Hubble Space Telescope Imaging of the CFRS and LDSS Redshift Surveys. III. Field Elliptical Galaxies at 0.2<z<1.0

Abstract

Two-dimensional surface photometry has been performed on a magnitude-limited sample of 46 field galaxies that are classified as ellipticals based on two-dimensional fitting of their luminosity profiles using Hubble Space Telescope imaging. These galaxies are described well by a de Vaucouleurs R1/4 profile. The sample was selected from the combined Canada-France and LDSS redshift surveys and spans the redshift range 0.20<z<1.00. This analysis reveals several clear evolutionary trends. First, the relationship between galaxy half-light radius and luminosity evolves with redshift such that a galaxy of a given size is more luminous by ΔMB=-0.97+/-0.14 mag at z=0.92 relative to the local cluster elliptical relation. Second, the mean rest-frame color shifts blueward with redshift by Δ(U-V)=-0.68+/-0.11 at z=0.92 relative to the same relation in the Coma Cluster. These shifts in color and luminosity of field elliptical galaxies are similar to those measured for cluster ellipticals. Approximately one-third of these elliptical galaxies (independent of redshift) exhibit [O II] 3727 emission lines with equivalent widths >15 Å, indicating ongoing star formation. Therefore, field elliptical galaxies are not composed entirely of very old stellar populations. Estimated star formation rates (SFR) together with stellar population evolutionary models imply that <=5% of the stellar mass in the elliptical galaxy population has been formed since z=1. We find some evidence that the dispersion in color among field ellipticals at z~0.55 may be larger than that seen among samples of cluster ellipticals and S0 galaxies at similar redshift. We see no evidence for a decline in the space density of early-type galaxies with look-back time. Both the <V/Vmax> statistics and a comparison with local luminosity functions are consistent with the view that the population of massive early-type galaxies was largely in place by z~1. This implies that merging is not required since that time to produce the present-day space density of elliptical galaxies. However, the statistics are poor: a larger sample is required to produce a decisive result. Based on observations with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy Inc., under NASA contract NAS 5-26555.