The Evolution and Structure of Early‐Type Field Galaxies: A Combined Statistical Analysis of Gravitational Lenses

We introduce a framework for simultaneously investigating the structure andluminosity evolution of early-type gravitational lens galaxies. The method isbased on the fundamental plane, which we interpret using the aperturemass-radius relations derived from lensed image geometries. We apply thismethod to our previous sample of 22 lens galaxies with measured redshifts andexcellent photometry. Modeling the population with a single mass profile andevolutionary history, we find that early-type galaxies are nearly isothermal(logarithmic density slope n = 2.06 +/- 0.17, 68% C.L.), and that their starsevolve at a rate of dlog(M/L)_B/dz = -0.50 +/- 0.19 (68% C.L.) in the restframe B band. For a Salpeter IMF and a concordance cosmology, this implies amean star formation redshift of > 1.5 at 95% confidence. While this modelcan neatly describe the mean properties of early-type galaxies, it is clearthat the scatter of the lens sample is too large to be explained byobservational uncertainties alone. We therefore consider statistical models inwhich the galaxy population is described by a distribution of star formationredshifts. We find that stars must form over a significant range of redshifts(Delta z_f > 1.7, 68% C.L.), which can extend as low as z_f = 1 for someacceptable models. However, the typical galaxy will still have an old stellarpopulation ( > 1.5). The lens sample therefore favors early star formationin field ellipticals -- even if we make no a priori assumption regarding theshape of the mass distribution in lenses, and include the range of possibledeviations from homology in the uncertainties. Our evolution results call intoquestion several recent claims that early-type galaxies in low-densityenvironments have much younger stars than those in rich clusters.Comment: 36 pages including 6 figures, (re-)submitted to Ap