A Deep Multicolor Survey VII. Extremely Red Objects and Galaxy Formation

Extremely Red Objects (EROs) offer a window to the universe at z ∼ 1 analogous to that provided by the Lyman Break galaxies at z = 3. Passive evolution and hierarchical galaxy formation models make very distinct predictions for the K (2.2μm) surface density of galaxies at z ∼ 1 and EROs are a powerful constraint on these theories. I present a study of nine resolved EROs with R − K ≥ 5.3 and K ≤ 18 mag found in the 185 arcmin of the Deep Multicolor Survey with near-infrared imaging. Photometric redshifts for these galaxies shows they all lie at z = 0.8 − 1.3. The relatively blue J − K colors of these galaxies suggest that most are old ellipticals, rather than dusty starbursts. The surface density of EROs in this survey (> 0.05 arcmin), which is a lower limit to the total z ∼ 1 galaxy surface density, is an order of magnitude below the prediction of passive galaxy evolution, yet over a factor of two higher than the hierarchical galaxy formation prediction for a flat, matter-dominated universe. A flat, Λ−dominated universe may bring the hierarchical galaxy formation model into agreement with the observed ERO surface density. Subject headings: galaxies: evolution – galaxies: photometry – galaxies: formation – cosmology: observations