Photometric mass and mass decomposition in early-type lens galaxies
  By using the SDSS multicolor photometry and lens modeling, we study stellar
mass properties and the luminous and dark matter composition of the 57
early-type lens galaxies analyzed by the SLACS Survey. We fit the lens SEDs
composed of ugriz magnitudes with a three-parameter grid of different composite
stellar population models, computed by adopting solar metallicity and various
IMFs, and we employ the best-fit values of the total projected mass enclosed
within the Einstein ring of each system. We measure size-stellar mass and
surface stellar mass density-stellar mass relations consistent with those
determined for non-lens galaxies. We find statistically significant evidence
that more massive lens galaxies tend to form in regions of higher galaxy
density, as for all early-type galaxies. We measure for the corresponding
stellar quantities the same scaling law between effective mass-to-light ratio
and mass, that is used to explain the "tilt" of the FP, and the same evolution
of the effective mass-to-light ratio with redshift, that can be derived from
the FP. We conclude that the lens total mass is linearly proportional to the
luminous mass, at a more than 99% CL. In addition, by assuming that the lens
galaxies are homologous systems, we study their distribution of dark matter and
estimate a value on the order of 30% for the dark over total projected mass
fraction contained within the average Einstein radius of approximately 4 kpc.
These results suggest that early-type lens galaxies are an unbiased subsample
representative of the whole sample of early-type galaxies. This allows us to
generalize our findings on dark matter in lens galaxies to the population of
massive early-type galaxies. We note that, for the assumed metallicity, a
Salpeter IMF describes well the lens sample.
