The phase space density in elliptical galaxies

The phase space mass densities of the cores of elliptical galaxies are found to generally exceed the phase space mass densities in galactic disks. Since the phase space density can never increase in a collisionless process, these data argue against the lower luminosity (MB fainter than -22) ellipticals being normally formed from stellar disks. It does leave open the possibility that subsequent dissipation of a gas component could increase the core densities to the large values required. A series of one-dimensional gravitational experiments shows a small region in the core which retains the initial phase space density. However, the observable phase density in the core is conserved only if the initial system has large random velocities.