Article - GCNEWS, Vol. 10, December 1999


A Newsletter for Galactic Center Research
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Volume 10, December 1999 - ARTICLE

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VLT Spectroscopy of the Sgr A* Stellar Cluster

Andreas Eckart, Thomas Ott & Reinhard Genzel
(Max-Planck-Institut für Extraterrestrische Physik, Garching)


We have obtained new near-infrared observations of the central stellar cluster of our Galaxy carried out with the infrared spectrometer ISAAC at the ESO VLT and the MPE speckle camera SHARP at the ESO NTT. With ISAAC we used a spectral resolution of lambda/Delta lambda ~ 3500 on the 2.058 micron He I, 2.165 micron Br-gamma emission lines, and 2.29 micron CO bandhead absorption line. The data demonstrate clearly that there is no strong CO bandhead absorption originating in the northern part (S1/S2 area) of the central stellar cluster at the position of Sgr A*. This makes it likely that the K~14.5 fast moving objects are O9 - B0.5 stars with masses of 15 to 20 Msun. Weak CO bandhead absorption in the southern part of the cluster (S10/S11 area) could be due to contributions from neighbouring stars. We have also detected Br-gamma and He I line emission at the position of the central stellar cluster which could be associated with the 'mini-spiral' rather than with the Sgr A* cluster itself. At the NTT we obtained another epoch to measure the proper motion structure and variability of sources in the central stellar cluster (see Ott, Eckart, Genzel 1999). The changes of source positions are consistent with the proper motion velocities derived from earlier epochs.

Figure 1: Left: Speckle image reconstruction of the new June 1999 SHARP data taken at the ESO NTT. Right: The central portion of a 5 minute, two dimensional spectroscopic exposure on the CO(2-0) bandhead absorption line taken in ~0.3'' IR seeing with ISAAC on the ESO VLT UT1 (ANTU). The position of Sgr A* (see Menten et al. 1997) is indicated by a cross in the SHARP image. (Click here for a PostScript version.)


The gas and stellar dynamics indicate the presence of a large unresolved central mass (Sellgren et al. 1990, Krabbe et al. 1995, Haller et al. 1996, Eckart and Genzel 1996, 1997, Genzel et al. 1996, 1997, Ghez et al. 1998, 1999). At its measured mass of about 2.6*106 Msun and density (>=2*1012 Msunpc-3) it cannot be stable and therefore is most likely present in the form of a massive black hole (Maoz 1998).

Genzel et al. (1997) reported first R=lambda/Delta lambda~35 speckle spectroscopy measurements on individual objects in the central ~1'' diameter stellar cluster at the position of Sgr A*(IR). In combination with other data this spectroscopic information can be used to derive a lower limit to the mass associated with the compact radio source. On 30 June and 1 July 1999 we obtained the first ESO VLT UT1 (ANTU) NIR spectroscopic data of the Sgr A* stellar cluster using ISAAC (Moorwood et al. 1998) at a spectral resolution of R~3500 on the 2.058 micron He I, 2.165 micron Br-gamma emission lines, and the 2.29 micron CO bandhead absorption lines. The combination of these spectroscopic data taken in excellent seeing (0.3'' to 0.5'') and our new speckle image reconstructions based on SHARP NTT data strengthen the case for a compact mass and add to our understanding of the stellar population near the center of the Galaxy. Details of the observations are given in Eckart, Ott, Genzel (1999, A&A Dec.) Here we summarize some of the first results.

Recombination lines

Our high spatial and spectral resolution data clearly show the presence of Br-gamma and He I emission which is apparently spatially coincident with the location of the Sgr A* central stellar cluster. From our Br-gamma data we find a line width of <120 km/s and a velocity gradient of about 35 km/s between the southern part (S10/S12-region) and the northern part (S1/S2-region) of the cluster. In both slit settings this line emission appears to be connected to the more extended line emission over the remaining central cluster. Combined with the small line width at any position in that region this indicates that the emitting gas is not necessarily associated with the Sgr A* stellar cluster. If the emission would be associated with the cluster we would expect a larger line width due to the higher gravitational potential indicated by the rapid motions of the stars. However, broad and weak emission components could still be present at very low flux levels.

CO absorption

Fig.1 shows our new speckle image reconstruction based on data taken with the MPE speckle camera SHARP at the NTT in June 1999. In this figure we compare the image with a section of the two dimensional ISAAC spectroscopic exposure on the CO(2-0) bandhead absorption line. One can clearly identify the individual sources that contributed to the flux density in the 0.6'' slit. At the high angular resolution (~0.3'' seeing) of the exposure one can distinguish between the northern and southern part of the Sgr A* stellar cluster as well as a star 1.12'' south of the the center with obvious bandhead absorption. In Fig.2 we show the corresponding spectra There is only very weak bandhead absorption on the northern Sgr A* cluster which is in agreement with an expected contribution from the more extended underlying stellar cluster. The bandhead absorption on the southern part of the central stellar cluster is probably due to a significant flux density contribution from late type stars in the immediate vicinity. These new findings are in full agreement with our initial results that we obtained via R~ 35 speckle spectroscopy measurements on the individual objects S1, S2, S8, and S11. It also indicates that most of the other S-sources - especially near the position of Sgr A* - can not be stars with strong CO bandhead absorption.


Figure 2: Spectra of the northern and southern part of the Sgr A* cluster as well as the late type star 1.12'' south of the center. No strong CO(2-0) and CO(3-1) bandhead absorption is measured towards the northern part containing the fast moving sources S1 and S2. The spectra of the southern part and the late type star have been shifted down by 150 and 300 units, respectively. (Click here for a PostScript version.)

Our new data indicate that the mK~14.5 sources in the central Sgr A* cluster are most likely moderately luminous (L~5,000 to 10,000 Lsun) early type stars. If these objects are on the main sequence they would have to be O9 - B0.5 stars with masses of 15 to 20 Msun. Backer (1996, 1999) and Reid et al. (1999) have shown that the proper motion of Sgr A* itself is < 16-20 km/s which is close to a factor of 100 smaller than the velocity of the fast moving stars in its vicinity. N-body simulations using 20 Msun as an upper limit of the mass distribution of these high velocity stars result in a lower limit of 103 Msun for Sgr A* (Reid et al. 1999, see also Genzel et al. 1999, 1997). The corresponding mass density is larger than 1018 Msunpc-3, if this mass is enclosed within the radio size of Sgr A* of \le 1 AU. At the measured mass and mass density any dark cluster of stellar remnants (neutron stars, stellar black holes), low luminosity stars (e.g. white dwarfs) or sub-stellar objects would have a lifetime less than ~107 years. This short live time in addition to the high compactness is inconsistent with any currently known dynamical system. It appears that the most probable configuration of the central mass concentration is a massive, but currently inactive black hole (e.g. Quataert, Narayan, Reid 1999, Falcke & Melia 1997, Blandford & Begelman 1999).


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