The Las Campanas Infrared Survey: Early-Type Galaxy Progenitors beyond z = 1

Abstract

We have identified a population of faint red galaxies from a 0.62 deg^2 region of the Las Campanas Infrared Survey whose properties are consistent with their being the progenitors of early-type galaxies. The optical and IR colors, number-magnitude relation, and angular clustering together indicate modest evolution and increased star formation rates among the early-type field population at redshifts between 1 and 2. The counts of red galaxies with H magnitudes between 17 and 20 rise with a slope that is much steeper than that of the total H sample. The surface density of red galaxies drops from roughly 3000 deg^(-2) at H = 20.5, I-H > 3 to ~20 deg^(-2) at H = 20, I-H > 5. The V-I colors are approximately 1.5 mag bluer on average than a pure old population and span a range of more than 3 mag. The strength of the angular clustering of the red galaxies is an order of magnitude larger than that of the full galaxy sample. The colors, and photometric redshifts derived from them, indicate that the red galaxies have redshift distributions adequately described by Gaussians with σ_z ≃ 0.2 centered near z = 1, with the exception that galaxies having V-I < 1.6 and I-H > 3 are primarily in the 1.5 ≾ z ≾ 2 range. We invert the angular correlation functions using these n(z) and find comoving correlation lengths of r_0 ≃ 9-10 h^(-1) Mpc at z ≃ 1, comparable to, or larger than, those found for early-type galaxies at lower redshifts. A simple photometric evolution model reproduces the counts of the red galaxies, with only an ~30% decline in the underlying space density of early-type galaxies at z ~ 1.2. The colors indicate characteristic star formation rates of ~1 M_☉ yr^(-1) per 10^(10) M_☉. We suggest on the basis of the colors, counts, and clustering that these red galaxies are the bulk of the progenitors of present-day early-type galaxies.