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 29 gravitational lenses spanning the redshift range 0 . z . 1. Based on the redshiftdependent intercept of the fundamental plane in the rest frame B band, we measure an evolution rate of d log(M/L)B/dz = −0.55 ± 0.04 (all errors are 1σ 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 = −0.49 ± 0.09. An evolution analysis of properlycorrected aperture mass-to-light ratios (defined by the lensed image separations) is closely related to the Faber-Jackson relation. In rest frame B band we find an evolution rate of d log(M/L)B/dz = −0.30 ± 0.21, a present-day characteristic magnitude of M∗0 = −19.86 + 5 log h± 0.29 (assuming a characteristic velocity dispersion of σDM∗ = 225 km s), and a Faber-Jackson slope of γFJ = 3.44 ± 0.58. The measured evolution rates favor old stellar populations (mean formation redshift 〈zf 〉 > 1.8 at 2σ confidence for a Salpeter initial mass function and a flat Ωm = 0.3 cosmology) among early-type field galaxies, and argue against significant episodes of star formation at z < 1. Subject headings: galaxies: elliptical and lenticular, cD – galaxies: evolution – gravitational lensing