Photometric mass and mass decomposition in early-type lens galaxies

Context. The largest spectroscopically selected sample of strong gravitational lens systems presented and analyzed to date is that of the Sloan Lens ACS (SLACS) survey. For the 57 massive early-type lens galaxies in the sample, photometric and spectroscopic measurements are available from the Sloan Digital Sky Survey (SDSS). Aims. By using the SDSS multicolor photometry and lens modeling, we study stellar-mass properties and the luminous and dark matter composition of the early-type lens galaxies in the sample. Methods. We fit the lens spectral energy distributions (SEDs) consisting of ugriz magnitudes with a three-parameter grid (age, star-formation timescale, and photometric mass) of Bruzual & Charlot's and Maraston's composite stellar-population models, computed by adopting solar metallicity and various initial mass functions (IMFs). We also utilize the best-fit parameters derived from the lens models of the total projected mass enclosed within the disk defined by the Einstein radius of each system. Results. We observe that early-type lens galaxies have the same physical properties as massive non-lens galaxies. In particular, we investigate the relationships between stellar mass and both the size and the surface stellar-mass density of the lens galaxies, which are consistent with those determined for non-lens galaxies in the local Universe. We find statistically significant evidence that more luminous and massive lens galaxies tend to form in regions of higher galaxy density, as for all early-type galaxies. Interestingly, for the corresponding stellar quantities we measure the same scaling law between effective mass-to-light ratio and mass used to explain the "tilt" of the Fundamental Plane (FP), and the same evolution in the effective mass-to-light ratio with redshift as derived from the FP. We conclude that the total (luminous+dark) mass of the lenses is linearly proportional to the luminous mass, at a confidence level of more than 99%. 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 ring of radius of approximately 4 kpc. Conclusions. These results suggest that early-type lens galaxies are an unbiased subsample representative of the complete sample of early-type galaxies. This allows us to generalize our findings on the amount and distribution of dark matter in lens galaxies to the population of massive early-type galaxies. For the assumed metallicity, we note that a Salpeter IMF is better suited than either a Chabrier or Kroupa IMF to describing the sample of lenses.