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

摘要

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 ρ_(DM)(r) and VD anisotropy β(r) to investigate radial stellar VD σ_(★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 ρ(r)/σ_(★r)~3(r) with total density ρ(r) = ρ_★(r) + ρ_(DM)(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 σ_(★r)(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 ρ(r)/σ_(★r)~3(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.