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Astron. Astrophys. 349, L49-L52 (1999)

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2. The Galactic Center environment

Before presenting our results, we briefly describe the most important radio emitting sources present in the region within [FORMULA] from SgrA* covered by our observation (see Morris & Serabyn 1996 for a recent review). These extended sources are collectively known as the Sgr A Complex (Fig. 1).

[FIGURE] Fig. 1. A schematic view of the main radio structures in the SgrA Complex. North is to the top, East to the left. 1 arcmin corresponds to about 2.5 pc at 8.5 kpc.

The most peculiar object is the compact radio source SgrA* (Balick & Brown 1974). It has a radio luminosity of [FORMULA] ergs s-1 and a flux density rising as [FORMULA] between a low frequency turnover at [FORMULA]800 MHz and a cut-off at 2000-4000 GHz (Mezger, Duschl & Zylka 1996). Lo et al. (1998) measured an intrinsic size of less than 5.4[FORMULA] cm with multi-wavelength VLBA observations. From the proper motion of the nearby IR stars a dark mass of 2.6[FORMULA] [FORMULA] has been deduced to reside within a central volume of 10-6 pc3 (Ghez et al. 1998). Sgr A* is generally considered to be a super-massive black hole due to its unique radio spectrum (see e.g. Krichbaum et al. 1998), its compactness, its location at the GC (Yusef-Zadeh, Choate & Cotton 1999) and its low proper motion ([FORMULA]40 km s-1, Reid et al. 1999). The high energy emission of SgrA* is surprisingly low, corresponding to a 2-10 keV luminosity of only [FORMULA]1035 ergs s-1 (Koyama et al. 1996, Sidoli et al. 1999b), well below the Eddington limit for a 2 million solar masses black hole.

SgrA* is embedded in the ionized gas of SgrA West, an HII region with a characteristic "mini-spiral" shape of thermal radio emission. With an angular extent of [FORMULA], SgrA West is the innermost and ionized part of the "Circumnuclear Disk", a structure of molecular gas extending from 1.7 pc up to about 7 pc from the GC and rotating around it (Marshall, Lasenby & Harris 1995).

Proceeding outward, we find the [FORMULA] shell of SgrA East (Eckers et al. 1983), which is characterized by a non-thermal radio emission with spectral index [FORMULA]. It is centered about [FORMULA] offset of SgrA* and is probably located behind SgrA West (Davies et al. 1976, Pedlar et al. 1989). Its synchrotron radio emission and its shell morphology were explained in terms of a supernova remnant. Mezger et al. (1989) discovered a dust ring compressed by the SgrA East shell, possibly indicating that this shell originated from an extremely energetic explosion (4[FORMULA] ergs), maybe associated with SgrA*, occurring inside a dense molecular environment (104 cm-3). Other alternative explanations are the simultaneous explosion of about 40 supernovae, or a single SN explosion inside a medium with a much lower density. In this case the dust shell could be due to the stellar wind from the normal supernova progenitor (Mezger et al. 1989). Another possibility is the tidal disruption of a star by the central supermassive black hole (Khokhlov & Melia 1996).

A triangular shaped non-thermal halo ([FORMULA]20 pc in diameter) surrounds in projection the SgrA East shell (Yusef-Zadeh & Morris 1987), but it is probably not physically related with it (see Pedlar et al. 1989 for a detailed discussion). Its origin is unknown. It could be centered at the SgrA* position and related with the high energy activity at the GC (Yusef-Zadeh et al. 1997, Melia et al. 1998). The total energy in relativistic particles estimated from radio observations is about 5[FORMULA] ergs s-1. This value, together with its non-thermal radio spectrum and its size, suggest the possibility that it is an evolved supernova remnant (Pedlar et al. 1989).

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© European Southern Observatory (ESO) 1999

Online publication: September 2, 1999