The DEEP Groth Strip Survey. X. Number Density and Luminosity Function of Field E/S0 Galaxies at z < 1

We present the luminosity function and color-redshift relation of a magnitude-limited sample of 145 mostly red field E/S0 galaxies at z<~1 from the DEEP Groth Strip Survey (GSS). Using nearby galaxy images as a training set, we develop a quantitative method to classify E/S0 galaxies based on smoothness, symmetry, and bulge-to-total light ratio. Using this method, we identify 145 E/S0's at 16.5<I<22 within the GSS, for which 44 spectroscopic redshifts (z_spec) are available. Most of the galaxies with spectroscopic redshifts (86%) form a ``red envelope'' in the redshift-color diagram, consistent with predictions of spectral synthesis models in which the dominant stellar population is formed at redshifts z>~1.5. We use the tight correlation between V-I and z_spec for this red subset to estimate redshifts of the remaining E/S0's to an accuracy of ~10%, with the exception of a small number (16%) of blue interlopers at low redshift that are quantitatively classified as E/S0's but are not contained within the red envelope. Constructing a luminosity function of the full sample of 145 E/S0's, we find that there is about 1.1-1.9 mag brightening in rest-frame B-band luminosity back to z~=0.8 from z=0, consistent with other studies. Together with the red colors, this brightening is consistent with models in which the bulk of stars in red field E/S0's formed before z_for>~1.5 and have been evolving rather quiescently, with few large starbursts since then. Evolution in the number density of field E/S0 galaxies is more difficult to measure, and uncertainties in the raw counts and their ratio to local samples might amount to as much as a factor of 2. Within that uncertainty, the number density of red E/S0's to z~=0.8 seems relatively static, being comparable to or perhaps moderately less than that of local E/S0's, depending on the assumed cosmology. A doubling of E/S0 number density since z=1 can be ruled out with high confidence (97%) if Ω_m=1. Taken together, our results are consistent with the hypothesis that the majority of luminous field E/S0's were already in place by z~1, that the bulk of their stars were already fairly old, and that their number density has not changed by large amounts since then.