A universal power-law profile of pseudo-phase-space density-like quantities in elliptical galaxies

Abstract

We study profiles of mass density, velocity dispersion (VD), and a combination of both using 2000 nearly spherical and rotation-free Sloan Digital Sky Survey galaxies. For observational stellar mass density ρ (r), we consider a range of dark matter (DM) distribution ρDM (r) and VD anisotropy β(r) to investigate radial stellar VD σr (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 with total density ρ(r) = ρ(r) + ρDM (r) is nearly universal. In the optical region, the negative logarithmic slope has a mean value of 〈χ〉 1.86-1.90 with a galaxy-to-galaxy rms scatter of 0.04-0.06, which is a few times smaller than that of ρ(r) profiles. The scatter of χ 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 σr (r) replaced by the rms velocity of stars v rms (r) including net rotation, exhibit a similar universality with a slope of χ = 1.93 ± 0.06. Remarkably, the inferred values of χ for ETGs match well the slope 1.9 predicted by N-body simulations of DM halos. We argue that the inferred universal nature of cannot be fully explained by equilibrium alone, implying that some astrophysical factors conspire and/or it reflects a fundamental principle in collisionless formation processes. © 2014. The American Astronomical Society. All rights reserved..