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 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 3 r (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 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.

On the apparent power law in CDM halo pseudo-phase space density profiles

“…From member galaxy kinematics and from lensing, X-ray, and kinematical mass profiles, a powerlaw Q(r) in agreement with simulations has been inferred for galaxy cluster Abell 2142 (Munari et al 2014). Measurements of the mass density ρ(r) and the radial stellar velocity dispersion σr, * (r) of ∼ 2000 SDSS elliptical galaxies yield a power-law Q(r) ∝ r −χ , with χ = 1.860 ± 0.035, in remarkable agreement with values for simulated CDM halos (Chae 2014;Chae et al 2014). This is particularly striking because density profiles for ellipticals are not universal and differ sharply from halo density profiles, largely because the stellar components of ellipticals are influenced by dissipative processes (e.g., gas cooling to form discs or the merger of disc galaxies to form a bulge).…”

Power-law Pseudo-phase-space Density Profiles of Dark Matter Halos: A Fluke of Physics?

“…2016; Nolting et al 2016;Gao et al 2012;Ludlow et al 2010;Navarro et al 2010;Ma et al 2009;Knollmann et al 2008;Hoffman et al 2007;Peirani et al 2006), halos computed through an iterative collisionless spherical collapse (Austin et al 2005), and even galaxies and clusters formed in the real Universe (Chae 2014;Munari et al 2014) are well characterized by a power law, ρ(r) σ 3 (r) ∝ r −α .…”

Self-gravitating Equilibria of Non-minimally Coupled Dark Matter Halos

“…Such values are indeed expected on the basis of simple self-similar arguments (see Bertschinger 1985;Lapi & Cavaliere al. 2009aNadler et al 2017) and also broadly consistent with observations (see Lapi & Cavaliere 2009b;Chae 2014;Munari et al 2014). On this basis, but to keep some degree of generality 4 , we adopt the EOS parametrization…”