THE MORPHOLOGICAL EVOLUTION OF MERGER REMNANTS

Elliptical galaxies formed in a major merger have a tendency to become more nearly spherical with time, thanks to the gravitational effect of their central black hole (or black holes). Observational results indicate that elliptical galaxies with older stellar populations (t > 7.5 Gyr) have rounder central isophotes than ellipticals with younger stellar populations. In addition, the older ellipticals tend to have core profiles, while the younger ellipticals have power-law profiles. Numerical simulations of galaxy mergers indicate that if one or both of the progenitors have a central black hole with mass ~ 0.2% of the stellar mass, then the effect of the black hole(s) is to make the central regions of the remnant rounder, with a characteristic time scale of a few gigayears.