The Galactic Center

P. Predehl

19.1 Introduction

The center of our Galaxy lies, within the constellation Sagittarius at a distance of —8 kpc, behind a large column density of gas and dust. This prevents observations over a wide band ranging from optical to soft X-ray wavelengths. The exploration of this region had to await the instrumental developments of, first, radio, later also of submillimeter and infrared, and most recently, X-ray- and y-ray astronomy. The Galactic Center is one of the scientifically most interesting regions for astrophysics. It is the closest galactic nucleus, a factor of 100 closer than the next external nucleus, M31. Thus, the relative proximity of our Galactic Center, combined with high angular resolution instruments now available, provides the opportunity to differentiate and study a wealth of highly unusual objects, possibly unique, but probably standard occupants of a normal galactic core.

19.1.1 Morphology of the Galactic Center

The first detection of the Galactic Center was made at radio frequencies ("Sgr A," Fig. 19.1). With observations at increasing resolution it turned out that Sgr A is a rather dense and complex region: at the dynamical center of the Galaxy, we have the nonthermal compact radio source Sgr A*. In recent years, observations made in the near-infrared revealed orbital motions of stars in the immediate vicinity of Sgr A*. A detailed study of these motions confirmed that Sgr A* is most probably a supermassive (—3 x 106 M0) black hole, e.g., [23]. Sgr A* is located at the center of the thermal radio source Sgr A West, which consists of a spiral-shaped group of thermal gas clouds. Sgr A West is surrounded by the circumnuclear disk (CND), a shell or ring of molecular matter. The radius of the CND is about 30". The CND is clumpy and rotating around IRS 16, a dense cluster of hot stars. The nonthermal shell-like radio source Sgr A East is surrounding Sgr A West in projection, but its center is offset by about 50". The nonthermal (and expanding) shell, in turn, is surrounded by a dust ring or molecular ridge. The molecular cloud M-0.02+0.07 (or "+50 km s^1 cloud") is located north-east of Sgr A East. At larger distances up

Fig. 19.1 VLA radio continuum image of the Galactic Center showing the shell-like structure of nonthermal Sgr A East and the spiral-shaped structure of Sgr A West at X = 6 cm with a resolution of 3.4" x 2.9" (left, [28]). Schematic diagram (right, [9]) showing the sky locations and rough sizes and shapes of Galactic Center sources. The coordinates are relative with respect to the compact nonthermal radio source Sgr A*. One arcminute corresponds to about 2.3 pc at the distance of 8kpc

Fig. 19.1 VLA radio continuum image of the Galactic Center showing the shell-like structure of nonthermal Sgr A East and the spiral-shaped structure of Sgr A West at X = 6 cm with a resolution of 3.4" x 2.9" (left, [28]). Schematic diagram (right, [9]) showing the sky locations and rough sizes and shapes of Galactic Center sources. The coordinates are relative with respect to the compact nonthermal radio source Sgr A*. One arcminute corresponds to about 2.3 pc at the distance of 8kpc to 100 pc, there is a variety of clusters of bright stars, synchrotron-bright filaments, supernova remnants, and giant molecular clouds.

The nonthermal radio emission from Sgr A East in the direction of Sgr A West is heavily absorbed by Sgr A West [28], a convincing indication of the fact that, along the line of sight, Sgr A West lies in front of the Sgr A East shell, although at an uncertain distance. Possibly, the front edge of the expanding shell has reached and passed through Sgr A*, at the center of Sgr A West. Whether the outward shock pushed gas toward the central black hole causing a bright flash about 1000 years ago, is under discussion. Once passed Sgr A*, this shock might have swept away the gas thereby being responsible for the currently dim nature of the black hole.

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