Compact Nuclear Radio Sources in our and External Galaxies

Abstract

The compact nonthermal radio source, Sgr A*, has long been considered a signpost of a massive black hole at the center of our Galaxy. It has properties similar to those of the powerful compact radio sources found in quasars and other active galactic nuclei (CNRS) that require massive black holes on energetic grounds. But, for Sgr A*, its low luminosity cannot exclude a stellar object such as a neutron star as the underlying energy source. We review the properties of Sgr A* and other CNRS in nearby “normal” galaxies. The continuity of properties of such radio sources with the powerful CNRS would suggest that massive black holes are common in galactic nuclei and that Sgr A* is due to a massive black hole. The compact radio sources in our and other nearby galaxies axe especially important since it is in such nearby sources that dynamical probes of the presence of a large point mass will become increasingly definitive. The capabilities of the Hubble Space Telescope for Sgr A* in this regard are described.