THE STRUCTURE AND DYNAMICS OF MASSIVE EARLY-TYPE GALAXIES: ON HOMOLOGY, ISOTHERMALITY, AND ISOTROPY INSIDE ONE EFFECTIVE RADIUS

Abstract

Based on 58 SLACS strong-lens early-type galaxies (ETGs) with direct total-mass and stellar-velocity dispersion measurements, we find that inside one effective radius massive elliptical galaxies with M eff 3 10 10 M are well approximated by a power-law ellipsoid, with an average logarithmic density slope of LD -d log( tot )/d log(r) = 2.085 +0.025 -0.018 (random error on mean) for isotropic orbits with r = 0, 0.1 (syst.) and 0.20 +0.04 -0.02 intrinsic scatter (all errors indicate the 68% CL). We find no correlation of LD with galaxy mass (M eff ), rescaled radius (i.e., R einst /R eff ) or redshift, despite intrinsic differences in density-slope between galaxies. Based on scaling relations, the average logarithmic density slope can be derived in an alternative manner, fully independent from dynamics, yielding SR = 1.959 0.077. Agreement between the two values is reached for r = 0.45 0.25, consistent with mild radial anisotropy. This agreement supports the robustness of our results, despite the increase in mass-to-light ratio with total galaxy mass: