ABSTRACT

The Galactic Center shows evidence for the presence of three important AGN ingredients: a black hole (M* ∼ 106 M⊙), an accretion disk (10-8.5 – 10-7 M⊙ yr-1) and a powerful jet (jet power ≥ 10% disk luminosity). However, the degree of activity is very low and can barely account for the energetics of the central region.

INTRODUCTION

The dynamical center of the Galaxy is the radio point source Sgr A*, which is also the center of the central star cluster (Eckart et al. 1993). Investigations of the enclosed mass in the central region show that there is evidence for a mass concentration of the order of 106 M⊙ within the central arcsecond (Genzel and Townes 1987). There is good reason to assume that this “dark mass” indeed is the mass of a massive black hole (BH) powering Sgr A*. The total spectrum of this source from radio to NIR was compiled by Zylka et al. (1992). There is a flat radio spectrum up to 7 mm, and a steeply rising submm spectrum, which Zylka et al. interpret as thermal emission from a dust torus surrounding the BH. In the FIR one finds a spectral break at 30 μm indicated by upper limits and a third spectral component rising in the NIR, which has been interpreted as emission from an accretion disk around the BH.

HERTZSPRUNG-RUSSELL DIAGRAM FOR THE SGR A* DISK

Because of strong obscuration in the galactic plane we probably will never be able to measure exactly the optical and UV part of Sgr A*, which is needed to discriminate between different disk models.