The cosmic evolution survey (COSMOS): The morphological content and environmental dependence of the galaxy color-magnitude relation at z similar to 0.7

We study the environmental dependence and the morphological composition of the galaxy color-magnitude diagram at z~0.7, using a pilot subsample of COSMOS. The sample includes ~2000 galaxies with IAB<24 and photometric redshift within 0.61<z<0.85, covering an area of 270 arcmin2. Galaxy morphologies are estimated via a nonparametric automatic technique. The (V-z') versus z' color-magnitude diagram shows a clear red sequence dominated by early-type galaxies and a remarkably well-defined ``blue sequence'' described by late-type objects. While the percentage of objects populating the two sequences is a function of environment, also following a clear morphology/color-density relation at this redshift, we establish that their normalization and slope are independent of local density. We identify and study a number of objects with ``anomalous'' colors, given their morphology, polluting the two sequences. Red late-type galaxies are found to be mostly highly inclined or edge-on spiral galaxies for which colors are dominated by internal reddening by dust. In a sample of color-selected red galaxies, these would represent 33% contamination with respect to truly passive spheroidals. Conversely, the population of blue early-type galaxies is composed of objects of moderate luminosity and mass, concurring to only ~5% of the mass in spheroidal galaxies. The majority of them (~70%) occupy a position in the μB-r50 plane not consistent with their being precursors of current-epoch elliptical galaxies. Their fraction with respect to the whole galaxy population does not depend on the environment, at variance with the general early-type class. In a color-mass diagram, color sequences are even better defined, with red galaxies covering in general a wider range of masses at nearly constant color, and blue galaxies showing a more pronounced dependence of color on mass. While the red sequence is adequately reproduced by models of passive evolution, the blue sequence is better interpreted as a specific star formation sequence. The substantial invariance of its slope and normalization with respect to local density suggests that the overall ``secular'' star formation is driven more by galaxy mass than by environment.