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 OIII, and 6750/400 AA including Ha, NII and SII. 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 ~0.35.
The HST images outline the detailed complex structure of the central ~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, 1.8'' (570 pc) and 2.3'' (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, 2.9 (0.9 kpc) and 3.6'' (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.
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Questions: Heino Falcke, hfalcke@mpifr-bonn.mpg.de