The Galactic Centre

The central 0.1pc of our Galaxy will be an important target for VLTI at wavelengths from 2 to 10microns. The resolution of the VLTI at 2microns is about 2mas, which at the Galactic Centre corresponds to 15AU or about 1500 times the Schwarzschild radius of a 10⁶M., black hole. The first and most important goal will be to test for the presence of a central massive black hole by measuring the three-dimensional velocity field of the star cluster centered on IRS16. The current astrometric program at the NTT (Eckart et al., 1995)could be continued with higher precision and radial velocities could be determined at very small distances from the center of the star cluster, where observations with single telescopes are limited by crowding.

Another very important goal for the VLTI will be detailed observations of the infrared sources close to the position of SgrA*. It is presently unclear whether any of the objects found at 2.2microns is the true counterpart of the compact radio source. The study of a potential IR counterpart of SgrA* would give completely new insights into the vicinity of the central object of our Galaxy, and could perhaps give us a direct view of the putative accretion disk. In addition, the high angular resolution of the VLTI will enable us to obtain infrared spectra of individual stars in the very crowded Galactic Centre region. It will thus be possible to make a census of the stellar population in this area, to check whether there is ongoing star formation in the vicinity of the Galactic Centre, and to search for ``peculiar'' stars, which may be the remnants of stellar collisions. Observations at 10microns would also reveal the distribution of warm dust associated with SgrA*.

Observing the Galactic Centre is quite a challenging task for the VLTI because of the high density of sources. Hybrid configurations formed by combining the UTs with the ATs will give good coverage of the (u,v) plane, in particular when the technique of multi-frequency synthesis is employed. The focal plane instruments needed for observations of the Galactic Centre are (i) an instrument for the mid-infrared with spectral resolution R~200, and (ii) a near-infrared instrument with R~2000. For the investigation of SgrA*, polarimetric capabilities would be very valuable.