Phase space density in elliptical galaxies

The high core densities of elliptical galaxies place a significant constraint on conditions at the time of star formation. Phase mixing in cores is investigated with a small series of one-dimensional, gravitational collapse experiments. It is found that very cold collapses, ones in which the initial random velocity is less than 20 percent of the virial velocity, do undergo a reduction in the core phase space density from the initial value. Consequently, purely stellar mergers are unable to produce the normal ellipticals. Mergers are allowed for the most luminous ellipticals, and with the controlled aid of a high-density bulge component or gas dissipating into the core, mergers could work for the lower mass ellipticals as well. 24 references