zCOSMOS 10k-bright spectroscopic sample ^⋆

Exploring mass and environment dependence in early-type galaxies

¹ Dipartimento di AstronomiaUniversitá degli Studi di Bologna, via Ranzani 1, 40127 Bologna, Italy
e-mail: michele.moresco@unibo.it
² INAF – Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
³ Institute of Astronomy, Swiss Federal Institute of Technology (ETH Hönggerberg), 8093 Zürich, Switzerland
⁴ INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica di Milano, via Bassini 15, 20133 Milano, Italy
⁵ Department of Astronomy, University of Massachusetts, 710 North Pleasant Street, Amherst, MA 01003, USA
⁶ Laboratoire d’Astrophysique de Marseille, Université d’Aix-Marseille, CNRS, 38 rue Frederic Joliot-Curie, 13388 Marseille Cedex 13, France
⁷ INAF – Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy
⁸ Laboratoire d’Astrophysique de Toulouse-Tarbes, Université de Toulouse, CNRS, 14 avenue Edouard Belin, 31400 Toulouse, France
⁹ Max Planck Institut für Extraterrestrische Physik, Giessenbachstrasse, 84571 Garching, Germany
¹⁰ Dipartimento di Astronomia, Universitá di Padova, Padova, Italy
¹¹ SUPA Institute for Astronomy, The University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh, EH9 3HJ, UK
¹² INAF – Osservatorio Astronomico di Brera, via Brera 28, 20121 Milano, Italy
¹³ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹⁴ Centre de Physique Thèorique, Marseille, Marseille, France
¹⁵ Institut d’Astrophysique de Paris, Université Pierre & Marie Curie, Paris, France
¹⁶ Universitats Sternwarte, Scheinerstrasse 1, 81679 Muenchen, Germany
¹⁷ INAF – Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monteporzio Catone, Italy
¹⁸ Spitzer Science Center, Pasadena, CA, USA
¹⁹ California Institute of Technology, MC 105-24, 1200 East California Boulevard, Pasadena, CA 91125, USA

Received: 12 January 2010
Accepted: 8 September 2010

Abstract

Aims. We present the analysis of the U − V rest-frame color distribution and some spectral features as a function of mass and environment for a sample of early-type galaxies up to z = 1 extracted from the zCOSMOS spectroscopic survey. This analysis is used to place constraints on the relative importance of these two properties in controlling galaxy evolution. Methods. We used the zCOSMOS 10k-bright sample, limitedto the AB magnitude range 15 < I < 22.5, from which we extracted two different subsamples of early-type galaxies. The first sample (“red galaxies”) was selected using a photometric classification (2098 galaxies), while in the second case (“ETGs”) we combined morphological, photometric, and spectroscopic properties to obtain a more reliable sample of elliptical, red, passive, early-type galaxies (981 galaxies). The analysis is performed at fixed mass to search for any dependence of the color distribution on environment, and at fixed environment to search for any mass dependence. Results. In agreement with the low redshift results of the SDSS, we find that the color distribution of red galaxies is not strongly dependent on environment for all mass bins, exhibiting only a weak trend such that galaxies in overdense regions (log ₁₀(1 + δ) ~ 1.2) are redder than galaxies in underdense regions (log ₁₀(1 + δ) ~ 0.1), with a difference of ⟨Δ(U − V)_rest⟩ = 0.027 ± 0.008 mag. On the other hand, the dependence on mass is far more significant, and we find that the average colors of massive galaxies (log ₁₀(M/M_⊙) ~ 10.8) are redder by ⟨Δ(U − V)_rest⟩ = 0.093 ± 0.007 mag than low-mass galaxies (log ₁₀(M/M_⊙) ~ 10) throughout the entire redshift range. We study the color-mass (U − V)_rest ∝ S_M·log ₁₀(M/M_⊙) relation, finding a mean slope ⟨S_M⟩ = 0.12 ± 0.005, while the color-environment (U − V)_rest ∝ S_δ·log ₁₀(1 + δ) relation is flatter, with a slope always smaller than S_δ ≈ 0.04. The spectral analysis that we perform on our ETGs sample is in good agreement with our photometric results: we study the 4000 Å break and the equivalent width of the Hδ Balmer line, finding for D4000 a dependence on mass (⟨ΔD4000⟩ = 0.11 ± 0.02 between log ₁₀(M/M_⊙) ~ 10.2 and log ₁₀(M/M_⊙) ~ 10.8), and a much weaker dependence on environment (⟨ΔD4000⟩ = 0.05 ± 0.02 between high and low environment quartiles). The same is true for the equivalent width of Hδ, for which we measure a difference of ΔEW₀(Hδ) = 0.28 ± 0.08 Å across the same mass range and no significant dependence on environment. By analyzing the lookback time of early-type galaxies, we support the possibility of a downsizing scenario, in which massive galaxies with a stronger D4000 and an almost constant equivalent width of Hδ formed their mass at higher redshift than lower mass ones. We also conclude that the main driver of galaxy evolution is the galaxy mass, the environment playing a subdominant role.