The zCOSMOS redshift survey: how group environment alters global downsizing trends^*

¹ INAF - Osservatorio Astronomico di Brera, via Brera, 28, 20159 Milano, Italy e-mail: angela.iovino@brera.inaf.it
² Laboratoire d'Astrophysique de Marseille, CNRS-Université d'Aix-Marseille, 38 rue Frederic Joliot Curie, 13388 Marseille, France
³ INAF - IASF Milano, via Bassini 15, 20133, Milano, Italy
⁴ Institute of Astronomy, ETH Zurich, 8093, Zürich, Switzerland
⁵ INAF - Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁶ Laboratoire d'Astrophysique de Toulouse-Tarbes, Université de Toulouse, CNRS, 14 Avenue Edouard Belin, 31400 Toulouse, France
⁷ INAF - Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy
⁸ Max-Planck-Institut für extraterrestrische Physik, 84571 Garching b. Muenchen, 85748, Germany
⁹ European Southern Observatory, Karl-Schwarzschild-Strasse 2, Garching b. Muenchen, 85748, Germany
¹⁰ Dipartimento di Astronomia, Università di Padova, vicolo Osservatorio 3, 35122 Padova, Italy
¹¹ INAF - Osservatorio Astronomico di Torino, Strada Osservatorio 20, 10025 Pino Torinese, Torino, Italy
¹² Dept. of Astronomy, University of Massachusetts, 710 North Pleasant Street, Amherst, MA 01003-9305, USA
¹³ Dipartimento di Astronomia, Universitá di Bologna, via Ranzani 1, 40127, Bologna, Italy
¹⁴ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹⁵ Physics Division, MS 50 R5004, Lawrence Berkeley National Laboratory, 1 Cyclotron Rd., Berkeley, CA 94720, USA
¹⁶ Centre de Physique Theorique, Campus de Luminy, Case 907 – 13288 Marseille, France
¹⁷ Institut d'Astrophysique de Paris, UMR 7095 CNRS, Université Pierre et Marie Curie, 98bis boulevard Arago, 75014 Paris, France
¹⁸ Universitäts-Sternwarte, Scheinerstrasse 1, Munich 81679, Germany
¹⁹ Argelander-Institut für Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany
²⁰ INAF, Osservatorio di Roma via di Frascati, 33, 00040 Monte Porzio Catone, Italy
²¹ Department of Astronomy, Columbia University, 550 West 120th Street, New York, NY 10027, USA
²² California Institute of Technology, MC 105-24, 1200 East California Boulevard, Pasadena, CA 91125, USA

Received: 22 May 2009
Accepted: 8 October 2009

Abstract

Context. Groups of galaxies are a common environment, bridging the gap between starforming field galaxies and quiescent cluster galaxies. Within groups secular processes could be at play, contributing to the observed strong decrease of star formation with cosmic time in the global galaxy population.

Aims. We took advantage of the wealth of information provided by the first ~10 000 galaxies of the zCOSMOS-bright survey and its group catalogue to study in detail the complex interplay between group environment and galaxy properties.

Methods. The classical indicator , i.e., the fraction of blue galaxies, proved to be a simple but powerful diagnostic tool. We studied its variation for different luminosity and mass selected galaxy samples, divided as to define groups/field/isolated galaxy subsamples.

Results. Using rest-frame evolving B-band volume-limited samples, the groups galaxy population exhibits significant blueing as redshift increases, but maintains a systematic difference (a lower ) with respect to the global galaxy population, and an even larger difference with respect to the isolated galaxy population. However moving to mass selected samples it becomes apparent that such differences are largely due to the biased view imposed by the B-band luminosity selection, being driven by the population of lower mass, bright blue galaxies for which we miss the redder, equally low mass, counterparts. By carefully focusing the analysis on narrow mass bins such that mass segregation becomes negligible we find that only for the lowest mass bin explored, i.e., , does a significant residual difference in color remain as a function of environment, while this difference becomes negligible toward higher masses.

Conclusions. Our results indicate that red galaxies of mass are already in place at z ~ 1 and do not exhibit any strong environmental dependence, possibly originating from so-called nature or internal mechanisms. In contrast, for lower galaxy masses and redshifts lower than z ~ 1, we observe the emergence in groups of a population of nurture red galaxies: slightly deviating from the trend of the downsizing scenario followed by the global galaxy population, and more so with cosmic time. These galaxies exhibit signatures of group-related secular physical mechanisms directly influencing galaxy evolution. Our analysis implies that these mechanisms begin to significantly influence galaxy evolution after z ~ 1, a redshift corresponding to the emergence of structures in which these mechanisms take place.