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\title{THE NATURE OF COMPACT RADIO CORES IN GALAXIES$^1$}

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\author{H.~FALCKE}
\institute{Max-Planck-Institut f\"ur Radioastronomie, Auf dem H\"ugel 69, D-53121 Bonn, Germany (hfalcke\atsign{}mpifr-bonn.mpg.de}
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\runningtitle{COMPACT RADIO CORES}

\begin{document}
\footnotetext[1]{invited
contribution, IAU Symp. 184, Kyoto 1997; to appear in: ``The Center of the
Galaxy and Galaxies'', Y.~Sofue (ed.), Kluwer}
\begin{abstract}
Compact radio cores, which are often assumed to mark the presence of a
super-massive black hole, are not only found in the nuclei of powerful
quasars but also in nearby galaxies. While in quasars they are
typically associated with relativistic jets, the nature of those cores
in low-luminosity AGN is less clear. Here, I will briefly mention some
of the recent theories (ADAFs or jets) and observations of the latter
class of objects. 
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\section{Introduction}
It is often said that VLBI radio cores indicate the presence of a
black hole in a galactic nucleus---but why? Three answers can be given
easily: a) Because Martin Rees has said so (e.g.~Lynden-Bell \& Rees
1971, where they predicted Sgr~A* and achieved something few theorists
ever will---they were right), b) What else should they be?  (radio
cores are unique objects in galactic nuclei, where super-massive black
holes are though to exist), and c) because it makes a good press
release (and justifies the amount of money spent on the necessary
expansion of VLBI). Of course we also would like to know whether there
are any other scientific arguments that prove the link between compact
radio cores and black holes.

\section{Compact Radio Cores in the Galaxy and Galaxies}
In a large survey Wrobel \& Heeschen (1991) discovered that a number
of elliptical galaxies have compact radio nuclei. Fabian \& Rees
(1995) interpret these as the emission from a hot ion torus. In a
survey of optically selected low-luminosity AGN (here only LINERs; see
also Ho 1997) Falcke et al.~(1997a) find that a quarter of nearby
LINER galaxies have compact, flat-spectrum radio cores. Comparison of
radio and H$\alpha$ flux shows that in LINERs with
steep-spectrum radio emission, radio and optical flux are not
correlated, while for LINERs with flat-spectrum cores there is a
marked trend for the radio flux to increase with the emission-line
flux. This is a clear sign that optical and radio activity in those
low-luminosity LINERs have a common origin---most likely an AGN.

Narayan (e.g.~1997)---conceptually not very different from Fabian \&
Rees---suggests that those radio cores could be explained by an
advection dominated accretion flow (ADAF). However, the radio spectral
indices we find in LINERs are usually substantially flatter than those
predicted in the ADAF model, and VLBI of some of those cores
(e.g. NGC~4258, Herrnstein 1997) have revealed jet structures similar
to those in more luminous AGN. Herrnstein, with his superb data, now
also puts strong upper limits on any advection dominated disk in NGC
4258. On the other hand, the newly discovered radio jet in NGC~4258
fits in many details very well the predictions of the jet-disk model
by Falcke (1996), as does the H$\alpha$/radio correlation for the
LINER sample (Falcke et al. 1997a).

Hence, despite the overwhelming momentum of the ADAF band wagon, I am
very confident, that most, if not all, of the more prominent radio
cores in nearby galaxies will turn out to be just scaled down AGN
jets---thus making the black hole/compact radio core link a rather
indirect one (there is a black hole, which accretes matter, which then
leads to the formation of a jet, and which then looks like a compact
radio core).

Only for the closest radio core, Sgr~A*, which hides behind a
scattering screen, the game is still wide open---but there is hope.
In a recent campaign to measure the simultaneous spectrum of Sgr~A*
(Falcke et al.~1997b) we were able to confirm the excess in its
spectrum at mm-wavelengths. This is best interpreted as an
ultra-compact emission region, very close to the event horizon of the
black hole.  This region could then in principle be resolved already
with the next generation of mm-VLBI experiments, which should have a
beam smaller than the expected `visual' size of the black hole, and
thus would not only discriminate between the various models but could
also finally prove the existence of a black hole.

\begin{thebibliography}{}  % Note the empty braces!

\bibitem[]{}Fabian, A.C., \& Rees, M.J. 1995, MNRAS 277, L55
\bibitem[]{}Falcke, H. 1996, ApJ 464, L67
\bibitem[]{}Falcke, H., Ho, L.C., Wilson, A.S. 1997a, in
``Relativistic Jets in AGN'', Cracow 1997, Ostrowski et al. (eds.)
\bibitem[]{}Falcke, H., Goss, W.M., Matsuo, H. et al.~1997b, ApJ Lett., subm.
\bibitem[]{}Herrnstein, J. 1997, this volume
\bibitem[]{}Ho, L.C. 1997, this volume
\bibitem[]{}Lynden-Bell, D., \& Rees, M.J. 1971, MNRAS 152, 461
\bibitem[]{}Narayan, R. 1997, this volume
\bibitem[]{}Wrobel, J.M., \& Heeschen, D.S. 1991, AJ 101, 148
\end{thebibliography}

\end{document}

