The Luminosity Function of Early-Type Field Galaxies at z ≈ 0.75

We measure the luminosity function of morphologically selected E/S0 galaxies from z = 0.5 to 1.0 using deep high-resolution Advanced Camera for Surveys (ACS) imaging data. Our analysis covers an area of 48 arcmin² (8 times the area of the Hubble Deep Field North) and extends 2 mag deeper (I ∼ 24 mag) than was possible in the Deep Groth Strip Survey (DGSS). Our fields were observed as part of the ACS Guaranteed Time Observations. At 0.5 < z < 0.75, we find M - 5 log h_0.7 = -21.1 ± 0.3 and α = -0.53 ± 0.2, and at 0.75 < z < 1.0, we find M - 5 log h_0.7 = -21.4 ± 0.2, consistent with 0.3 mag of luminosity evolution (across our two redshift intervals). These luminosity functions are similar in both shape and number density to the luminosity function using morphological selection (e.g., DGSS), but are much steeper than the luminosity functions of samples selected using morphological proxies such as the color or spectral energy distribution (e.g., CFRS, CADIS, or COMBO-17). The difference is due to the "blue," (U - V)₀ < 1.7, E/S0 galaxies, which make up to ∼30% of the sample at all magnitudes and an increasing proportion of faint galaxies. We thereby demonstrate the need for both morphological and structural information to constrain the evolution of galaxies. We find that the blue E/S0 galaxies have the same average sizes and Sersic parameters as the "red," (U - V)₀ > 1.7, E/S0 galaxies at brighter luminosities (M_B < -20.1), but are increasingly different at fainter magnitudes, where blue galaxies are both smaller and have lower Sersic parameters. We find differences in both the size-magnitude relation and the photometric plane offset for red and blue E/S0s, although neither red nor blue galaxies give a good fit to the size-magnitude relation. Fits of the colors to stellar population models suggest that most E/S0 galaxies have short star formation timescales (τ < 1 Gyr), and that galaxies have formed at an increasing rate from z ∼ 8 until z ∼ 2, after which there has been a gradual decline.