A journey from the outskirts to the cores of groups

I. Color- and mass-segregation in 20K-zCOSMOS groups^⋆

¹ Universitá degli Studi dell’Insubria, via Valleggio 11, 22100 Como, Italy
e-mail: valentina.presotto@brera.inaf.it
² INAF – Osservatorio Astronomico di Brera, via Brera, 28, 20159 Milano, Italy
³ INAF – IASF Milano, via Bassini 15, 20133 Milano, Italy
⁴ INAF – Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy
⁵ Institute of Astronomy, ETH Zurich, 8093 Zürich, Switzerland
⁶ INAF – Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁷ UCO/Lick Observatory, Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064, USA
⁸ Max-Planck-Institut für extraterrestrische Physik, 84571 Garching b, 85748 Muenchen, Germany
⁹ IPMU, Institute for the Physics and Mathematics of the Universe, 5-1-5 Kashiwanoha, 277-8583 Kashiwa, Japan
¹⁰ MPA – Max Planck Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85741 Garching, Germany
¹¹ Institut de Recherche en Astrophysique et Planétologie, CNRS, 14, avenue Edouard Belin, 31400 Toulouse, France
¹² IRAP, Université de Toulouse, UPS-OMP, Toulouse, France
¹³ Laboratoire d’Astrophysique de Marseille, CNRS-Université d’Aix-Marseille, 38 rue Frederic Joliot Curie, 13388 Marseille, France
¹⁴ European Southern Observatory, Karl-Schwarzschild-Strasse 2, Garching b. Muenchen, 85748, Germany
¹⁵ Dipartimento di Astronomia, Università di Padova, vicolo Osservatorio 3, 35122 Padova, Italy
¹⁶ SUPA Institute for Astronomy, The University of Edinburgh, Royal Observatory, Edinburgh, EH9 3HJ, UK
¹⁷ University of Vienna, Department of Astronomy, Tuerkenschanzstrasse 17, 1180 Vienna, Austria
¹⁸ Instituto de Astrofisìca de Andalucìa, CSIC, Apartado de correos 3004, 18080 Granada, Spain
¹⁹ Instituto de Astrofsica de Canarias, Vía Lactea s/n, 38200 La Laguna, Tenerife, Spain
²⁰ Dipartimento di Astronomia, Universitá di Bologna, via Ranzani 1, 40127 Bologna, Italy
²¹ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
²² Institut d’Astrophysique de Paris, UMR 7095 CNRS, Université Pierre et Marie Curie, 98bis boulevard Arago, 75014 Paris, France
²³ Institut d’Astrophysique Spatiale, Batiment 121, CNRS & Univ. Paris Sud XI, 91405 Orsay Cedex, France
²⁴ INAF – IASF Bologna, via P. Gobetti 101, 40129 Bologna, Italy

Received: 18 October 2011
Accepted: 22 December 2011

Abstract

Context. Studying theevolution of galaxies located within groups may have important implications for our understanding of the global evolution of the galaxy population as a whole. The fraction of galaxies bound in groups at z ~ 0 is as high as 60% and therefore any mechanism (among the many suggested) that could quench star formation when a galaxy enters group environment would be an important driver for galaxy evolution.

Aims. Using the group catalog obtained from zCOSMOS spectroscopic data and the complementary photometric data from the COSMOS survey, we explore segregation effects occurring in groups of galaxies at intermediate/high redshifts. Our aim is to reveal if, and how significantly, group environment affects the evolution of infalling galaxies.

Methods. We built two composite groups at intermediate (0.2 ≤ z ≤ 0.45) and high (0.45 < z ≤ 0.8) redshifts, and we divided the corresponding composite group galaxies into three samples according to their distance from the group center. The samples roughly correspond to galaxies located in a group’s inner core, intermediate, and infall region. We explored how galaxy stellar masses and colors – working in narrow bins of stellar masses – vary as a function of the galaxy distance from the group center.

Results. We found that the most massive galaxies in our sample (log (ℳ_gal/ℳ_⊙) ≥ 10.6) do not display any strong group-centric dependence of the fractions of red/blue objects. For galaxies of lower masses (9.8 ≤ log (ℳ_gal/ℳ_⊙) ≤ 10.6) there is a radial dependence in the changing mix of red and blue galaxies. This dependence is most evident in poor groups, whereas richer groups do not display any obvious trend of the blue fraction. Interestingly, mass segregation shows the opposite behavior: it is visible only in rich groups, while poorer groups have a a constant mix of galaxy stellar masses as a function of radius.

Conclusions. These findings can be explained in a simple scenario where color- and mass-segregation originate from different physical processes. While dynamical friction is the obvious cause for establishing mass segregation, both starvation and galaxy-galaxy collisions are plausible mechanisms to quench star formation in groups at a faster rate than in the field. In poorer groups the environmental effects are caught in action superimposed to secular galaxy evolution. Their member galaxies display increasing blue fractions when moving from the group center to more external regions, presumably reflecting the recent accretion history of these groups.