. \cite{Axon98}; Bicknell et al. \cite{Bicknell98}).
	 
	 One of the primary manifestations of a jet can be the presence of a hot
	 expanding cocoon of shocked jet material, driving shocks into the ambient
	 medium. This interaction between the cocoon and the ambient medium may
	 produce clumps of dense gas in a shell-like structure around the cocoon
	 (Steffen et al. \cite{Steffen97a}). For a cylindrical cocoon, the predicted
	 emission-line profiles are symmetric and we would have to be observing only
	 one side of the cocoon to see the redshifts and blueshifts associated with
	 SE1-red and NW1-blue, respectively. Such a situation can arise if the jet is
	 propagating in a stratified medium (e.g. the expected gradient of density
	 perpendicular to the galaxy disk), with the denser side dominating the
	 emitted spectrum. However, in Mrk~573, the northwest side of the emission-line
	 region is less reddened and believed to be in front of the galaxy disk
	 (Tsvetanov \& Walsh \cite{Tsvetanov92}; Storchi-Bergmann et al.
	 \cite{Storchi96}). Thus, the dense side of the northwest cocoon would be
	 expected to be receeding, whereas the observations show that NW1-blue is
	 blueshifted. Therefore, it seems unlikely that the inner knots reflect the
	 expansion of a jet cocoon.
	 
	 Bright optical knots and filaments (as well as radio hot spots) can also be
	 created during the interaction of a jet with ambient gas clouds (e.g.
	 Blandford \& K\"{o}nigl \cite{Blandford79}; Wilson \cite{Wilson82}; Allen
	 \cite{Allen84}). This scenario is supported by the fact that the bright
	 emission-line knots are associated with radio hot spots, as well as with
	 changes in the direction of the radio `jet' (Falcke et al. \cite{Falcke98};
	 see Sect.~\ref{SectionResultsImaging}). This suggests that the knots may
	 trace the deflection of the radio jet by individual clouds (as modelled by,
	 e.g. Bicknell et al. \cite{Bicknell98}). An interaction between a jet and a
	 dense might easily account for the velocity shifts seen at SE1-red and
	 NW1-blue, with the southeast jet receding and the northwest jet coming toward
	 us. Although the \OIII/\OII\ ratio increases by more than a factor of two at
	 the location of one of the knots of NW1-knots (Fig.~3, bottom panel, in
	 Capetti et al. \cite{Capetti96}), other line ratios, such as
	 \OIII/(\Ha+\NIIwb), are roughly constant in the central 1\arcsec-1\farcsec5
	 (Fig.~\ref{FigureHST1}, bottom left). Therefore, in Mrk~573, such a
	 jet-cloud interaction is not associated with changes in the gaseous
	 excitation as dramatic as seen in NGC~1068 (e.g. cloud G, see Fig.~3 in
	 P\'econtal et al. \cite{Pecontal97}). High spatial resolution observations of
	 higher excitation lines would be helpful in this connection. In this
	 scenario, the association between SE2-knots and SE2-red would trace the
	 deflection of a weaker jet (created by an older nuclear ejection), perhaps
	 driving a bow shock seen as SE2-arc. Last, the arc-like sub-structure in
	 NW1-knots (see Fig.~1 in Capetti et al. \cite{Capetti96}) is very similar to
	 what is obtained in the numerical simulations of Steffen et al.
	 (\cite{Steffen96}) if the head of the jet is currently impinging on a dense
	 cloud.
      
      \subsubsection{Central source photoionization models for the inner arcs}
	 In the following, we consider models in which the gas in SE1-arc and NW1-arc
	 is photoionized by a compact nuclear source. In these models, the arcs could
	 be either pre-existing gaseous structures unrelated to the nuclear activity
	 (e.g. rings or spiral arms), or the post-shock cooling regions of bow shocks
	 driven by the radio ejecta. In this scenario, the outflow has enough power to
	 create shocks and the observed emission-line structures,but not enough power
	 to provide the ionizing radiation (e.g. Whittle et al. \cite{Whittle88};
	 Wilson, Ward \& Haniff \cite{Wilson88}). An argument in favor of central
	 source photoionization models is the good agreement between the ends of the
	 arcs and the edges of the wedge-shaped ionization structure (see
	 Fig.~\ref{FigureHST1}).
	 
	 \paragraph{Photon budget:}
	    \label{SectionDiscussionNLRPhoton}
	    As we need high spatial resolution data to measure accurately the
	    emission-line fluxes in the arcs, we have used the \OIII\ fluxes
	    derived from the HST image (see Table.~\ref{TableSpecSpec}), to
	    compute the absolute \Hb\ luminosity, ${\cal L}_{\mbox{\scriptsize
	    H}\beta}$, of SE1-arc and NW1-arc. Assuming a distance to Mrk~573 of
	    65~Mpc, and including a reddening correction factor of 2.7 (E(B-V) =
	    0.3~mag, Tsvetanov \& Walsh \cite{Tsvetanov92}; extinction curves from
	    Cardelli, Clayton, \& Mathis \cite{Cardelli89} with R$_V$ = 3.1), we
	    derive absolute \OIII\ fluxes in the arcs of 2.1 $\times$ \ten{41} \ergs\
	    (SE1-arc) and 2.3 $\times$ \ten{41} \ergs\ (NW1-arc). Using the observed
	    \OIII/ \Hb\ ratio of $\sim$10.5 , this yields absolute \Hb\ luminosities
	    ${\cal L}_{\mbox{\scriptsize H}\beta}$ of 2.0 and 2.2 $\times$ \ten{40}
	    \ergs\ for SE1-arc and NW1-arc, respectively.
	    
	    Using the case B recombination rates and assuming a temperature of
	    \ten{4}~K, the total number of recombinations per second in the arc
	    (${\cal N}_{rec}$) and the total number of ionizing photons
	    reaching the arc per second (${\cal N}_{ion}$), are:
	    \begin{equation}
	    {\cal N}_{ion}\,\left[\,\mbox{\small photon s}^{-1}\,\right]
	    \; = \; 
	    {\cal N}_{rec}\,\left[\,\mbox{\small recombination s}^{-1}\,\right] 
	    \;=\; 
	    2.09 \,\times\, 10^{12} \; 
	    {\cal L}_{\mbox{\scriptsize H}\beta}\,\left[
	    \,\mbox{\small erg s}^{-1}\,\right]
	    \end{equation}
	    For a nuclear source, the ionizing photon flux, ${\cal F}_{ion}$, at the
	    radius of the arc is then: 
	    \begin{equation}
	    {\cal F}_{ion}\,\left[\,\mbox{\small photon s}^{-1}
	    \,\mbox{\small cm}^{-2}\,\right]
	    \,=\,
	    2.09 \;\; 10^{12}
	    \;\;
	    \frac
	    {
	    {\cal L}_{\mbox{\scriptsize H}\beta}\,\left[
	    \,\mbox{\small erg s}^{-1}\,\right]
	    }    
	    {
	    4\pi R^{2}\,\left[ \,\mbox{\small cm}^{2}\, \right]
	    }
	    \;\;
	    \left( \frac{\Omega\,\left[\,\mbox{\small sr}\,\right]}{4\pi} 
	    \right)^{-1}
	    \;\; \varepsilon^{-1}
	    \end{equation}
	    where $R$ is the radius of the arc, $\Omega$ is the solid angle
	    subtended by the arc, as seen from the nucleus, and $\varepsilon$ the
	    covering factor of the arc material. Assuming that the direction of
	    nuclear ejection lies in the plane of the sky\footnote{Otherwise, ${\cal
	    F}_{ion}$ in equation (2) would have to be multiplied by
	    cos$^2$($\theta$), where $\theta$ is the angle between the direction of
	    ejection and the plane of the sky.}, and that the arcs are circular `bow
	    shocks' viewed edge-on, this equation gives ionizing photon fluxes, ${\cal
	    F}_{ion}$, at the location of the arcs of 2 $\times$ \ten{10}
	    $\varepsilon^{-1}$~\pcms\
	    and 1.6 $\times$ \ten{10} $\varepsilon^{-1}$~\pcms\ for SE1-arc ($R$ =
	    1\farcsec8 = 570~pc, $\Omega$/4$\pi$ = 0.054) and NW1-arc ($R$ =
	    2\farcsec3 = 725~pc, $\Omega$/4$\pi$ = 0.047), respectively.

	 \paragraph{Ionization parameter:}
	    \label{SectionDiscussionNLRU}
	    In order to estimate the ionization parameter of the gas in SE1-arc and
	    NW1-arc from the observed \NIIwb/\Ha\ and \SIIwwb/\Ha\ line ratios, we
	    have used Mappings Ic (see Appendix A in Ferruit et al. \cite{Ferruit97a})
	    to compute a sequence of simple photoionization models with ionization
	    parameters U (defined in equation (3) below) varying between \ten{-4} and
	    1. All the models involved a plane-parallel, radiation-bounded slab of
	    gas with constant density (500~\cmc) and solar abundances, which is
	    photoionized by an AGN-type central source. The spectrum of the ionizing
	    radiation has been modelled as a broken power-law (f$_{\nu} \propto
	    \nu^{\alpha}$) with different values of $\alpha$ at the low and high
	    energies. For energies $<$~50~eV, we have used the spectral index $\alpha$
	    = --0.9 measured by Kinney et al. \cite{Kinney91}) from an IUE
	    spectrum of Mrk~573. For energies $>$~50~eV, we have used a steeper
	    power-law with $\alpha$ = --2, corresponding to a rough average of the set
	    of model-dependent values of $\alpha$ derived by Turner, Urry, \&
	    Mushotzky (\cite{Turner93}, typical photon index $\Gamma$ = 3 = 1 --
	    $\alpha$) from ROSAT/PSPC observations of Mrk~573 in the 0.1-2.0~keV
	    band. The \NIIwb/\Ha, \SIIwwb/\Ha\ and \OIwa/\Ha\ line ratios inferred
	    from these models for \ten{-4} $<$ U $<$ 1 are shown in
	    Fig.~\ref{FigureRatioU}. For SE1-arc and NW1-arc, the line ratios are
	    \NII/\Ha\ $\sim$ 1 and \SII/\Ha\
	    $\sim$ 0.65 (see Table~\ref{TableSpecSpec} and Fig.~\ref{FigureRatio}),
	    which implies ionization parameters of 9 $\times$ \ten{-4} and 5 $\times$
	    \ten{-4}, respectively. 
	    
	    The ionizing photon flux (${\cal F}_{ion}$) at the location of SE1-arc and
	    NW1-arc can be expressed as a function of the ionization parameter U and
	    the gas density N$_{\mbox{\scriptsize H}}$:  
	    \begin{equation}
	    {\cal F}_{ion}\,\left[\,\mbox{\small photon s}^{-1} \,\mbox{\small
	    cm}^{-2}\,\right] \,=\, 
	    \mbox{c}\,\left[\,\mbox{\small cm s}^{-1}\,\right]
	    \;\;
	    \mbox{U}
	    \;\;
	    \mbox{N}_{\mbox{\scriptsize H}}
	    \,\left[ \,\mbox{\small cm}^{-3}\,\right]
	    \end{equation}
	    where c is the speed of light. Using the values of U determined above and
	    the measured electron densities (Table~\ref{TableSpecSpec} and
	    Fig.~\ref{FigureRatio}), this equation gives ionizing photon fluxes of
	    1.3--2.2 $\times$ \ten{10} \pcms\
	    (SE1-arc, U = 5--9 $\times$ \ten{-4}, N$_{\mbox{\scriptsize H}}$ =
	    850~\cmc) and 0.9-1.5 $\times$ \ten{10} \pcms\
	    (NW1-arc, U = 5--9 $\times$ \ten{-4}, N$_{\mbox{\scriptsize H}}$ =
	    580~\cmc), in good agreement with the values of ${\cal F}_{ion}$
	    derived from the absolute \Hb\ luminosities of the arcs (2 $\times$
	    \ten{10} $\varepsilon^{-1}$ and 1.6 $\times$ \ten{10}
	    $\varepsilon^{-1}$~\pcms\
	    for SE1-arc and NW1-arc, respectively, see previous section). Under the
	    assumption of central source photoionization, this implies that the
	    arc-like features have a covering factor $\varepsilon$ $\sim$1, i.e. they
	    absorb all or most of the ionizing photons incident on them, only a small
	    fraction of the area of the putative `bow shocks', as seen from the
	    nucleus, containing low density `holes' which are porous to ionizing
	    radiation.
	    
	    According to the MB-IB models of Binette et al. (\cite{Binette96}),
	    changes in the \NII/\Ha\ and \SII/\Ha\ ratios could be a result of changes
	    in the ratio of the solid angle subtended by MB clouds to the solid angle
	    subtended by IB clouds (A$_{\mbox{\scriptsize M/I}}$), rather than the
	    ionization parameter as assumed in our calculations. However, in the MB-IB
	    models, any A$_{\mbox{\scriptsize M/I}}$-sequence is accompanied by
	    changes in the \HeII/\Hb\ ratio, contrary to what happens for a
	    U-sequence (see Fig.~7 in Binette et al. \cite{Binette96}). In Mrk~573, the
	    nuclear/circumnuclear and external regions have very similar
	    \HeII/\Hb\ ratios (see Fig.~7, open circles in Binette et al.
	    \cite{Binette96}), indicating that the changes in line ratios actually
	    reflect changes in the ionization parameter, and not in
	    A$_{\mbox{\scriptsize M/I}}$, supporting our interpretation.
	 
	 \paragraph{Bow shock models of the inner arcs:}
	    \label{SectionDiscussionNLRBowShock}
	    A possible description of the inner arcs is provided by the geometrically
	    thin bow shock models developed by Taylor, Dyson, \& Axon
	    (\cite{Taylor92}) and Ferruit et al. (\cite{Ferruit97a}). These models
	    assume that the thin arcs generated by the bow shocks are photoionized by
	    nuclear radiation and are transparent to this radiation. To test if  the
	    observed properties of the inner arcs could be reproduced using the bow
	    shock models of Ferruit et al. (\cite{Ferruit97a}), we have computed a
	    grid of bow shock models, manually adjusting the input parameters to try
	    to find the best match to the observed spectral characteristics (line
	    ratios and absolute fluxes) of the arcs. Following the same procedure as
	    used for NGC~5929 (Ferruit et al. \cite{Ferruit97b}), we have first set
	    the geometrical input parameters (D$_Z$ and C, see Ferruit et al.
	    \cite{Ferruit97a}), but this time using the \OIII\ image as a reference.
	    In Fig.~\ref{FigureHST1}, the lateral and longitudinal (along the jet
	    axis) extents of the arcs are 1\farcsec5 ($\sim$470~pc) and 0\farcsec5
	    ($\sim$160~pc), respectively for SE1-arc, and  1\farcsec7 ($\sim$530~pc)
	    and 0\farcsec8 ($\sim$250~pc), respectively for NW1-arc. These geometries
	    have been reproduced by using values of D$_Z$ (the distance from the shock
	    apex along the jet axis at which this distance is equal to the radius r of
	    the bow shock) of 290~pc (SE1-arc) and 360~pc (NW1-arc) and setting C = 1
	    (Z $\propto$ r$^2$ bow shock profile, see Ferruit et al.
	    \cite{Ferruit97a}).
	    
	    As for the photoionization models (see Sect.~\ref{SectionDiscussionNLRU}),
	    the spectrum of the nuclear ionizing source has been modelled by a broken
	    power-law (f$_{\nu} \propto \nu^{\alpha}$) with $\alpha$ = --0.9 for
	    energies $<$~50~eV, and --2 for energies $>$~50~eV. Additional ionizing
	    radiation generated `in situ' by hot shocked gas has not been included
	    (see Sect.~4.5.2 in Ferruit et al. \cite{Ferruit97a}).
	    
	    We have included in the model only the emission from the observed arcs.
	    The ends of the arcs (i.e. the `tails' of the bow shocks in this
	    interpretation) are observed to correspond roughly to the edges of the
	    wedge shaped ionization structure (Fig.~\ref{FigureHST1}). In those
	    `tails', the fluxes of emission-lines like \OIII\ or \NII\ and \Ha\ are
	    dominated by emission from the cool, dense photoionized post-shock gas
	    (see Fig.~11b in Ferruit et al. \cite{Ferruit97a}). Thus, in the adopted
	    case of central source photoionization of the bow shock,  the absence of
	    line emission outside the observed arcs is best explained by a dearth of
	    nuclear ionizing photons, with the gas at large distance from the jet axis
	    flowing outside of the ionization cone.
	    
	    A grid of bow shock models was then constructed by varying the shock
	    velocity, the pre-shock density, the magnetic parameter and the pre-shock
	    ionization parameter. We have not been able to reproduce simultaneously
	    the observed strengths of medium excitation lines like \OIII\ and those of
	    low excitation lines like \OII, \NII, \SII\ and especially \OI\
	    (\OIwa/\Ha\ $\sim$ 0.13, Tsvetanov \& Walsh \cite{Tsvetanov92}). This is a
	    direct consequence of the relative locations of the low and medium
	    excitation photoionization regions in the bow shock models. In the
	    evolutionary track of a gas cell, the low ionization parameter region (in
	    which the low excitation lines are emitted) starts immediately after the
	    particle catastrophic cooling, where the gas density is maximum. Then, as
	    the gas flows along the bow shock, its ionization parameter increases
	    slowly as its density decreases, allowing the existence of medium to high
	    excitation ionic species and their emission lines, like \OIII. To obtain
	    the high \OIII/\Hb\ ratios observed in the inner arcs, the pre-shock
	    density [and/or the ionization parameter] has to be low [high] enough for
	    the medium excitation regions to exist before the truncation imposed by
	    the observed geometry of the arcs. This cannot be done without weakening
	    the low excitation lines to levels inconsistent with the observations.
	    Therefore, the bow shock model, as it stands in Ferruit et al.
	    (\cite{Ferruit97a}), fails to reproduce the observed line ratios.
	    
	    The models of Ferruit et al. (\cite{Ferruit97a}) assume the cool
	    post-shock gas is transparent to the ionizing radiation of the central
	    source, i.e. the ionization of the gas is matter-bounded. A future
	    development, which could improve the agreement with observations, would be
	    the inclusion of absorption of ionizing radiation in the post-shock gas,
	    i.e. allowing the gas to be ionization-bounded to photons from the central
	    source.
      
      \subsubsection{Photoionizing shock models of the inner arcs}
	 \label{SectionDiscussionNLRFastShocks}
	 
	 Recently, Dopita \& Sutherland (\cite{Dopita95}, \cite{Dopita96}) have
	 calculated the emission-line spectra of radiative shock waves with velocities
	 in the range 150 $\leq$ V$_S$ $\leq$ 500~\kms. In this sub-section, we
	 compare the energetic, spectral and structural properties of these
	 shock + precursor models with those of SE1-arc and NW1-arc.
	 
	 Using equations (2.2) and (2.4) of Dopita \& Sutherland (\cite{Dopita95}),
	 the total luminosity of the shock + precursor in \Hb, ${\cal
	 L}_{\mbox{\scriptsize H}\beta}$ (erg s$^{-1}$), may be expressed in terms of
	 the shock velocity V$_S$, the shock area ${\cal A}$, and the pre-shock
	 density n$_{\mbox{\scriptsize 0}}$:
	 \begin{equation}
	    {\cal L}_{\mbox{\scriptsize H}\beta} = 
	    \left[ 
	    7.44 \times 10^{-6} \; 
	    \left( \frac{\mbox{\small V}_S}{\mbox{\small 100 km s}^{-1}}
	    \right)^{2.41} +
	    9.85\times 10^{-6}
	    \left( \frac{\mbox{V}_S}{\mbox{\small 100 km s}^{-1}} \right)^{2.28}
	    \right] 
	    \,
	    \left( \frac{\mbox{n}_{\mbox{\scriptsize 0}}}{\mbox{\small cm}^{-3}} \right)
	    \,
	    \left( \frac{\cal A}{\mbox{\small cm}^{2}} \right)
	 \end{equation}
	 Assuming that the SE1-arc and NW1-arc features represent bow shocks viewed
	 edge-on and that their line of sight depths are equal to their extents in the
	 plane of the sky (cylindrically symmetric bow shocks), we can use the
	 bow shock geometrical parameters derived in
	 Sect.~\ref{SectionDiscussionNLRBowShock} to estimate the shock areas of
	 these two arcs. This yields shock areas of ${\cal A}_{\mbox{\scriptsize
	 SE1-arc}}$ = 0.24~kpc$^2$ and ${\cal A}_{\mbox{\scriptsize NW1-arc}}$ =
	 0.38~kpc$^2$, similar to the geometric areas used in
	 Sect.~\ref{SectionDiscussionNLRU}. For the parameters needed to power the
	 line emission from the arcs (n$_{\mbox{\scriptsize 0}}$ $>$ 10~\cmc, V$_S$
	 $\sim$ 500~\kms, see below), the cooling length  ${\cal L}_{\mbox{\scriptsize
	 cool}}$ = 1.4 $\times$ \ten{-9} {V$_{S}$}$^{4}$ n$_{\mbox{\scriptsize
	 o}}^{-1}$ pc (where V$_S$ is in \kms\
	 and n$_{\mbox{\scriptsize 0}}$ in \cmc, Whittle et al. \cite{Whittle86}) $<$
	 9~pc, much smaller than the spatial resolution of the HST images, justifying
	 our use of the emission-line images to estimate the shock area.
	 
	 Next, to derive estimates of the pre-shock densities, we have computed the
	 shock compression factors appropriate to the \SII-emitting zone. These
	 factors were derived from the \SII\ line ratios listed for the grids of
	 models of Dopita \& Sutherland (\cite{Dopita96}, their Tables~8 and 10), for
	 various combinations of reasonable shock velocities (V$_S$ = 200, 300, 400
	 and 500~\kms) and magnetic parameters (B$_{\mbox{\scriptsize
	 0}}$/{n$_{\mbox{\scriptsize 0}}$}$^{1/2}$ = 0, 1, 2 and 4 $\mu$G cm$^{3/2}$).
	 Given the densities of 850~\cmc\ (SE1-arc) and 580~\cmc\ (NW1-arc) inferred
	 from the observed \SII\ ratio (see Table~\ref{TableSpecSpec}), we have used
	 these compression factors to build a table of pre-shock densities as a
	 function of the shock velocity and magnetic parameter for SE1-arc and
	 NW1-arc. The results are tabulated in Table~\ref{TableNH} and have been used,
	 together with the shock areas ${\cal A}_{\mbox{\scriptsize SE1-arc}}$ and
	 ${\cal A}_{\mbox{\scriptsize NW1-arc}}$, to compute the corresponding \Hb\
	 luminosities of SE1-arc and NW1-arc predicted by the photoionizing shock
	 models (see Table~\ref{TableLHb}).
	 
	 Absolute \Hb\ luminosities comparable to the observed ones (20 and 22
	 $\times$ \ten{39} \ergs\ for SE1-arc and NW1-arc, respectively, see
	 Sect.~\ref{SectionDiscussionNLRPhoton}) can only be produced by the
	 combination of high shock velocities (400-500~\kms), high pre-shock densities
	 ($>$ 10~\cmc) and  high magnetic parameter (2-4~$\mu$G cm$^{3/2}$). The same
	 range of velocities is suggested by the observed \OIIIwb/\Hb\ ratio
	 ($\sim$10--11) which can only be produced in shocks with velocities between
	 400 and 500~\kms\ (for solar abundances). Therefore, in the following we will
	 consider only shock velocities, pre-shock densities and magnetic parameters
	 in these ranges.
	 
	 A first problem with these models is the lack of kinematic perturbations at
	 the locations of SE1-arc and NW1-arc in our velocity and line width maps of
	 \NIIww, \Ha\ and \SIIwwb\ (Fig.~\ref{FigureNIIHa} and \ref{FigureSII}). In
	 models with shock velocities of 400-500~\kms\ and magnetic parameters of
	 2-4~$\mu$G cm$^{3/2}$, about $\sim$50~\% of \Ha, $\sim$94~\% of \NIIww\ and
	 $\sim$93~\% of \SIIwwb\  come from the shock region and should therefore
	 exhibit broadened line profiles relative to the \OIIIwb\ line, more than
	 $\sim$80~\% of which comes from the kinematically quiescent precursor. This
	 extra broadening of the low excitation lines is not seen in our observations
	 (see Table~\ref{TableSpecSpec}). In fact, the opposite effect is seen in
	 NW1-arc, where the \NII, \Ha\ and \SII\ lines are {\it narrower} than the
	 \OIII\ line (see Table.~\ref{TableSpecSpec}).
	 
	 Another problem comes from the spatial extent of the precursor. Equation
	 (4.3) in Dopita \& Sutherland (\cite{Dopita96}) gives the extent of the
	 Str\"{o}mgren region of the precursor, yielding lengths of ${\cal L}$ = 610
	 $\times$ 1/n$_{\mbox{\scriptsize 0}}$ pc (V$_S$ = 400~\kms), where
	 n$_{\mbox{\scriptsize 0}}$ is the pre-shock density in \cmc. Given our
	 estimates of the pre-shock densities in the arcs (8-20~\cmc\ and 5-13~\cmc\
	 for SE1-arc and NW1-arc, respectively, see Table~\ref{TableNH}) and for this
	 conservative shock velocity of 400~\kms\ (the extent of the precursor has to
	 be multiplied by a factor of 2 for a 500~\kms\ shock), this yields typical
	 extents of 30-80~pc (SE1-arc) and 50-120~pc (NW1-arc) for the \HII\ region of
	 the precursor, which corresponds roughly to the extent of the \OIII\ zone
	 (see Fig.~4d in Dopita \& Sutherland \cite{Dopita96}). Thus, the \OIII
	 -emitting region should be at least partially resolved in the HST
	 observations, whereas the arcs are not transversally resolved in our \OIII\
	 WFC image (30~pc per pixel), and probably not in the \OIII\ FOC image (30~pc
	 resolution) of Capetti et al. (\cite{Capetti96}, the arcs are termed
	 `narrow'). Furthermore, the \OIIw\ emission comes partly from the partially
	 ionized region of the precursor and partly from the post-shock region. Given
	 that these two regions are, respectively, upstream of the precursor \HII\
	 region and downstream of the shock (see Fig.~4d in Dopita \& Sutherland
	 \cite{Dopita96}), a clear split of the arcs should be seen in the \OII\ HST
	 images, with the \OIII\ arcs lying inbetween. In contrast, the observations
	 by Capetti et al. (\cite{Capetti96}, see their Fig.~1) show that the
	 structures of NW1-arc in \OIIw\ and \OIIIww\ are almost identical. Any
	 systematic shift between the arcs in these two lines is $<$ 0\farcsec05
	 (17~pc), in contradiction to the predictions of the model. Projection effects
	 will tend to create transversely extended \OII\
	 arcs, still in disagreement with the observations.
	 
	 These considerations rule out a description of the arcs as photoionizing
	 radiative shocks. This kind of model will always require the existence of a
	 photoionized precursor to produce the high \OIII/\Hb\ ratios observed in the
	 arcs, and will suffer from the problems described in the previous paragraph.
	 We conclude that, if the inner arcs are shock features, the shock velocity
	 must be low ($<$ 300~\kms, to prevent line widths in excess of those
	 observed and the formation of a spatially resolved precursor) and the
	 ionizing photons must be provided by an external source, such
	 as the nucleus.
	          
  \subsection{The outer arcs and the origin of the ionizing photons}
     \subsubsection{General considerations}
	Our observations clearly show that the second pair of arcs (SE2-arc and
	NW2-arc) marks the transition between the denser, high-velocity dispersion NLR
	(\OIII\ FWHM $>$ 200~\kms), and the more rarefied, lower velocity dispersion
	ENLR of Mrk~573. These two arcs have a lower gas density and narrower
	lines than the gas in the NLR (Table~\ref{TableSpecSpec}), but larger
	densities and broader lines (\OIII\ FWHM of 120-150~\kms) than the ENLR gas
	(\OIII\ FWHM $<$ 45~\kms, Unger et al. \cite{Unger87}). They seem to
	participate in the ambient velocity field, as does the ENLR gas (Unger et al.
	\cite{Unger87}; Whittle et al. \cite{Whittle88}; Durret \& Warin
	\cite{Durret90}; Tsvetanov \& Walsh \cite{Tsvetanov92}), although they have a
	steeper velocity gradient than measured from regions further out (Durret \&
	Warrin \cite{Durret90}; see Sect.~\ref{SectionResultsOIII}).
	
	It is tempting to infer a common origin for the inner and outer arcs, such as
	bow shocks driven by a jet. However, the increasing distortion of the arcs
	from bow shock shapes with increasing radius (Fig.~\ref{FigureHST2} and
	\ref{FigureLabel}; see also Pogge \& De Robertis \cite{Pogge95}), and their
	kinematics indicate that SE2-arc and NW2-arc are significantly affected by
	rotational shearing of the gas (Pogge \& De Robertis \cite{Pogge95}). These
	two outer arcs also have lower densities than SE1-arc and NW1-arc (see
	Table~\ref{TableSpecSpec}), as well as a much more diffuse, filamentary
	morphology (see Fig.~\ref{FigureHST2} and \ref{FigureLabel}). All this
	suggests that, even though the outer arcs may share a similar origin with the
	inner arcs (shocks driven by an intermittent nuclear outflow?), they are less
	prominent due to a lower pre-shock density and are currently merging with the
	ambient interstellar medium.
	
	Capetti et al. (\cite{Capetti96}) have suggested that this filamentary
	structure of the outer arcs, as well as the absence (or
	faintness\footnote{Falcke et al. (\cite{Falcke98}) claim the detection of very
	faint radio emission associated with the outer arcs in their tapered 3.5~cm
	map.}) of associated radio emission could correspond to the rapid expansion of
	a radio lobe associated with a sudden drop in interstellar gas density, as
	inferred in IRAS~04210+040 (Holloway et al. \cite{Holloway96}). However, we do
	not observe the high velocities and large broadening of the lines expected
	in this case. The observations suggest a much less violent process, involving
	a progressive weakening of the nuclear outflow and the associated shocks with
	increasing distance from the nucleus.
	
     \subsubsection{Viability of a central source photoionization model}
	\label{SectionDiscussionCentralSource}
	The outer arcs (SE2-arc and NW2-arc) have higher \OIII/\OII\ and \OIII/\Ha\
	ratios than the inner arcs (SE1-arc and NW1-arc). Combining this result with
	the gas densities inferred by Tsvetanov \& Walsh (\cite{Tsvetanov92}), Capetti
	et al. (\cite{Capetti96}) have argued that a local source of ionization is
	required to explain the ionization structure of Mrk~573. However, this
	comparison depends on reddening-sensitive line ratios and a combination of
	line ratios and density measurements obtained from observations with very
	different spatial resolutions. A more complete study can be conducted
	using our original (not deconvolved) R configuration data set (\NII+\Ha+\SII),
	which provides homogeneous measurements of line ratios which are sensitive to
	the ionization parameter U, but insensitive to reddening (\NIIwb/\Ha\ and
	\SIIwwb/\Ha), and of the gas density (as derived from the \SII\ ratio) with a
	much better spatial coverage and resolution than what previously
	obtained by Tsvetanov \& Walsh (\cite{Tsvetanov92}).
	
	We have therefore used the simple photoionization models described in
	Sect.~\ref{SectionDiscussionNLRU} (see also Fig.~\ref{FigureRatioU}) to
	convert the observed maps of the \NIIwb/\Ha\ and \SIIwwb/\Ha\ ratios (original
	maps, i.e. not deconvolved) into maps of the ionization parameter U. The
	resulting map of U derived from the \NII/\Ha\ ratio and the dependence of U on
	distance from the nucleus are shown in Fig.~\ref{FigureU} (upper left and
	upper right panels, respectively). At a given radius, the ionization parameter
	derived from the \NII/\Ha\ ratio is systematically higher than the ionization
	parameter obtained using the \SII/\Ha\ ratio, indicating that the simple
	photoionization models used to derive U are not a very accurate description of
	the actual situation. The \SII/\Ha\ ratio, in particular, is also sensitive to
	the precise density used in the models, due to collisional deexcitation of the
	$^2$D$_{3/2}$ and $^2$D$_{5/2}$ levels of \sii. However, the two distributions
	show a similar behavior as a function of radius. As the \NII/\Ha\ ratios
	have a better signal to noise ratio than the \SII/\Ha\
	ratios and are unlikely to be affected by collisional deexcitation (except
	perhaps in the unresolved nucleus), only the ionization parameters derived
	from the \NII/\Ha\ ratios will be used in the following analysis.
	
	As clearly apparent in Fig.~\ref{FigureU} (upper panels), the ionization
	parameter displays a minimum at the locations of SE1-arc and NW1-arc, before
	increasing further out in SE2-arc and NW2-arc. The ionization parameter in
	NW2-arc displays a strong asymmetry, higher values of U being observed in
	the southern part of the arc (Fig.~\ref{FigureU}, upper left). The same
	asymmetry is seen in the \OIII/(\Ha+\NIIwb) map derived from our HST
	observations (Fig.~\ref{FigureHST1}, bottom left panel). There is also an area
	of lower U at the north-end of NW1-arc. However, the range of U along the
	NW1-arc is only a factor of $\sim$1.5.
	
	As each point in Fig.~\ref{FigureU}, upper right panel, was obtained from a
	spectrum which also provides a measurement of the density, we have computed
	for each point the ionizing photon flux, ${\cal F}_{ion}$, using equation (3).
	The resulting dependence of ${\cal F}_{ion}$ on radius is shown in
	Fig.~\ref{FigureU} (lower left). Locations where the observed \SII\ ratio is
	consistent with the low density limit are indicated in light red or light
	blue. For these points, the actual density could be much lower than the
	nominal estimate, and we would overestimate ${\cal F}_{ion}$ in our
	calculation. Points within 0\farcsec5 of the nucleus (gray rectangle in the
	figure) are also subject to uncertainty as they can be affected by (spatial)
	resolution and collisional deexcitation effects. We find that ${\cal F}_{ion}$
	decreases with increasing projected radius from the nucleus, up to a radius of
	$\sim$2\farcsec5. Beyond that, there is a wide spread of ${\cal F}_{ion}$ at a
	given radius, but most of the density information at these radii comes from
	\SII\ line ratios consistent with the low density limit.
	
	We have then computed, for each point, the rate of emission of ionizing
	photons, ${\cal Q}_{ion}$, by a central compact source which corresponds to
	the measured photon flux, ${\cal F}_{ion}$. For a given deprojected radius R,
	and assuming that the nuclear ionizing radiation does not suffer any
	absorption on its way from the nucleus, we have:
	\begin{equation}
	{\cal Q}_{ion}\,\left[\,\mbox{\small photon s}^{-1}\right]
	\,=\,	
	4\pi\, \left( \mbox{R}\,\left[\,\mbox{\small cm}\,\right]\,\right)^2
	\;
	{\cal F}_{ion}\,\left[\,\mbox{\small photon s}^{-1} \,\mbox{\small
	cm}^{-2}\,\right] 
	\end{equation}
	Assuming that the emission-line region lies in the plane of the sky and using
	equation (3), ${\cal Q}_{ion}$ becomes:
	\begin{equation}
	{\cal Q}_{ion}\,\left[\,\mbox{\small photon s}^{-1}\right]
	\,=\,
	3.6\,\times\, 10^{53} \;\;
	\left( \mbox{r}\,\left[\,\mbox{\small \arcsec}\,\right]\,\right)^2
	\;
	\mbox{U}
	\;
	\mbox{N}_{\mbox{\scriptsize H}}
	\,\left[ \,\mbox{\small cm}^{-3}\,\right]	    
	\end{equation} 
	The resulting dependence of ${\cal Q}_{ion}$ on r is shown in
	Fig.~\ref{FigureU} (right panel). As for ${\cal F}_{ion}$, the estimates of
	${\cal Q}_{ion}$ in the inner 0\farcsec5 region (grey rectangle) are not
	reliable and should be treated with caution.
	
	If the central ionizing source has not varied\footnote{The dependence of
	${\cal Q}_{ion}$ on radius could be explained if the nuclear ionizing flux
	has decreased by more than a factor 10 in the last 4,000 years.}, ${\cal
	Q}_{ion}$ must be constant, or decrease with increasing radius if there is any
	absorption of the nuclear radiation on its way to the outer parts of the
	emission-line nebulosities. Instead, ${\cal Q}_{ion}$ increases by more than
	one order of magnitude between the central regions and the outer arcs
	4\arcsec\ away from the nucleus (Fig.~\ref{FigureU}, lower right panel). This
	indicates that a non-variable, central source photoionization model is not
	viable in Mrk~573. An additional, extra-nuclear source of ionizing photons
	is required to explain the observed ionization and excitation structure of
	the emission-line region, in agreement with the conclusions of Capetti et al.
	(\cite{Capetti96}). Judging by the increase of ${\cal Q}_{ion}$ in the inner
	2\arcsec, such extra ionizing photons appear to be required in SE1-arc and
	NW1-arc.

     \subsubsection{Are the additional ionizing photons generated by hot stars ?}
	Recently, Gonz\'alez-Delgado et al. (\cite{Gonzalez98}) have provided
	direct evidence for the existence of nuclear starbursts in three Seyfert 2
	galaxies and shown that they account for the `featureless continuum' component
	called FC2. An extended, diffuse excess of near-UV (3600~\AA) light is
	detected in Mrk~573 at the locations of the outer arcs (see Fig.~3 in Pogge \&
	De Robertis \cite{Pogge93}) and could be interpreted in terms of a starburst
	in the vicinity of the outer arcs. The hot stars in such a starburst could
	provide the required, spatially extended source of ionizing photons. However,
	between 1\arcsec\
	and 3\arcsec-4\arcsec, ${\cal Q}_{ion}$ increases by almost a factor 10
	(Fig.~\ref{FigureU}, bottom right panel) indicating that, in this
	interpretation, hot stars would be the dominant source of ionizing photons at
	the locations of the outer arcs. SE2-arc and NW2-arc should therefore exhibit
	line ratios typical of \HII\ regions, in contradiction with the
	observations which show that they display Seyfert-like line ratios. Therefore,
	hot stars are probably not the spatially extended source of ionizing photons.
     
     \subsubsection{Are the additional ionizing photons generated by shocks ?}
	In the last sections, we argued that a central source photoionization model
	cannot account for the ionization structure of the emission-line region of
	Mrk~573 and showed that the extended source of ionizing photons is probably
	not hot stars. In the AGN context, the second plausible candidate for the
	generation of an extended source of ionizing photons is fast shocks,
	presumably generated by the propagation of the radio ejecta through the
	insterstellar medium of Mrk~573. The procedure used to check if
	photoionizing shocks can power the line emission in SE1-arc and NW1-arc
	(see Sect.~\ref{SectionDiscussionNLRFastShocks}) cannot be safely applied to
	SE2-arc and NW2-arc, which arcs are ill-defined (especially NW2-arc) and the
	\SII\ ratio is too close to its low density limit to provide reliable
	estimates of the density.
	
	However, as for the inner arcs, any shock+precursor structure should display
	distinctive morphological signatures in the high spatial resolution HST \OIII, \OII\ and
	\Ha+\NII\ images. The observed \OIIIwb/\Hb\ ratio of $\sim$10 (see
	Fig.~\ref{FigureRatio}) yields typical shock velocities higher than 400~\kms\
	(for solar abundances). In addition, an upper limit to the pre-shock density
	of 5~\cmc\ is obtained using the minimum expected compression
	factor of $\sim$46 in the \SII\ emitting zone (400-500~\kms\ shock with a
	magnetic parameter of 0-4 $\mu$G cm$^{3/2}$, Dopita \& Sutherland
	\cite{Dopita96}; see Sect.~\ref{SectionDiscussionNLRFastShocks}), and the
	density of 200~\cmc\ obtained by Tsvetanov \& Walsh (\cite{Tsvetanov92}, their
	Fig.~7) 4\arcsec\ away from the nucleus as an upper-limit on the (post-shock)
	density in the outer arcs. Treating the shock velocity of 400~\kms\ as a lower
	limit, and using this upper limit on pre-shock density, we can derive a lower
	limit to the size of the precursor \HII\ region, which is ${\cal L}$ = 610
	$\times$ 1/n$_{\mbox{\scriptsize 0}}$ pc (Dopita \& Sutherland
	\cite{Dopita96}). This gives a minimum extent for the \HII\ region of the
	precursor (where roughly 90~\% of the total \OIII\ flux of the shock+precursor
	structure is emitted) of 120~pc (0\farcsec4). Therefore, as in the case of
	SE1-arc and NW1-arc (see Sect.~\ref{SectionDiscussionNLRFastShocks}), the
	outer arcs should display strong morphological differences between images in
	\OIII\ and lower excitation lines like \Ha, \OII\ and \NII. This is not the
	case (see our Fig.~\ref{FigureHST1} and Fig.~1 in Capetti et al.
	\cite{Capetti96}). We thus conclude that photoionizing radiative shocks are
	ruled out for both the inner and outer arcs of Mrk~573.
	
	The remaining possibility is that the spatially extended source of ionizing
	photons is faster shocks located in the vicinity of the arcs, but not directly
	associated with them. Such shocks could correspond to the jet/cloud
	interactions suspected to take place in the strings of knots (SE2-knots,
	SE1-knots and NW1-knots, see Sect.~\ref{SectionDiscussionNLRKnots}). In the
	inner regions, where the density is high enough for these fast shocks to be
	radiative, and for their internal structure to be spatially unresolved even at
	HST resolution, they show up in the HST emission-line images as strings of
	kinematically disturbed emission-line knots (SE1-knots, NW1-knots and in a
	lesser extent SE2-knots). However, no emission-line signature of such shocks
	is seen near NW2-arc, neither in the HST images, nor in our spectroscopic
	data. Therefore, in the vicinity of NW2-arc, any shock producing the ionizing
	photons must be fast enough and/or propagating in a sufficiently low density
	medium to be non-radiative, i.e. for the shocked gas not to cool down to
	temperatures where it would emit strong optical lines. These shocks will have
	extended photoionized precursors, which will certainly have a low
	surface brightness and may not be detected in the emission-line images, except
	in regions of enhanced density such as NW2-arc. For the shock to be
	non-radiative, the cooling length ${\cal L}_{\mbox{\scriptsize cool}}$ must be
	larger than the distance between the outer arcs and the nucleus
	($\sim$4\arcsec, i.e. $\sim$1.3~kpc). Assuming a pre-shock density of 1~\cmc\
	and using the equation for ${\cal L}_{\mbox{\scriptsize cool}}$ given in
	Sect.~\ref{SectionDiscussionNLRFastShocks}, we find a minimum shock velocity
	of 1000~\kms. High shock velocities are also necessary to produce the ionizing
	photon fluxes of ${\cal F}_{ion}$ $\sim$ \ten{10} \pcms\ seen by the gas in
	SE2-arc and NW2-arc (Fig.~\ref{FigureU}, bottom left panel), as a 500~\kms\
	shock propagating into a 1~\cmc\ medium produces a ionizing flux of only 2.4
	$\times$ \ten{9} \pcms, even before any geometrical dilution or absorption is
	taken into account. We envisage the jet as progressively weakening through
	such interactions, so the bow shock at the end of the jet, identified with the
	arcs, is of relatively low velocity.

\section{Summary and conclusion}
   We have reported the results of observations of the emission-line region of Mrk~573
   with the WFPC2/HST in the \OIII\ and \Ha+\NII\ emission-lines as well as with the
   3D spectrograph TIGER in the \Hb+\OIII\ and \Ha+\NII+\SII\ wavelength domains.
   The HST \OIII/(\Ha+\NIIwb) line ratio image shows a well defined bi-cone.
   The central nucleus and the various arcs and knots seen in the HST emission-line
   images are shown to exhibit very different kinematical and spectral behaviors. The
   results for each component can be summarized as follow:
   
   \noindent
   $\bullet$ {\it The nucleus:} The velocity of the central \OIII\ peak is in very
   good agreement with the stellar systemic velocity. The absence of clear kinematical
   signatures of shocks (no velocity shift and relatively narrow lines) suggests that
   the gas is probably photoionized by the active nucleus.\\
   $\bullet$ {\it The strings of knots:} Strong velocity perturbations are found in
   the strings of knots, in the vicinity of each radio component, and are interpreted
   as signatures of the interaction/deflection of the radio jet with/by individual
   clouds.\\  
   $\bullet$ {\it The inner arcs:} We have ruled out the notion that the line
   emissions of the inner arcs (SE1-arc and NW1-arc, see Fig.~\ref{FigureLabel})
   represent photoionizing (`autoionizing') shocks, as modelled by (Dopita \&
   Sutherland \cite{Dopita95}, \cite{Dopita96}), by a comparison of the predicted and
   observed morphological and spectral properties. Specifically, the arcs have a
   similar morphological structure in the medium excitation \OIIIwb\
   line as in low excitation lines such as \OIIw\ and \NIIww. This similarity is not
   expected in the photoionizing shock model, in which most of the \OIIIwb\ emission
   comes from the precursor and most of the low excitation line emission comes from
   the post-shock gas. Further, the expected kinematic structure of the shock is not
   seen. These arcs are therefore photoionized by an external source. We find good
   agreement between the ionizing photon fluxes inferred from the absolute \Hb\
   luminosity and from the observed ionization parameter and gas density, indicating
   that in the frame of a central source photoionization model, the covering factor
   of the material of the inner arcs is close to unity. The bow shock models of
   Ferruit et al. (\cite{Ferruit97a}) with central source photoionization fail to reproduce the observed line ratios, possibly because they assume that
   the ionization of the cool post-shock gas is matter-bounded.\\
   $\bullet$ {\it The outer arcs:} SE2-arc and NW2-arc (see Fig.~\ref{FigureLabel})
   are shown to mark the transition between the NLR and ENLR gas in Mrk~573 and could
   represent shock structures similar to the inner arcs, but which are currently
   dissipating.
   
   From the spatial variation of the ionization parameter and gas density and
   assuming that the compact central source of ionizing photons has not varied in the
   last 4000 years, we show that this central source alone cannot account for the
   ionization of the outer arcs and may account only partially for the ionization of
   the inner arcs. The spectra of the outer arcs are Seyfert-like, suggesting that hot
   stars cannot be the spatially extended source of ionizing photons. As in the case
   of the inner arcs, photoionizing radiative shocks fail to explain the spectral and
   morphological properties of the outer arcs and cannot be the source of the
   additional ionizing photons. Alternatively, the ionizing photons could be supplied
   by fast shocks located in the vicinity of the arcs but separate from them. These
   shocks would be radiative in the dense inner regions, their presence suggested by
   the emission-line knots associated with the radio ejecta, and non-radiative in the
   low density outer regions where they could supply the ionizing photons without
   being seen in the optical and, therefore, without suffering the morphological and
   spectral problems described above.
      
   This high-spatial resolution work constitutes one of the most extensive studies of
   the circumnuclear ionized gas in a Seyfert galaxy to date. In the
   future, STIS spectrographic observations combining the high-spatial resolution of
   HST with an extensive UV-optical coverage (and ideally a good spatial coverage by
   using multiple slit positions and slitless spectroscopy) will be a key to confirm
   and strengthen our results and to investigate in detail the origin of the ionizing
   photons.

\acknowledgements We thank L.B. Lucy for telling  us how to modify his original
deconvolution algorithm to take into account the additional constraints provided by
the HST narrow-band images. PF thanks Eric Emsellem, Carole G. Mundell and Neil Nagar
(who proposed the variable central source possibility) for useful discussions and
comments, Fran\c{c}ois Simien for providing references on the rotation curves of S0
galaxies, and L. Binette for providing his code Mappings Ic and constructing the
Mappings Ic source file for the nuclear ionization spectrum of Mrk~573. HF thank the
STScI staff -- especially J. Biretta, M. McMaster, and K. Rudloff -- for their support
in the LRF calibrations. This research was supported by NASA under grants NAGW-3268
and NAG8-1027 and by a grant from NASA administered by the American Astronomical
Society. We also acknowledge support from HST grants GO~5411, 6006 and 6419. HF
acknowledges support by a DFG grant Fa 358/1-1\&2. This research has made an extensive
use of NASA's Astrophysics Data System Abstract Service.


\begin{thebibliography}{DUM}

\bibitem[1984]{Allen84} Allen, A. J. 1984, MNRAS, 210, 147

\bibitem[1998]{Allen98} Allen, M. G., Dopita, M. A., \& Tsvetanov, Z. I. 1998, ApJ,
493, 571

\bibitem[1998]{Alonso98} Alonso-Herrero, A., Simpson, C., Ward, M. J., \& Wilson, A.
S. 1998, ApJ, 495, 196

\bibitem[1993]{Antonucci93} Antonucci, R. 1993, ARA\&A, 31, 473

\bibitem[1998]{Axon98} Axon, D. J., Marconi, A., Capetti, A., Macchetto, F. D.,
Schreier, E., \& Robinson, A. 1998, ApJ, 496, 75

\bibitem[1995]{Bacon95} Bacon, R., et al. 1995, A\&AS, 113, 347

\bibitem[1998]{Bicknell98} Bicknell, G. V., Dopita, M. A., Tsvetanov, Z. I., \&
Sutherland, R. S. 1998, ApJ, 495, 680

\bibitem[1996]{Binette96} Binette, L., Wilson, A. S., \& Storchi-Bergmann, T. 1996,
A\&A, 312, 365

\bibitem[1979]{Blandford79} Blandford, R. D.,\& K\"{o}nigl, A. 1979, Ap.Letts, 20, 15

\bibitem[1996]{Capetti96} Capetti, A., Axon, D. J., Macchetto, F., Sparks, W.
B., \& Boksenberg, A. 1996, ApJ, 469, 554

\bibitem[1989]{Cardelli89} Cardelli, J. A., Clayton, G. C., \& Mathis, J. S.
1989, ApJ, 345, 245

\bibitem[1995]{Dopita95} Dopita, M. A., \& Sutherland, R. S. 1995, ApJ, 455, 468

\bibitem[1996]{Dopita96} Dopita, M. A., \& Sutherland, R. S. 1996, ApJS, 102, 161

\bibitem[1990]{Durret90} Durret, F., \& Warin, F. 1990, A\&A, 238, 15

\bibitem[1996]{Falcke96} Falcke, H., Wilson, A. S., Simpson, C., \& Bower, G. A.
1996, ApJ, 470, L31

\bibitem[1998]{Falcke98} Falcke, H., Wilson, A. S., \& Simpson, C. 1998, ApJ, 502, 199

\bibitem[1997a]{Ferruit97a} Ferruit, P., Binette, L., Sutherland, R. S., \&
P\'econtal, E. 1997a, A\&A, 322, 73

\bibitem[1997b]{Ferruit97b} Ferruit, P., P\'econtal, E., Wilson, A. S., \& Binette, L.
1997b, A\&A, 328, 493

\bibitem[1998]{Gonzalez98} Gonz\'alez Delgado, R. M., Heckman, T., Leitherer, C.,
Meurer, G., Krolik, J., Wilson, A. S., Kinney, A., \& Koratkar, A. 1998, ApJ, 505,174

\bibitem[1988]{Haniff88} Haniff, C. A., Wilson, A. S., \& Ward, M. J. 1988, ApJ, 334,
104

\bibitem[1991]{Haniff91} Haniff, C. A., Ward, M. J., \& Wilson, A. S. 1991, ApJ, 368,
167

\bibitem[1996]{Holloway96} Holloway, A. J., Steffen, W., Pedlar, A., Axon, D. J.,
Dyson, J. E., Meaburn, J., \& Tadhunter, C. N. 1996, MNRAS, 279, 171

\bibitem[1991]{Kinney91} Kinney, A. L., Antonucci, R. R. J., Ward, M. J., Wilson, A.
S., \& Whittle, M. 1991, ApJ, 377, 100

\bibitem[1978]{Koski78} Koski, A. T. 1978, ApJ, 223, 56

\bibitem[1997]{Kotilainen97} Kotilainen, J. K., \& Ward, M. J. 1997, A\&AS, 121, 77

\bibitem[1996]{Leitherer96} Leitherer, C., et al. 1996, \pasp, 108, 996

\bibitem[1974]{Lucy74} Lucy, L. B. 1974, AJ, 79, 745

\bibitem[1997]{Lucy97} Lucy, L. B. 1997, private communication

\bibitem[1998]{Malkan98} Malkan, M. A., Gorjian, V., \& Tam, R. 1998, ApJS, 117, 25

\bibitem[1998]{Nagar98} Nagar, N. M., Wilson, A. S., Mulchaey, J. S., \& Gallimore, J.
F. 1998, in preparation

\bibitem[1995]{Nelson95} Nelson, C. H., \& Whittle, M. 1995, ApJS, 99, 67

\bibitem[1996]{Osterbrock96} Osterbrock, D. E., Fulbright, J. P., Martel, A. R.,
Keane, M. J., Trager, S. C., \& Basri, G. 1996, PASP, 108, 2770

\bibitem[1993]{Osterbrock93} Osterbrock, D. E. 1993, ApJ, 404, 551

\bibitem[1997]{Pecontal97} P\'econtal, E., Ferruit, P., Binette, L., \& Wilson, A. S.
1997, A\&SS, 248, 167

\bibitem[1993]{Pogge93} Pogge, R. W., \&  DeRobertis, M. M. 1993, ApJ, 404, 563

\bibitem[1995]{Pogge95} Pogge, R. W., \&  DeRobertis, M. M. 1995, ApJ, 451, 585

\bibitem[1972]{Richardson72} Richardson, W. H. 1972, J. Opt. Soc. Am., 62, 55

\bibitem[1992]{Rousset92} Rousset, A. 1992, Contribution des m\'ethodes
num\'eriques au d\'epouillement des donn\'ees du spectrographe int\'egral de
champ TIGER, PhD Thesis, Univ. J.Monnet de Saint-Etienne

\bibitem[1996]{Schmitt96} Schmitt, H. R., \& Kinney, A. L. 1996, ApJ, 463, 498

\bibitem[1996]{Steffen96} Steffen, W., G\'omez, J. L., Raga, A. C., \& Pedlar, A.
1996, in Jet-Cloud Interactions in Active Galaxies, ed. N. Clark (Univ. of Sheffield),
on-line proceedings

\bibitem[1997a]{Steffen97a} Steffen, W., G\'omez, J. L., Williams, R. J. R., Raga, A.
C., \& Pedlar, A. 1997a, MNRAS, 286, 1032

\bibitem[1997b]{Steffen97b} Steffen, W., G\'omez, J. L., Raga, A. C., \& Williams, R.
J. R. 1997b, ApJ, 491, L73

\bibitem[1996]{Storchi96} Storchi-Bergmann, T., Wilson, A. S., Mulchaey, J. S., \&
Binette, L. 1996, A\&A, 312, 357

\bibitem[1992]{Taylor92} Taylor, D., Dyson, J. E., \& Axon, D. J. 1992, MNRAS, 255,
351

\bibitem[1992]{Tsvetanov92} Tsvetanov, Z., \& Walsh, J. R. 1992, ApJ, 386, 485

\bibitem[1993]{Turner93} Turner, T. J., Urry, C. M., \& Mushotzky, R. F. 1993, ApJ,
418, 653

\bibitem[1984]{Ulvestad84} Ulvestad, J. S., \& Wilson, A. S. 1984, ApJ, 278, 544

\bibitem[1987]{Unger87} Unger, S. W., Pedlar, A., Axon, D. J., Whittle, M., Meurs, E.
J. A., \& Ward, M. J. 1987, MNRAS, 228, 671

\bibitem[1995]{Urry95} Urry, C. M., \& Padovani, P. 1995, PASP, 107, 803

\bibitem[1989a]{Viegas89} Viegas--Aldrovandi, S. M., \& Contini, M. 1989, ApJ,
339, 689

\bibitem[1986]{Whittle86} Whittle, M., et al. 1986, MNRAS, 222, 189

\bibitem[1988]{Whittle88} Whittle, M., Pedlar, A., Meurs, E. J. A., Unger, S. W.,
Axon, D. J., and Ward, M. J. 1988, ApJ, 326, 125

\bibitem[1980]{Wilson80} Wilson, A. S., \& Willis, A. G. 1980, ApJ, 240, 429

\bibitem[1982]{Wilson82} Wilson, A. S. 1982, In Extragalactic Radio Sources, IAU
Symposium Nr. 97, eds D. Heeschen and C. Wade, p. 179 (Reidel, Dordrecht)

\bibitem[1988]{Wilson88} Wilson, A. S., Ward, M. J., \& Haniff, C. A. 1988, ApJ, 334, 121

\bibitem[1994]{Wilson94} Wilson, A. S., \& Tsvetanov, Z. I. 1994, AJ, 107, 1227

\bibitem[1997]{Wilson97} Wilson, A. S., Binette, L., \& Storchi-Bergmann, T. 1997,
ApJ, 482, L131

\end{thebibliography}

\clearpage
\begin{table}
\caption[]{\label{TableTigerLog} Log file of the TIGER spectrographic exposures in
the blue (B) and red (R) spectral configurations. In this table, $\lambda_0$ is the
central wavelength, $\Delta\lambda$ the filter bandpass, $\delta\lambda$ the
spectral resolution, $\delta v_{\lambda_0}$ the corresponding velocity resolution at
$\lambda_0$, and t$_i$ the integration time.} \scriptsize
\begin{tabular}{lccccc|l}
   \\
   \tableline\tableline
   ID & $\lambda_0$/$\Delta\lambda$ & $\delta\lambda$ & $\delta v_{\lambda_0}$
   & t$_i$ & airmass & Comments\\
   \tableline
   B1 & 5000/400\,\AA & 3.6\,\AA & 216\,\kms & 3600\,s & 1.006 &
   \OIII+\Hb\ configuration, centered east of the nucleus\\
   B2 & 5000/400\,\AA & 3.6\,\AA & 216\,\kms & 3600\,s & 1.102 &
   \OIII+\Hb\ configuration, centered west of the nucleus\\
   R1 & 6750/400\,\AA & 3.6\,\AA & 160\,\kms & 2819\,s & 1.092 &
   \NII+\Ha+\SII\ configuration, centered east of the nucleus\\
   R2 & 6750/400\,\AA & 3.6\,\AA & 160\,\kms & 2821\,s & 1.198 &
   \NII+\Ha+\SII\ configuration, centered west of the nucleus\\
   \tableline
\end{tabular}
\end{table}

\clearpage
\begin{deluxetable}{lrlrlrrrrr}
\small
\tablecaption{\label{TableHSTFluxes} \OIII\ fluxes of the Emission Line Region
components in Mrk~573}
\tablehead{
\colhead{Component}&
\colhead{Offset}&
\colhead{Area}&
\colhead{\OIII\ flux}&
\colhead{Relative}&
\colhead{\OIII\,/}\\
&arcsec [pc]& arcsec$^2$ [kpc$^2$] &erg/sec/cm$^2$& flux &(\Ha+\NII)&
}
\tablecolumns{6}
\startdata
SE2-arc & 2.9 [910] & 2.6 [0.26] & $1.2\times10^{-13}$ & 8\% & 2.8--3.5 \nl
SE2-knots & 2.5 [790] & 0.42 [0.04] & $0.13\times10^{-13}$ & 1\%&2.5 \nl
SE1-arc & 1.6 [500] & 1.32 [0.13] & $1.5\times10^{-13}$ & 10\% &1.8--2.1 \nl
SE1-knots & 0.9 [280] & 0.55 [0.05] & $1.0\times10^{-13}$ & 6\% & 2--3 \nl
Nucleus & 0 [0] & 0.55 [0.05] & $3.5\times10^{-13}$ & 24\% & 2.7--2.9 \nl
NW1-knots & 0.7 [220] & 0.63 [0.06] & $0.7\times10^{-13}$ & 5\% & 1.9--2.2 \nl
NW1-arc & 1.6 [500] & 1.84 [0.18] & $1.7\times10^{-13}$ & 12\% & 1.5--1.7 \nl
NW2-arc & 3.6 [1130] & 2.28 [0.23] & $0.7\times10^{-13}$ & 5\% & 1.8--3.5 \nl
Total &&& $1.5\cdot10^{-12}$ & 100\% &
\tablecomments{
(1) -- Designation of component,
(2) -- Projected radial distance from nucleus,
(3) -- Area of component,
(4) -- \OIIIwb\ flux,
(5) -- \OIIIwb\ flux as percentage of total \OIII\ flux,
(6) -- Range of \OIII/(\Ha+\NII) ratio
}
\enddata
\end{deluxetable}

\clearpage

\clearpage
\begin{table}
\caption[]{\label{TableSpecSpec} \small Results of single Gaussian fitting of spectra
averaged over selected 0\farcsec2 diameter areas in each of the deconvolved data
cubes (see Fig.~\protect\ref{FigureOIIIProfile} for their locations in Mrk~573). All
the spectra are displayed in Fig.~\ref{FigureOIIISpec}, \ref{FigureHbSpec},
\ref{FigureNIIHaSpec} and \ref{FigureSIISpec}. The spatial resolution is different for
the different deconvolved data cubes. The parameters of the weak lines \Hb\ and \SII\ (see Fig.~\protect\ref{FigureHbSpec} and \protect\ref{FigureSIISpec}) at SE2-red, SE1-red and NW1-blue are not reliable and should be treated with caution.} 
\scriptsize
\begin{tabular}{llrrrr}
   \\
   \tableline\tableline
   && SE2-arc & SE2-red & SE1-arc & SE1-red \\
   \tableline
   Location & $\Delta\alpha$ (\arcsec\ [pc]) & -2.8 [-880] & -1.8 [-570]
   & -1.7 [-530] & -0.6 [-190] \\
   & $\Delta\delta$ (\arcsec\ [pc]) & -1.5 [-470] & -1.0 [-310] &
   -0.6 [-190] & -0.2 [-60] \\
   \tableline
   \Hbw & (v -- 5150)/FWHM (\kms) & -180/150 & -50/unresolved & -60/350 & +135/460 \\
   \tableline
   \OIIIwb & (v -- 5150)/FWHM (\kms) & {\bf -130/120} & {\bf +30/350} & {\bf -90/290}
   & {\bf +110/470}
   \\
   \tableline
   \Haw & (v -- 5150)/FWHM (\kms) & -100/90 & -50/260 & -80/340 & +70/430 \\
   \NIIwb & (v -- 5150)/FWHM (\kms) & -90/170 & -50/190 & -70/340 & +70/470 \\
   \NIIwb/\Ha && 0.5 & 0.8 & 1.0 & 0.7 \\
   \tableline
   \SIIwwb & (v -- 5150)/FWHM (\kms) & -90/50 & -90/310 & -100/300 & -10/550 \\
   \SIIwc/\SIIwd && 1.5 & 0.9 & 0.9 & 0.7 \\
   Density & (\cmc) & $<$ 100 & 850 & 850 & 2000 \\
   \tableline\tableline
   && Nucleus & NW1-blue & NW1-arc & NW2-arc \\
   \tableline
   Location & $\Delta\alpha$ (\arcsec\ [pc]) & 0 [0] & +0.8 [+250] &
   +2.1 [+660] & +3.4 [+1070] \\
   & $\Delta\delta$ (\arcsec\ [pc]) & 0 [0] & +0.8 [+250] &
   +0.8 [+250] & +2.2 [+690] \\
   \tableline
   \Hbw & (v -- 5150)/FWHM (\kms) & -10/440 & -230/110 & -20/330 & +90/unresolved \\
   \tableline
   \OIIIwb & (v -- 5150)/FWHM (\kms) & {\bf -10/350} & {\bf -290/440} & {\bf +40/230}
   & {\bf +90/150}\\
   \tableline
   \Haw & (v -- 5150)/FWHM (\kms) & +20/380 & -250/360 & +60/140 & +100/180 \\
   \NIIwb & (v -- 5150)/FWHM (\kms) & +20/360 & -260/400 & +70/160 & +90/240 \\
   \NIIwb/\Ha && 0.6 & 0.9 & 1.0 & 0.4\\
   \tableline
   \SIIwwb & (v -- 5150)/FWHM (\kms) & +10/330 & -370/540 & +50/140 & +50/150 \\
   \SIIwc/\SIIwd && 0.6 & 1.3 & 1.0 & 1.3 \\
   Density & (\cmc) & 3600 & 130 & 580 & 130 \\
   \tableline\tableline
\end{tabular}
\end{table}

\clearpage
\begin{table}
\caption[]{\label{TableNH} \small SE1-arc and NW1-arc pre-shock densities (\cmc) as a
function of the shock velocity (200~\kms $<$ V$_S$ $<$ 500~\kms) and magnetic
parameter (0~$\mu$G cm$^{3/2}$ $<$ B$_{\mbox{\scriptsize 0}}$/n$_{\mbox{\scriptsize 0}}^{1/2}$ $<$ 4~$\mu$G cm$^{3/2}$). See Sect.~\protect\ref{SectionDiscussionNLRFastShocks} for details.} \small
\begin{tabular}{llcccc}
   \\
   \tableline\tableline
   \multicolumn{2}{r}{V$_S$} & 200~\kms & 300~\kms & 400~\kms &  500~\kms \\
   \multicolumn{2}{l}{B$_{\mbox{\scriptsize 0}}$/n$_{\mbox{\scriptsize 0}}^{1/2}$ ($\mu$G cm$^{3/2}$)} \\
   \tableline
   0 & SE1-arc & 2.5 & 1.1 & 0.8 & 0.6 \\
   & NW1-arc & 1.7 & 0.8 & 0.6 & 0.4 \\
   \tableline
   1 & SE1-arc & 9.7 & 6.8 & 5.1 & 4.2 \\
   & NW1-arc & 6.7 & 4.6 & 3.5 & 2.9 \\
   \tableline
   2 & SE1-arc & 15.5 & 12.0 & 8.9 & 7.9 \\
   & NW1-arc & 10.5 & 8.2 & 6.0 & 5.4 \\
   \tableline
   4 & SE1-arc & 37.0 & 26.6 & 18.3 & 15.7 \\
   & NW1-arc & 25.2 & 18.0 & 12.5 & 10.8 \\
   \tableline\tableline
\end{tabular}
\end{table}
\clearpage

\begin{table}
\caption[]{\label{TableLHb} \small Predicted SE1-arc and NW1-arc \Hb\ luminosities
(${\cal L}_{\mbox{\scriptsize H}\beta}$, in units of \ten{39} \ergs) as a function of
the shock velocity (200~\kms $<$ V$_S$ $<$ 500~\kms) and magnetic parameter (0~$\mu$G
cm$^{3/2}$ $<$ B$_{\mbox{\scriptsize 0}}$/n$_{\mbox{\scriptsize 0}}^{1/2}$ $<$ 4~$\mu$G cm$^{3/2}$). The luminosities are the sum of the contributions of the shock and precursor in the photoionizing shock models of Dopita \& Sutherland (\cite{Dopita95},
\cite{Dopita96})} \small
\begin{tabular}{llcccc}
   \\
   \tableline\tableline
   \multicolumn{2}{r}{V$_S$} & 200~\kms & 300~\kms & 400~\kms &  500~\kms \\
   \multicolumn{2}{l}{B$_{\mbox{\scriptsize 0}}$/n$_{\mbox{\scriptsize 0}}^{1/2}$ ($\mu$G cm$^{3/2}$)} \\
   \tableline
   0 & SE1-arc & 0.5 & 0.6 & 0.8 & 1.0 \\
   & NW1-arc & 0.8 & 0.7 & 1.0 & 1.1 \\
   \tableline
   1 & SE1-arc & 1.9 & 3.5 & 5.1 & 7.1 \\
   & NW1-arc & 2.1 & 3.8 & 5.6 & 7.8 \\
   \tableline
   2 & SE1-arc & 3.1 & 6.2 & 8.9 & 13.4 \\
   & NW1-arc & 3.4 & 6.7 & 9.6 & 14.6 \\
   \tableline
   4 & SE1-arc & 7.4 & 13.8 & 18.3 & 26.7 \\
   & NW1-arc & 8.1 & 14.8 & 20.0 & 29.2 \\
   \tableline\tableline
\end{tabular}
\end{table}
\clearpage

\figcaption[f1.eps]{Synthetic emission-line sky spectrum used in the sky
subtraction. The horizontal dashed line marks the 1~$\sigma$ noise level in individual
spectra. \label{FigureSky}}

\figcaption[f2.eps]{{\bf Top row:} \OIIIwb\ intensity (left), centroid velocity
(middle) and FWHM (right) maps, as derived from a single component Gaussian fitting of
the line in each spectrum of the original (i.e. not deconvolved) \OIII\ TIGER data
cube. {\bf Second row:} As top row, but derived from the \OIII\ data cube by means of
a Richardson-Lucy deconvolution (150 iterations, see
Sect.~\protect\ref{SectionTigerDeconvolution}). {\bf Third row:} As top row, but
derived from the \OIII\ data cube by means of a `weakly guided' deconvolution (150
iterations, $\alpha$ = 0.1, see Sect.~\ref{SectionTigerDeconvolution}). {\bf Bottom
row:} As top row, but derived from the \OIII\ data cube by means of a `strongly
guided' deconvolution (150 iterations, $\alpha$ = 1, see
Sect.~\ref{SectionTigerDeconvolution}). \label{FigureComparison}}

\figcaption[f3.eps]{{\bf Top left panel:} Continuum subtracted \Ha+\NII\ image of
Mrk~573 taken with the Wide Field Camera (0\farcsec0996 pixels). The intensity scale
is proportional to the square root of the brightness. The location of the continuum
peak of the galaxy is marked by a cross, and the (B1950) coordinates were
obtained as described in Falcke et al. (\protect\cite{Falcke98}).  {\bf Top right
panel:} Continuum subtracted \OIIIwb\ image of Mrk~573 taken with the Wide Field
Camera (0\farcsec0996 pixels). The intensity scale is proportional to the square
root of the brightness. {\bf Bottom left panel:} Excitation map of Mrk~573,
obtained by dividing the \OIII\ map by the \Ha+\NII\ map. Darker shades represent
regions of higher \OIII/(\Ha+\NII) ratio. The linear grayscale ranges from
\OIII/(\Ha+\NII) = 1 to 5.5. {\bf Bottom right panel:} Broad-band F606W image of
Mrk~573 taken with the Planetary Camera (0\farcs0455 pixels), overlaid with logarithmic isophotes. \label{FigureHST1}}

\figcaption[f4.eps]{Emission-line images of Mrk~573 obtained on the Planetary
Camera with broad-band filters (F569W minus F814W, includes emission-lines such as
\Hb\ and \OIIIww, right panel; F675W minus F814W, includes emission-lines such as
\OIww, \Ha, \NIIww, and \SIIwwb, left panel; see
Sect.~\protect\ref{SectionHSTReduction}). The intensity scale is proportional to the
square root of the brightness. \label{FigureHST2}}

\figcaption[f5.eps]{The HST continuum-subtracted \OIII\ image shown in
Fig.~\protect\ref{FigureHST1} (upper right panel). The locations and adopted names of
the various structures in the emission-line region are indicated. \label{FigureLabel}}

\figcaption[f6.eps]{{\bf Top left panel:} \OIII\ HST map of Mrk~573 with contours of
the 2\,cm radio map at 0.5, 1 and 1.5 mJy (beam)$^{-1}$ superimposed. {\bf Top right
panel:} \OIIIwb\ intensity map derived from single component Gaussian fitting of the
deconvolved TIGER \OIII\ data cube, with contours of the 3.5\,cm radio map at
0.5, 1 and 1.5 mJy (beam)$^{-1}$ superimposed. {\bf Middle left panel:} Superposition
of the \OIIIwb\ intensity (color image) and centroid velocity (contours from 4830 to
5280~\kms\ with a 50~\kms\ increment) maps. Both were derived from single
component Gaussian fitting of the deconvolved TIGER \OIII\ data cube. {\bf Middle
right panel:} \OIIIwb\ centroid velocity map (color image) derived from single
component Gaussian fitting of the deconvolved TIGER \OIII\ data cube, with contours of
the 3.5\,cm radio map at 0.5, 1 and 1.5 mJy (beam)$^{-1}$ superimposed. {\bf Bottom
panels:} Same as middle panels, but for the FWHM instead of the centroid velocity.
Contours from 3~\AA\ to 10~\AA, with a 1~\AA\ increment. \label{FigureOIII}}

\figcaption[f7.eps]{{\bf Central image:} \OIIIwb\ emission-line map reconstructed
from single component Gaussian fitting of the TIGER deconvolved data cube (`weakly
guided' method, 300 iterations), with the \OIII\ isovelocity contours (see
Fig.~\ref{FigureOIII}) superimposed. {\bf Profiles:} \OIIIwb\ velocity profiles  from
eight selected locations (averaged over 0\farcsec2 diameter circular areas, i.e. over 3-4
spectra of the deconvolved data cube). Properties of the emission-lines at these locations are given in Table~\protect\ref{TableSpecSpec}. The origin corresponds to the stellar systemic
velocity of 5150~\kms\ (Nelson \& Whittle \protect\cite{Nelson95}).
\label{FigureOIIIProfile}}

\figcaption[f8.eps]{Continuum subtracted \OIIIwb\ spectra (thick lines) at eight
selected 0\farcsec2 diameter locations (see Fig.~\protect\ref{FigureOIIIProfile} and
Table~\protect\ref{TableSpecSpec}) with the fitted Gaussian superimposed
(thin lines). The spectra were obtained from the \OIII\ data cube through 300
iterations of `weakly guided' deconvolution (see
Sect.~\ref{SectionTigerDeconvolution}). \label{FigureOIIISpec}}

\figcaption[f9.eps]{\Hb\ intensity (left), velocity (middle) and FWHM (right) maps
derived from a single component Gaussian fitting of each spectrum in the original
(i.e. not deconvolved) \Hb+\OIII data cube. \label{FigureHb}}

\figcaption[f10.eps]{Continuum subtracted \Hb\  spectra (thick lines) at
selected 0\farcsec2 diameter locations (see Fig.~\protect\ref{FigureOIIIProfile} and
Table~\protect\ref{TableSpecSpec}) with the fitted Gaussians
superimposed (thin lines). The spectra were obtained from B configuration observations
processed with 150 iterations of Richardson-Lucy deconvolution (see
Sect.~\ref{SectionTigerDeconvolution}). \label{FigureHbSpec}}

\figcaption[f11.eps]{{\bf Top panels:} \Ha\ intensity (left), velocity (middle) and
FWHM (right) maps as derived from a single component Gaussian fitting of each spectrum
in the deconvolved \Ha+\NII\ data cube (`weakly guided' deconvolution -- 300
iterations). {\bf Bottom panels:} \NIIwb\ intensity (left), velocity (middle) and
FWHM (right) maps as derived from a single Gaussian fitting of each spectrum in the
deconvolved \Ha+\NII\ data cube (`weakly guided' deconvolution -- 300 iterations).
\label{FigureNIIHa}}

\figcaption[f12.eps]{Continuum subtracted \NIIww\ + \Ha\  spectra (thick lines) at
selected 0\farcsec2 diameter locations (see Fig.~\protect\ref{FigureOIIIProfile} and
Table~\protect\ref{TableSpecSpec}) with the fitted Gaussians superimposed (thin lines).
The spectra were obtained from the \Ha+\NII\ data cube through 300 iterations of
`weakly guided' deconvolution (see Sect.~\ref{SectionTigerDeconvolution}).
\label{FigureNIIHaSpec}}

\figcaption[f13.eps]{{\bf Top panels:} \SIIwc\ (left) and \SIIwd\ (right) intensity
maps as derived from single component Gaussian fitting of each spectrum in the
original (i.e. not deconvolved) R configuration data cube. {\bf Middle panels:}
\SIIwwb\ velocity (left) and FWHM (right) as derived from single component Gaussian
fitting of each spectrum in the original (i.e. not deconvolved) R configuration data
cube. {\bf Bottom panels:} Maps of the density sensitive \SIIwc/\SIIwd\ ratio
overlaid with the isophotes of the \SIIwc\ intensity map (left) and the \SII\
isovelocity contours (right). The isovelocity contours range from 5000 to
5250~\kms, with a contour interval of 50~\kms. \label{FigureSII}}

\figcaption[f14.eps]{Continuum subtracted \SIIwwb\  spectra (thick lines) at
selected 0\farcsec2 diameter locations (see Fig.~\protect\ref{FigureOIIIProfile} and
Table~\protect\ref{TableSpecSpec}) with the fitted Gaussians
superimposed (thin lines). The spectra were obtained from the \SII\ data cube
processed  with 150 iterations of Richardson-Lucy deconvolution (see
Sect.~\ref{SectionTigerDeconvolution}).  \label{FigureSIISpec}}

\figcaption[f15.eps]{{\bf Top left panel:} Plot of the distribution of the \OIIIwb\
peak intensity (in \ten{-15} \ergscmAarcsec) as a function of the projected radial
distance from the nucleus. Negative (positive) radii correspond to the southeast
(northwest) side of the emission-line region. Secondary peaks in the \OIII\ intensity
are seen at the locations of the two pairs of arcs and are marked. Each point in
this plot corresponds to one spectrum in the original (not deconvolved) TIGER data
cube. {\bf Top right panel:} The original (not deconvolved) TIGER \OIII\ image with
the various arcs indicated. The line in PA 35\degr\ separates the southeast and
northwest sides. {\bf Middle (left and right ) and bottom (left) panels:} Plots of
the \OIIIwb/\Hb\ (middle left; black crosses -- no correction for \Hb\
absorption; red and blue symbols : corrected for \Hb\ absorption), \NIIwb/\Ha\
(middle right) and \SIIwwb/\Ha\ (bottom left) line ratios as a function of the
projected radial distance from the nucleus. The positions of individual arcs are
labelled. Lines with a fitted intensity peak below 0.4 $\times$ \ten{-15}
\ergscmAarcsec\ (i.e. $\sim$13~$\sigma$) were omitted from the flux ratio
calculations. {\bf Bottom right panel:} Plot of the density, as derived from the
\SIIwc/\SIIwd\ ratio, as a function of the radial distance from the nucleus. Spectra
with a fitted intensity peak in the \SIIwc\ or \SIIwd\ lines below 0.2 $\times$
\ten{-15} \ergscmAarcsec\ (i.e. $\sim$7~$\sigma$) were not used to calculate the
density. The light red and light blue symbols correspond to density measurements for
which the error bars on the \SII\ ratio reach the low density limit of the \SII\
ratio. \label{FigureRatio}}

\figcaption[f16.eps]{Variation of the \NIIwb/\Ha\ (solid line), \SIIwwb/\Ha\ (dotted
line) and \OIwa/\Ha\ (dashed line) line ratios with the ionization parameter U, as
derived from simple plane-parallel photoionization models with Mappings Ic (see
Sect.~\protect\ref{SectionDiscussionNLRU}). \label{FigureRatioU}}

\figcaption[f17.eps]{{\bf Top left panel:} Map of the ionization parameter U,
as derived from the \NII/\Ha\ ratio (see
Sections~\protect\ref{SectionDiscussionNLRU} and
\protect\ref{SectionDiscussionCentralSource}), with isophotes of the HST \OIII\
image superimposed. White dots indicate regions where the \SII\ ratio is consistent
with the low density limit to within the errors. {\bf Top right panel:}  Variation of
the ionization parameter, U, as a function of the projected radial distance from the
nucleus, as derived from \NII/\Ha\ (diamonds) and \SII/\Ha\ (squares). Lines
with a fitted peak intensity below 0.4 $\times$ \ten{-15} \ergscmAarcsec\ (i.e.
$\sim$13~$\sigma$) are not displayed. {\bf Bottom left panel:} Ionizing photon flux,
${\cal F}_{ion}$ (as derived from the \NII/\Ha\ line ratio), as a function of the projected radial distance from the nucleus. Points where, given the error bars, the \SII\
ratio is consistent with the low density limit (white dots in the top left panel)
are marked with light red or blue diamonds. The gray-shaded rectangle corresponds to the nuclear regions (projected radius $<$~0\farcsec5), where there may be a wide range of density, and where our measurements of the density cannot be safely used to estimate ${\cal F}_{ion}$.
{\bf Bottom right panel:} Plot of the number of ionizing
photons, ${\cal Q}_{ion}$, emitted per second by a compact central source versus radius. \label{FigureU}}

\end{document}


