The Cosmic Evolution Survey (cosmos): the Morphological Content and Enviromental Dependence of the Galaxy Color-magnitude Relation at Z

We study the enviromental dependence and the morphological composition of the galaxy colormagnitude diagram at z ∼ 0.7, using a pilot sub-sample of the COSMOS survey. The sample includes ∼ 2000 galaxies with IAB < 24 and photometric redshift within 0.61 < z < 0.85, covering an area of 270 square-arcmin. Galaxy morphologies are estimated via a non-parametric automatic technique. The (V − z) vs. z color-magnitude diagram shows a clear red-sequence dominated by early-type galaxies and also 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, following a clear morphology/color-density relation also 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 a 33% contamination with respect to truly passive spheroidals. Conversely, the population of blue early-type galaxies is composed by 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 them 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. Subject headings: large-scale structure: general — clusters of galaxies: general — galaxies: morphology — galaxies: general — galaxies: evolution ⋆ Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA Inc, under NASA contract NAS 5-26555; also based on data collected at : the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan; the XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA; the European Southern Observatory, Chile; Kitt Peak National Observatory, Cerro Tololo Inter-American Observatory, and the National Optical Astronomy Observatory, which are operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under cooperative agreement with the National Science Foundation; the National Radio Astronomy Observatory which is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc ; and the CanadaFrance-Hawaii Telescope operated by the National Research Council of Canada, the Centre National de la Recherche Scientifique de France and the University of Hawaii. 1 INAF-IASF Milano, via Bassini 15, I-20133 Milano, Italy 2 Dipartimento di Astronomia, Università di Padova, Vicolo dell’Osservatorio 2, I-35122, Padova, Italy 3 INAF-Osservatorio Astronomico di Brera, via Bianchi 46, I23807, Merate (LC), Italy 4 California Institute of Technology, MC 105-24, 1200 East California Boulevard, Pasadena, CA 91125 5 Visiting Astronomer, Univ. Hawaii, 2680 Woodlawn Dr., Honolulu, HI, 96822 6 Space Telescope Science Institute, 3700 SanMartin Drive, Baltimore, MD 21218 7 Institut d’Astrophysique de Paris, UMR7095 CNRS, Université Pierre et Marie Curie, 98 bis Boulevard Arago, 75014 Paris,