We combine in a self-consistent way the constraints from both gravitational lensing and stellar kinematics to perform a detailed investigation of the internal mass distribution , amount of dark matter , and dynamical structure of the sixteen early-type lens galaxies from the SLACS Survey , at z = 0.08 - 0.33 , for which both HST /ACS and NICMOS high-resolution imaging and VLT VIMOS integral-field spectroscopy are available .
Based on this data set , we analyze the inner regions of the galaxies , i.e .
typically within one ( three-dimensional ) effective radius r _ { \mathrm { e } } , under the assumption of axial symmetry and by constructing dynamical models supported by two-integral stellar distribution functions ( DFs ) .
For all systems , the total mass density distribution is found to be well approximated by a simple power-law \rho _ { \mathrm { tot } } \propto m ^ { - \gamma ^ { \prime } } ( with m being the ellipsoidal radius ) : this profile is on average slightly super-isothermal , with a logarithmic slope \langle \gamma ^ { \prime } \rangle = 2.074 ^ { +0.043 } _ { -0.041 } ( errors indicate the 68 % CL ) and an intrinsic scatter \sigma _ { \gamma ^ { \prime } } = 0.144 ^ { +0.055 } _ { -0.014 } , and is fairly round , with an average axial ratio \langle q \rangle = 0.77 \pm 0.04 .
The lower limit for the dark matter fraction ( f _ { \mathrm { DM } } ) inside r _ { \mathrm { e } } ranges , in individual systems , from nearly zero to almost a half , with a median value of 12 % .
By including stellar masses derived from stellar population synthesis ( SPS ) models with a Salpeter initial mass function ( IMF ) , we obtain an average f _ { \mathrm { DM } } = 31 \% , and the corresponding stellar profiles are physically acceptable , with the exception of two cases where they only marginally exceed the total mass profile .
The f _ { \mathrm { DM } } rises to 61 % if , instead , a Chabrier IMF is assumed .
For both IMFs , the dark matter fraction increases with the total mass of the galaxy ( correlation significant at the 3-sigma level ) .
Based on the intrinsic angular momentum parameter calculated from our models , we find that the galaxies can be divided into two dynamically distinct groups , which are shown to correspond to the usual classes of the ( observationally defined ) slow and fast rotators .
Overall , the SLACS systems are structurally and dynamically very similar to their nearby counterparts , indicating that the inner regions of early-type galaxies have undergone little , if any , evolution since redshift z \approx 0.35 .