The evolution of a mass-selected sample of early-type field galaxies

We investigate the evolution of mass-selected early-type field galaxies using a sample of 28 gravitational lenses spanning the redshift range 0 less than or similar to z less than or similar to 1. Based on the redshift-dependent intercept of the fundamental plane in the rest-frame B band, we measure an evolution rate of d log (M/L)(B)/dz = -0.56 +/- 0.04 (all errors are 1 sigma unless noted) if we directly compare to the local intercept measured from the Coma Cluster. Refitting the local intercept helps minimize potential systematic errors and yields an evolution rate of d log (M/L)(B)/dz = -0.54 +/- 0.09. An evolution analysis of properly corrected aperture mass-to-light ratios (defined by the lensed image separations) is closely related to the Faber-Jackson relation. In the rest-frame B band, we find an evolution rate of d log(M/L)(B)/dz = -0.41 +/- 0.21, a present-day characteristic magnitude of M*0 = -19.70 + 5 log h +/- 0.29 (assuming a characteristic velocity dispersion of sigma(DM*) = 225 km s(-1)), and a Faber-Jackson slope of gamma(FJ) = 3.29 +/- 0.58. The measured evolution rates favor old stellar populations ( mean formation redshift [z(f)] > 1.8 at 2 sigma confidence for a Salpeter initial mass function and a. at Omega(m) = 0.3 cosmology) among early-type field galaxies and argue against significant episodes of star formation at z < 1.