% 164examp.tex % % sample file for manuscripts for proceedings of IAU Colloquium 164 % see: 164instr.tex for instructions % and: 164examp.tex for an extended example % % 24april97: Version 1.0 / Anton Zensus \documentstyle[11pt,paspconf,164page,164def,psfig]{article} \pagestyle{myheadings} \begin{document} \title{Sgr A* and Company - Multiwavelength observations of Sgr A* and VLA search of ``Sgr A*'s'' in LINERs\footnote{Talk presented at IAU Coll.~164, A.~Zensus, G.~Taylor, \& J.~Wrobel (eds.), PASP Conf.~Proc.}} \label{falck} % running head stuff \markboth{Falcke et al.}{Sgr A* and Co.} \author{ H. Falcke\altaffilmark{1,2}, W.M. Goss\altaffilmark{3}, L.C. Ho\altaffilmark{4}, H. Matsuo\altaffilmark{5}, P. Teuben\altaffilmark{1}, A.S. Wilson\altaffilmark{1}, J.-H. Zhao\altaffilmark{4}, R. Zylka\altaffilmark{2} } \altaffiltext{1}{Dept. Astronomy, University of Maryland, College Park, MD 20742-2421, USA} \altaffiltext{2}{Max-Planck-Institut f\"ur Radioastronomie, Auf den H\"ugel 69, D-53121 Bonn, Germany.} \altaffiltext{3}{NRAO Socorro} \altaffiltext{4}{Harvard/CfA} \altaffiltext{5}{NRO/Japan} \begin{abstract} We report first results from a multiwavelength campaign to measure the simultaneous spectrum of Sgr A* from cm to mm wavelengths. The observations confirm that the previously detected submm-excess is not due to variability; the presence of an ultracompact component with a size of a few Schwarzschild radii is inferred. In a VLA survey of LINER galaxies, we found Sgr A*-like nuclei in one quarter of the galaxies searched, suggesting a link between those low-power AGN and the Galactic Center. \end{abstract} \section{Introduction} The closest compact, flat spectrum radio core in the center of a galaxy is Sgr A* in the Galactic Center (GC). NIR observations of the GC have convincingly demonstrated the presence of a dark mass of $2.5\cdot10^6 M_{\odot}$ (Eckart \& Genzel 1996) which is most likely due to a black hole associated with Sgr A*. Here we present the results of two observational campaigns that may shed further light on the nature of Sgr A* and of flat spectrum radio cores in galactic nuclei in general. %\index{Sgr A*} %\index{compact radio cores} \section{Multiwavelength campaign for Sgr A*} Early mm- and submm-observations of Sgr A* suggested the presence of a submm-excess in the spectrum of Sgr A* (Zylka et al. 1992). Since Sgr A* can be variable it was, however, not clear, whether this excess was due to non-simultaneous measurements, a systematic error, or an intrinsic up-turn of the spectrum. Nevertheless, so far the excess at submm-wavelengths has persisted in non-simultaneous measurements by various other groups (see Morris \& Sera\-byn, p. 685). As a further step, we have now performed a campaign to measure quasi-simultaneously the spectrum of Sgr~A* to exclude the effects of variability on the broad-band radio spectrum. %\index{spectrum!Sgr A*} \begin{figure}[th] \cl{ \psfig{figure=falckf1.ps,width=0.5\textwidth,bbllx=3.4cm,bburx=18.3cm,bblly=2.5cm,bbury=25.4cm,angle=-90} } \cl{\footnotesize log $\nu$ [GHz]} \caption{Spectrum of Sgr A* averaged over all telescopes at each wavelength for October 25-27, 1996.\label{falckf1}} \end{figure} The observation were carried out at four different telescopes---VLA A-Array (20, 6, 3.6, 2, 1.3, \& 0.7 cm), IRAM (3, 2, \& 1.3 mm), BIMA C-Array (3mm), and Nobeyama 45m (3 \& 2 mm)---on three consecutive days on October 25/26/27, 1996. The data were reduced by the individual groups following standard procedures. For BIMA we did not use the longer baselines because of coherence problems; the IRAM 1.3mm observations were affected by weather. The resulting spectrum is shown in Fig.~1 where we have averaged the fluxes at each wavelength from all days and telescopes. The VLA spectrum shows a marked break at 10 GHz. Below the break it can be described as a powerlaw $S_{\nu}\propto\nu^{\alpha}$ with $\alpha=0.16$ and above the break as a powerlaw with $\alpha=0.28$. The combined 3 and 2mm fluxes seem to be significantly above the extrapolation from the VLA fluxes. As discussed in Falcke (1996) such an excess, if due to self-absorption, indicates an ultra-compact region of $\sim2-3$ Schwarzschild radii. This region could in principle be resolved by future, global (sub)mm-VLBI experiments and thus one could directly probe the black hole nature of Sgr~A*. \section{VLA survey of nearby LINER galaxies} To see whether Sgr A* is really unique, we have surveyed a sample of 48 nearby LINER galaxies, selected from Ho et al.~(1995), with the VLA in A-configuration at 2cm. We found that a quarter (mostly spirals) of the galaxies surveyed showed a compact, flat-spectrum core above a 5 $\sigma$ detection limit of $\sim1$ mJy. Hence, we conclude that a nucleus like Sgr~A* is not a unique feature of our galaxies, but can be found in other nearby galaxies, especially in those with signs of optical nuclear activity. Since the cores we found show a good radio/H$\alpha$ correlation, it is very likely that they are indeed directly associated with the nuclear engine. %\index{jets in!LINER galaxies} %\begin{figure}[h] %\cl{ %\psfig{figure=corr2.ps,width=0.3\textwidth,bbllx=2.5cm,bburx=19.2cm,bblly=6.1cm,bbury=21.8cm} %} %\cl{\footnotesize $L_{\rm disk}$ [erg/sec]} %\caption{Radio/optical correlation for flat-spectrum radio nuclei as %given in Falcke \& Biermann (1996) (log $\nu L_{5{\rm GHz}}$ vs. log $L_{\rm %disk}=\log{L_{{\rm H}\alpha}^{\rm narrow}+2.1}$) --- the radio cores in LINER galaxies we %found are given as big, filled dots in the range $42