BLACK HOLE CANDIDATES AT GALACTIC CENTRES

Observations with the WHT and the ISIS spectrograph have greatly improved our knowledge of the rotation properties of a number of nearby galaxies. Two of the galaxies observed, the Andromeda nebula M31, and its compact elliptical companion M32, have been proposed as black hole candidates on the basis of their rapid central rotation. In excellent seeing conditions rotation velocities in the centres were measured with unprecedented spatial resolution. Despite the excellent seeing the central velocity in M31 is still unresolved, strengthening the case for a central black hole of at least 10⁷ and probably 10⁸ solar masses in this galaxy. In M32 the rotation gradient is much less steep, but the case for a central black hole of around 8x10⁶ solar masses is still strong.

Based on the success of these observations, researchers have searched for similarly rotating cores in other galaxies. Two very strong candidates have emerged, NGC3115, a lenticular, and NGC5813, an elliptical. In the later case the rotation gradient is also unresolved, and this is a particularly strong candidate for a black hole. NGC205 is a different kind of galaxy, a dwarf spheroidal companion on M31. It is elongated with an axial ratio of two to one. Observations with the intermediate dispersion spectrograph on the INT have shown that this galaxy does not rotate at all. It must be flattened by an anisotropic velocity dispersion, which is suprising for such a small galaxy.