Astron. Astrophys. 348, 768-782 (1999)

1. The central region of the Galaxy: an overview

Our present knowledge of the Galactic Center Region has been reviewed by Mezger et al. (1996; in the following referred to as MDZ96). To keep this introduction as concise as possible we quote here only the most relevant or recent papers and refer otherwise to the corresponding sections of this review.

The Central Region extends out to a distance of kpc where the Galactic Disk with its spiral structure begins. Throughout this paper a distance kpc to the Galactic Center is adopted. The Central Region consists of the Galactic Bulge () and the Nuclear Bulge (R kpc). Subunits of the Nuclear Bulge which in the following will be referred to are: the Central Cavity (pc) which is located inside the Circum-Nuclear Disk and specifically the central (linear size pc) surrounding the compact synchrotron source Sgr A* which is associated with a massive () Black Hole. Contained within the Nuclear Bulge is the K band mosaic, an area of with an equivalent radius pc approximately centered on Sgr A* which has been covered to date by our NIR survey. This mosaic includes the subareas Sgr A East and M-0.13-0.08 GMC indicated in Fig. 1b.

Fig. 1. a  K band mosaic of the central centered approximately on Sgr A*. The black square indicates "no usable data". White framed contours indicate subareas of the mosaic each of size , which are referred to in the text. The upper rectangle is centered on the synchrotron source Sgr A East, the lower irregularly shaped area is centered on the compact cloud core M-0.13-0.08. b  Overlaid on the K band mosaic is a contour map representing the dust emission observed at 1.2 mm with MPIfR bolometer arrays in the IRAM 30-m MRT (). Contour levels are: 90, 240, 390, 540, 690, 840, 990, 1180, 1680, 2180, 2680, 3180 and 3680 mJy/ beam. 100 mJy/ beam corresponds to a for and K. The corresponding m opacity is .

The integrated physical characteristics of both the Galactic Bulge, Nuclear Bulge and - for comparison - of the Galactic Disk are given in MDZ96, Table 5. Referred to the total masses of stars or Interstellar Matter (ISM) these integral characteristics are rather similar in the Nuclear Bulge and Galactic Disk, but in the Nuclear Bulge the volume densities of matter and of radiation are about two magnitudes higher. This is the reason why practically all interstellar gas in the Nuclear Bulge exists in molecular form and why there the dust temperatures are considerably higher. The Galactic Bulge, on the other hand, contains little ISM and shows little signs of recent star formation. Evidence for a bar structure in the Galactic Bulge and the outer regions of the Nuclear Bulge are reviewed in MDZ96, Sect. 2.3.

The Circum-Nuclear Disk extends from 1.7 to 7 pc and contains 10⁴ of ISM mainly in form of clumps of hydrogen mass 30 with a central visual extinction of mag. The inner edge of the Circum-Nuclear Disk rotates once in 1.510⁵ yr around the dynamical center. Part of the gas inside the Central Cavity is ionized and forms the HII region Sgr A West. In recent years the inner of the Central Cavity has become one of the best investigated areas in the Galaxy. Observational results related to Circum-Nuclear Disk and central are summarized in MDZ96, Sects. 4 and 5.

In a series of papers with the general title "Anatomy of the Sgr A complex I - V" (Zylka et al. 1990; 1992; Gordon et al. 1993; Zylka et al. 1995; Beckert et al. 1996) our group has investigated the physical state of the ISM in the central part of the Nuclear Bulge and specifically the nature of the enigmatic source Sgr A*. Most of the physical characteristics observed in Active Galactic Nuclei (AGN) are also found in the Central Region of our Galaxy: A massive () Black Hole, a central mass flow of M_ yr^-1 for radii pc (MDZ96, Sect. 3.5) and indications of mild star bursts yrs ago. But the energy produced at present by this Black Hole is negligible compared to the luminosity of a cluster of evolved early-type stars in the immediate vicinity of Sgr A*.

© European Southern Observatory (ESO) 1999

Online publication: August 13, 199
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