2.2 Disk Heating and Massive Black Holes
The phenomenon of disk heating, i.e. the increase in velocity dispersion of stellar populations as function of age is well known for the solar neighbourhood. Stars are thought to be borne with a velocity dispersion similar to the gas, i.e. 10 km s-1. Due to scattering of the stars in the fluctuations of the galactic potential, the peculiar velocities of the star with respect to the circular velocity increase, and an ensemble of stars of a given age has thus a higher velocity dispersion with increasing age.
Several mechanisms have been proposed for the disk heating : 1) massive black holes as being the constituents of dark matter halos 2) giant molecular clouds, and 3) shearing bits and pieces of spiral arms. Predictions for the shape of the velocity ellipsoid (the normalized length of the velocity vectors in the three principal directions) have been made by Lacey (1984, 1991) for each of these three scenario's. The new HIPPARCOS results will certainly lead to a more profound examination of this problem for the solar neighbourhood (e.g. Gomez et al. 1997, Dehnen & Binney 1998).
Lacey & Ostriker (1985) analyse the effect of massive black holes, assuming they are the only constituent of the dark halo, on the dynamical heating of the disk. They derive an upper limit of about 106 M for our Galaxy. Rix & Lake (1993) point out that for small Sc galaxies this upper limit will have to be lowered to 104 M, since their potential well is much shallower. Very heavy black holes could even wreck the fragile disk of those galaxies altogether. In view of this effect, most people exclude massive black holes as an important constituent of dark matter.