Evolution of the early-type galaxy fraction in clusters since z = 0.8 ^*,**

¹ National Research Council of Canada, Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, British Columbia, Canada
² Ohio University, Department of Physics and Astronomy, Clippinger Lab 251B, Athens, OH 45701, USA
³ California Institute of Technology, MS 320-47, Pasadena, CA 91125, USA
⁴ University of Washington, Department of Astronomy, Box 351580, Seattle, WA 98195-1580, USA
⁵ Kavli Institute for Particle Astrophysics and Cosmology, PO Box 20450, MS 29, Stanford, CA 94309, USA
⁶ Max-Planck-Institut für Astrophysik, Karl-Schwarschild-Str. 1, Postfach 1317, 85741 Garching, Germany
⁷ INAF – Astronomical Observatory of Padova, Italy
⁸ School of Physics and Astronomy, University of Nottingham, Nottingham, NG7 2RD, UK
⁹ INAF – Astronomical Observatory of Trieste, via Tiepolo 11, 34143 Trieste, Italy
¹⁰ INAF – Osservatorio Astronomico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy
¹¹ Observatoire de l'Université de Genève, Laboratoire d'Astrophysique de l'École Polytechnique Fédérale de Lausanne (EPFL), 1290 Sauverny, Switzerland
¹² Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
¹³ The Royal Library/Copenhagen University Library, Research Department, Box 2149, 1016 Copenhagen K, Denmark
¹⁴ Max-Planck Institut für extraterrestrische Physik, Giessenbachstrasse 85748 Garching, Germany
¹⁵ Laboratoire d'Astrophysique de Toulouse-Tarbes, CNRS, Université de Toulouse, 14 avenue Édouard Belin, 31400 Toulouse, France
¹⁶ The University of Kansas, Department of Physics and Astronomy, Malott room 1082, 1251 Wescoe Hall Drive, Lawrence, KS, 66045, USA
¹⁷ NOAO, 950 North Cherry Avenue, Tucson, AZ 85719, USA
¹⁸ Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ, 85721, USA

Received: 18 October 2007
Accepted: 6 October 2009

Abstract

We study the morphological content of a large sample of high-redshift clusters to determine its dependence on cluster mass and redshift. Quantitative morphologies are based on PSF-convolved, 2D bulge+disk decompositions of cluster and field galaxies on deep Very Large Telescope FORS2 images of eighteen, optically-selected galaxy clusters at observed as part of the ESO Distant Cluster Survey (“EDisCS”). Morphological content is characterized by the early-type galaxy fraction f_et, and early-type galaxies are objectively selected based on their bulge fraction and image smoothness. This quantitative selection is equivalent to selecting galaxies visually classified as E or S0. Changes in early-type fractions as a function of cluster velocity dispersion, redshift and star-formation activity are studied. A set of 158 clusters extracted from the Sloan Digital Sky Survey is analyzed exactly as the distant EDisCS sample to provide a robust local comparison. We also compare our results to a set of clusters from the Millennium Simulation. Our main results are: (1) the early-type fractions of the SDSS and EDisCS clusters exhibit no clear trend as a function of cluster velocity dispersion. (2) Mid-z EDisCS clusters around σ = 500 km s^-1 have f_et  0.5 whereas high-z EDisCS clusters have f_et  0.4. This represents a ~25% increase over a time interval of 2 Gyr. (3) There is a marked difference in the morphological content of EDisCS and SDSS clusters. None of the EDisCS clusters have early-type galaxy fractions greater than 0.6 whereas half of the SDSS clusters lie above this value. This difference is seen in clusters of all velocity dispersions. (4) There is a strong and clear correlation between morphology and star formation activity in SDSS and EDisCS clusters in the sense that decreasing fractions of [OII] emitters are tracked by increasing early-type fractions. This correlation holds independent of cluster velocity dispersion and redshift even though the fraction of [OII] emitters decreases from to in all environments. Our results pose an interesting challenge to structural transformation and star formation quenching processes that strongly depend on the global cluster environment (e.g., a dense ICM) and suggest that cluster membership may be of lesser importance than other variables in determining galaxy properties.