| Summary: | 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 [> over ~] 3 × 10[superscript 10] M[superscript ☉] are well approximated by a power-law ellipsoid, with an average logarithmic density slope of 〈γ'LD〉 ≡ −dlog(ρ[subscript tot]) [over dlog(r)] = 2.085[superscript +0.025 subscript −0.018] (random error on mean) for isotropic orbits with β[subscript r] = 0, ±0.1 (syst.) and σ[subscript γ'] [< over ~] 0.20 [superscript +0.04 subscript -0.02] intrinsic scatter (all errors indicate the 68% CL). We find no correlation of γ'[subscript LD] with galaxy mass (M[subscript eff]), rescaled radius (i.e., R[subscript einst] [over R[subscript 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 〈γ'[subscript SR]〉 = 1.959 ± 0.077. Agreement between the two values is reached for 〈β[subscript 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: M[subscript eff] ∝ L[superscript 1.363±0.056 subscript V,eff]. We conclude that massive ETGs are structurally close to homologous with close to isothermal total density profiles ([< over ~]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.
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