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 Meff 3 × 10 10 M� are well approximated by a power-law ellipsoid, with an average logarithmic density slope of � γ � LD �≡ −d log(ρtot)/d log(r) = 2.085 +0.025 −0.018 (random error on mean) for isotropic orbits with βr = 0, ±0.1 (syst.) and σγ � 0.20 +0.04 −0.02 intrinsic scatter (all errors indicate the 68% CL). We find no correlation of γ � LD with galaxy mass (Meff), rescaled radius (i.e., Reinst/Reff) 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 � γ � SR �= 1.959 ± 0.077. Agreement between the two values is reached for � βr �= 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: Meff ∝ L 1.363±0.056 V, eff . We conclude that massive ETGs are structurally close to homologous with close to isothermal total density profiles (10% intrinsic scatter) and have at most some mild radial anisotropy. Our results provide new observational limits on galaxy formation and evolution scenarios, covering 4 Gyr look-back time.