A UNIVERSAL POWER-LAW PROFILE OF PSEUDO-PHASE-SPACE DENSITY-LIKE QUANTITIES IN ELLIPTICAL GALAXIES

We study profiles of mass density, velocity dispersion (VD), and a combination of both using similar to 2000 nearly spherical and rotation-free Sloan Digital Sky Survey galaxies. For observational stellar mass density. rho(*)(r), we consider a range of dark matter (DM) distribution rho(DM)(r) and VD anisotropy beta(r) to investigate radial stellar VD sigma(*r)(r) using the spherical Jeans equation. While mass and VD profiles vary appreciably depending on DM distribution and anisotropy, the pseudo-phase-space density-like combination rho(r)/sigma(3)(*r) with total density rho(r) = rho(*)(r) + rho(DM)(r) is nearly universal. In the optical region, the negative logarithmic slope has a mean value of approximate to 1.86-1.90 with a galaxy-to-galaxy rms scatter of approximate to 0.04-0.06, which is a few times smaller than that of.(r) profiles. The scatter of. can be increased by invoking wildly varying anisotropies that are, however, less likely because they would produce too large a scatter of line of sight VD profiles. As an independent check of this universality, we analyze stellar orbit-based dynamical models of 15 early-type galaxies (ETGs) of the Coma cluster provided by J. Thomas. Coma ETGs, with sigma(*r)(r) replaced by the rms velocity of stars v(*rms)(r) including net rotation, exhibit a similar universality with a slope of chi = 1.93 +/- 0.06. Remarkably, the inferred values of chi for ETGs match well the slope approximate to 1.9 predicted by N-body simulations of DM halos. We argue that the inferred universal nature of rho(r)/sigma(3)(*r)(r) cannot be fully explained by equilibrium alone, implying that some astrophysical factors conspire and/ or it reflects a fundamental principle in collisionless formation processes.