Based on 58 SLACS strong-lens early-type galaxies with direct total-mass and stellar-velocity dispersion measurements , we find that inside one effective radius massive elliptical galaxies with M _ { eff } \gtrsim 3 \cdot 10 ^ { 10 } M _ { \odot } are well-approximated by a power-law ellipsoid with an average logaritmic density slope of \langle \gamma ^ { \prime } _ { LD } \rangle \equiv - d \log ( \rho _ { tot } ) / d \log ( r ) = 2. % 085 ^ { +0.025 } _ { -0.018 } ( random error on mean ) for isotropic orbits with \beta _ { r } = 0 , \pm 0.1 ( syst . )
and \sigma _ { \gamma ^ { \prime } } \lesssim 0.20 ^ { +0.04 } _ { -0.02 } intrinsic scatter ( all errors indicate the 68 % CL ) .
We find no correlation of \gamma ^ { \prime } _ { 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 \langle \gamma ^ { \prime } _ { SR } \rangle = 1.959 \pm 0.077 .
Agreement between the two values is reached for \langle \beta _ { r } \rangle = 0.45 \pm 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 } \propto L _ { V, eff } ^ { 1.363 \pm 0.056 } .
We conclude that massive early-type galaxies are structurally close-to homologous with close-to isothermal total density profiles ( \lesssim 10 % intrinsic scatter ) and have at most some mild radial anisotropy .
Our results provide new observational limits on galaxy formation and evolution scenarios , covering four Gyr look-back time .