Gravitational lensing and stellar dynamics provide two complementary , nearly orthogonal , constraints on the mass distribution of early-type lens galaxies .
This allows the luminous and dark-matter distribution in higher-redshift ( z > 0.1 ) galaxies to be studied beyond the limitations of each individual method .
Two surveys have been initiated to compile a large sample of early-type galaxies suitable to lensing and dynamical studies : ( 1 ) The Lenses Structure & Dynamics ( LSD ) Survey and ( 2 ) the Sloan Lens ACS ( SLACS ) Survey .

Using spherically symmetric mass models , I illustrated how lensing and dynamical constraints can be used to measure the “ effective ” density slope ( \gamma ^ { \prime } ) of galaxies inside their Einstein radii and estimate the typical error on this determination .

The main results from the LSD survey thus far are : ( i ) Massive ( typically > L _ { * } ) early-type galaxies at z \approx 0.5–1 contain a significant fraction f _ { CDM } = 0.4 - 0.7 of dark matter inside their Einstein radii .
[ The null-hypothesis , f _ { CDM } = 0 , is excluded at the > 99 \% in all analyzed systems . ]
( ii ) The inner CDM density slope is \gamma _ { CDM } = 1.3 ^ { +0.2 } _ { -0.3 } ( 68 % CL ) for \rho _ { CDM } \propto r ^ { - \gamma _ { CDM } } .
( iii ) The total density slope \gamma ^ { \prime } = 1.9 \pm 0.1 ( with 0.3 rms scatter in the sample ; \rho \propto r ^ { - \gamma ^ { \prime } } ) .
The intrinsic scatter of 15 % in \gamma ^ { \prime } is consistent with local dynamical studies and can lead to a 30 % rms scatter in inferred values of H _ { 0 } from lens time-delays , when purely isothermal mass models are assumed .

Hence , the common practice to assume that lens galaxies are perfectly isothermal should be abandoned , especially in cases where this assumption is critical .