THE GALACTIC-CENTER - A LABORATORY FOR AGN

This paper reviews the physical state of stars and Interstellar Matter in the Galactic Bulge (radius R similar to 0.3 - 3 kpc from the dynam ical center of the Galaxy), in the Nuclear Bulge (R < 0.3 kpc) and in the Sgr A Radio and GMC Complex, i.e. the central similar to 50 pc of our Galaxy. The Galactic Bulge is devoid of cold Interstellar Matter a nd consists mainly of old stars, while the Nuclear Bulge accounts for similar to 10% of the mass of all of the Interstellar Matter in the Ga laxy. A similar ratio holds for the formation rate of medium and high mass stars in Bulge and Disk. The metal abundance of the Interstellar Matter in the Galactic Bulge is found to be (Z/Z.) similar to 2. The H -2-to-(CO)-C-12 conversion factors to be applied to molecular gas in t he Central Region are by factors 3 (Arimoto et al. 1996) to 10 (Sodros ki et al. 1995) lower than in the solar vicinity. Hence, most H-2 mass es derived for the Central Region appear to be considerably overestima ted. The Nuclear Bulge is pervaded by a thermal plasma (T-e less than or equal to 7000 K) which is responsible for the diffuse free-free emi ssion. Lyman continuum photon and dust TR luminosity of the Nuclear Bu lge again account for similar to 10% of the respective total luminosit ies of the Galaxy. Magnetic fields in the Nuclear Bulge are strong (up to similar to 2 mG) as compared with the Galactic Disk (a few tens of mu G). The field lines are oriented parallel to the galactic plane in side giant molecular clouds and perpendicular to the plane in the inte rcloud medium. The compact source Sgr A is close to or at the dynamic al center of the Galaxy. Its radio spectrum with a high frequency cut- off at nu(c) similar to 2000 - 4000 GHz, a low frequency turnover at n u similar to 0.8 GHz and a S-nu proportional to nu(1/3) flux density d ependence in between can be explained by synchrotron emission from qua si-monoenergetic relativistic electrons. Due to an extinction between Sun and Galactic Center corresponding to A(v) similar to 31(m), an int rinsic weakness of this source in the near infrared, and a strong back ground emission from warm dust there are only upper limits available f or the flux density of Sgr A in the far, mid and near infrared and X- ray regime. The size of Sgr A in the radio regime is less than or equ al to 2.5 - 4 10(13) cm, its dereddened K-band flux density is < 9 mJy , its luminosity has upper limits of < 10(3) L. (if radiation comes fr om an Accretion Disk) and less than or equal to 5 10(4) L. (if black-b ody radiation from an object with a single temperature of similar to 4 10(4) K is assumed. If anyone of the soft X-ray sources detected by R OSAT actually coincides with Sgr A, its X-ray luminosity would be les s than a few 10(2) L.. With a dark mass of similar to 2 - 3 10(6) M. S gr A is the best candidate for a starving black hole, although there are no observational indications for the presence of a (Standard) Accr etion Disk. While the radio/IR spectrum of Sgr A is purely nonthermal , the spectrum integrated over the central parsec resembles that of a Seyfert galaxy. Sgr A is embedded in the HII region Sgr A West with p art of the ionized gas forming a minispiral. Sgr A West is surrounded by the Circum Nuclear Disk, an irregular shaped assembly of similar to 10(4) M. molecular gas which extends from R similar to 1.7 - 7 pc and rotates around the Galactic Center with an estimated dynamical time s cale of similar to 10(5) yr. The total luminosity of similar to 10(8) L. of the central parsec is due to the radiation of early-type stars o f which similar to 24 have now been directly identified as luminous bl ue supergiants. It is still debated, however, if these stars can also account for all of the ionization of Sgr A West. In addition, the cent ral parsec contains red giants, AGE stars, and a few super giants of w hich the brightest similar to 500 are now identified by direct imaging . These stars - together with a few million low mass main sequence sta rs - account for the bulk of the 2.2 mu m emission. The spatial distri butions of the three stellar populations in the central similar to 1 - 2 pc are remarkably different. Sgr A is - along the line-of-sight - presumably located close to the center of the HII region Sgr A West, w hich in turn is located in front of the extended (similar to 10 pc) sy nchrotron source Sgr A East, which appears to be the remnant of a giga ntic explosion (of the order of similar to 40x the energy of a single supernova explosion) which rook place similar to 5 10(4) yr ago inside the GMC Sgr A East Core. X-ray observations show within +/- 80 pc a p ervasive hot (kT similar to 10 keV) plasma of expansion age of similar to 10(5) yr. Both phenomena - as well as the formation of the Circum Nuclear Disk - may have the same origin. Influx of material is observe d within the Nuclear Bulge on all distance scales. In the Nuclear Bulg e (R less than or equal to 200 pc) as well as in the Circum Nuclear Di sk (R less than or equal to 7 pc) inflow towards the Galactic Center o ccurs primarily in the galactic plane and amounts to a few 10(-2) M., yr(-1). The accretion rate into the central Black Hole, deduced from t he luminosity of Sgr A, however, appears to be lower by at least live orders of magnitude (assuming standard disk accretion). But in an equ ilibrium state only part of the infalling mass which is not accreted b y the Black Hole can be consumed by star formation. A mass inflow rate varying with time is a more natural explanation. Comparing the physic al state of the Center of our Galaxy with that of Active Galactic Nucl ei derived from observations and modelling, we find that most of the b asic characteristics of an AGN are also present in the Galactic Center . Lacking are, however, both the evidence for a standard Accretion Dis k and a hard UV spectrum with accompanying high excitation emission li nes in the Galactic Center which are characteristic for AGN. The lumin osity of the central parsec, similar to 10(8) L., amounts to only simi lar to 0.3% of the total luminosity of the Galaxy of similar to 4 10(1 0) L.. Seen from a distance of M31 (D similar to 700 kpc) with an angu lar resolution of similar to 0.'' 5 (corresponding to a lineal size of similar to 1 pc) the Center of our Galaxy would appear as a mildly ac tive nucleus with some starburst activity and would probably be classi fied as a weak Seyfert galaxy. The synchrotron spectrum of Sgr A. how ever, would be completely masked by reprocessed stellar ligh