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The Cosmic Evolution Survey (COSMOS): The morphological content and enviromental dependence of the galaxy color-magnitude relation at z~0.7

P. Cassata·L. Guzzo·A. Franceschini·N. Scoville·P. Capak·R. S. Ellis·A. Koekemoer·H. J. McCracken·B. Mobasher·A. Renzini·E. Ricciardelli·M. Scodeggio·Y. Taniguchi·D. Thompson
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Abstract

We study the enviromental dependence and the morphological composition of the galaxy color-magnitude diagram at z~0.7, using a pilot sub-sample of ~2000 galaxies from the COSMOS surve, with I_AB<24, photometric redshift within 0.61<z<0.85 and galaxy morphologies based on the HST-ACS data. The C-M 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, with a clear morphology/color-density relation, their normalization and slope are independent of local density. We identify and study a number of objects with "anomalous'' colors, given their morphology. Red late-type galaxies are found to be mostly highly-inclined spiral galaxies, with colors dominated by internal reddening (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 surface-brightness/effective-radius plane not consistent with them being precursors of current epoch elliptical galaxies. 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. Its substantial invariance with respect to local density suggests that the overall, "secular'' star formation is driven more by galaxy mass than by environment. [...]

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P. Cassata, L. Guzzo, A. Franceschini, N. Scoville, P. Capak, R. S. Ellis, A. Koekemoer, H. J. McCracken, B. Mobasher, A. Renzini, E. Ricciardelli, M. Scodeggio, Y. Taniguchi, D. Thompson. 2007-01-17. The Cosmic Evolution Survey (COSMOS): The morphological content and enviromental dependence of the galaxy color-magnitude relation at z~0.7. https://doi.org/10.1086/516591

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