  Elliptical galaxies formed in a major merger tend 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, older
ellipticals tend to have core profiles, while younger ellipticals tend to have
power-law profiles. Numerical simulations of galaxy mergers indicate that if
one or both of the progenitors have a central black hole with a mass around 0.2
percent of the mass in stars, then the effect of the black hole(s) is to make
the central regions of the remnant rounder, with a characteristic time of a few
gigayears.
