Hubble Space Telescope imaging of the CFRS and LDSS redshift surveys.: III.: Field elliptical galaxies at 0.2 < z < 1.0

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 R-1/4 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 Delta M-B = -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 Delta(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 Angstrom, 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 less than or equal to 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 similar to 0.55 may be larger than that seen among samples of cluster ellipticals and SO 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/V-max] 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 similar to 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.