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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.