The Structure and Dynamics of Massive Early-Type Galaxies: On Homology, Isothermality, and Isotropy Inside One Effective Radius

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 gsim 3 × 10¹⁰ M _sun are well approximated by a power-law ellipsoid, with an average logarithmic density slope of langγ'_LDrang ≡ -dlog(ρ_tot)/dlog(r) = 2.085^+0.025 _-0.018 (random error on mean) for isotropic orbits with β _r = 0, ±0.1 (syst.) and σ_{γ'} ≲ 0.20^{+0.04}_{-0.02} intrinsic scatter (all errors indicate the 68% CL). We find no correlation of γ'_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 langγ'_SRrang = 1.959 ± 0.077. Agreement between the two values is reached for langβ _r rang = 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: M _eff vprop L ^1.363±0.056 _V,eff. We conclude that massive ETGs are structurally close to homologous with close to isothermal total density profiles (lsim10% 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.