The Sloan Lens ACS Survey. II. Stellar Populations and Internal Structure of Early‐Type Lens Galaxies

We derive Fundamental Plane parameters of 15 early-type lens galaxiesidentified by the Sloan Lens ACS (SLACS) Survey. The size of the sample allowsus to investigate for the first time the distribution of lens galaxies in theFP space. After correcting for evolution, we find that lens galaxies occupy asubset of the local FP. The edge-on projection (approximately M vs M/L) isindistinguishable from that of normal early-type galaxies. However -- withinthe fundamental plane -- the lens galaxies appear to concentrate at the edge ofthe region populated by normal early-type galaxies. We show that this is aresult of our selection procedure (approximately velocity dispersionsigma>240km/s). We conclude that SLACS lenses are a fair sample of highvelocity dispersion early-type galaxies. By comparing the central stellarvelocity dispersion that of the best fit lens model, we find== =1.01+-0.02 with 0.065 rms scatter. We conclude thatwithin the Einstein radii the SLACS lenses are very well approximated byisothermal ellipsoids, requiring a fine tuning of the stellar and dark matterdistribution (bulge-halo ``conspiracy''). Interpreting the offset from thelocal FP in terms of evolution of the stellar mass-to-light ratio, we find forthe SLACS lenses d log M/L_B/dz=-0.69+-0.08 (rms 0.11) consistent with the ratefound for field early-type galaxies and with a scenario where most of the starswere formed at high redshift (>2) with secondary episodes of star formationproviding less than ~10% of the stellar mass below z=1. We discuss starformation history and structural homogeneity in the context of formationmechanisms such as collisionless (``dry'') mergers. [Abridged]Comment: 2006, ApJ, 604, 622; 13 pages, 7 figures, 2 tables. Replaced Table 2, since the previous version was incorrectly sorted. Updated references. No changes in plots or content. More info available at SLACS website www.slacs.or