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Impact of different selection criteria on observed properties of passive galaxies in zCOSMOS-20k^⋆ sample

¹ Dipartimento di Fisica e AstronomiaUniversitá degli Studi di Bologna, V.le Berti Pichat, 6/2, 40127 Bologna, Italy
e-mail: michele.moresco@unibo.it
² INAF – Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
³ Institut de Recherche en Astrophysique et Planétologie, CNRS, 31400 Toulouse, France
⁴ Institute for Astronomy, ETH Zurich, 8093 Zurich, Switzerland
⁵ IRAP, Université de Toulouse, UPS-OMP, 31000 Toulouse, France
⁶ Laboratoire d’Astrophysique de Marseille, Aix-Marseille Université, CNRS, 13388 Marseille, France
⁷ European Southern Observatory, 85748 Garching, Germany
⁸ INAF – Osservatorio Astronomico di Padova, 35122 Padova, Italy
⁹ INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica, 20162 Milano, Italy
¹⁰ INAF – Osservatorio Astronomico di Roma, 00040 Monteporzio Catone, Italy
¹¹ Institute for Astronomy, The University of Edinburgh, Royal Observatory, Edinburgh, EH93 HJ, UK
¹² INAF – Osservatorio Astronomico di Brera, Milano, Italy
¹³ Space Telescope Science Institute, Baltimore, MD 21218, USA
¹⁴ Kavli Institute for the Physics and Mathematics of the Universe (WPI), Todai Institutes for Advanced Study, The University of Tokyo, 5-1-5 Kashiwanoha, 277-8583 Kashiwa, Japan
¹⁵ INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica, Bologna, Italy
¹⁶ Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
¹⁷ Institut d’Astrophysique de Paris, Université Pierre & Marie Curie, 75014 Paris, France
¹⁸ UC Santa Cruz/UCO Lick Observatory, 1156 High Street, Santa Cruz, CA 95064, USA
¹⁹ Minnesota Institute for Astrophysics, School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455, USA
²⁰ California Institute of Technology, MC 249-17, 1200 East California Boulevard, Pasadena, CA 91125, USA
²¹ Institut d’Astrophysique Spatiale, Bâtiment 121, CNRS & Univ. Paris Sud XI, 91405 Orsay Cedex, France
²² Centro de Estudios de Física del Cosmos de Aragoón, Plaza San Juan 1, planta 2, 44001 Teruel, Spain
²³ Kapteyn Astronomical Institute, University of Groningen, P.O. Box 800, 9700 AV Groningen, The Netherlands
²⁴ University of Vienna, Department of Astrophysics, Tuerkenschanzstrasse 17, 1180 Vienna, Austria

Received: 30 April 2013
Accepted: 19 July 2013

Abstract

Aims. We present the analysis of photometric, spectroscopic, and morphological properties for differently selected samples of passive galaxies up to z = 1 extracted from the zCOSMOS-20k spectroscopic survey. This analysis intends toexplore the dependence of galaxy properties on the selection criterion adopted, study the degree of contamination due to star-forming outliers, and provide a comparison between different commonly used selection criteria. This work is a first step to fully investigating the selection effects of passive galaxies for future massive surveys such as Euclid.

Methods. We extracted from the zCOSMOS-20k catalog six different samples of passive galaxies, based on morphology (3336 “morphological” early-type galaxies), optical colors (4889 “red-sequence” galaxies and 4882 “red UVJ” galaxies), specific star-formation rate (2937 “quiescent” galaxies), a best fit to the observed spectral energy distribution (2603 “red SED” galaxies), and a criterion that combines morphological, spectroscopic, and photometric information (1530 “red & passive early-type galaxies”). For all the samples, we studied optical and infrared colors, morphological properties, specific star-formation rates (SFRs), and the equivalent widths of the residual emission lines; this analysis was performed as a function of redshift and stellar mass to inspect further possible dependencies.

Results. We find that each passive galaxy sample displays a certain level of contamination due to blue/star-forming/nonpassive outliers. The morphological sample is the one that presents the higher percentage of contamination, with ~12−65% (depending on the mass range) of galaxies not located in the red sequence, ~25−80% of galaxies with a specific SFR up to ~25 times higher than the adopted definition of passive, and significant emission lines found in the median stacked spectra, at least for log (M/M_⊙) < 10.25. The red & passive ETGs sample is the purest, with a percentage of contamination in color <10% for stellar masses log (M/M_⊙) > 10.25, very limited tails in sSFR, a median value ~20% higher than the chosen passive cut, and equivalent widths of emission lines mostly compatible with no star-formation activity. However, it is also the less economic criterion in terms of information used. Among the other criteria, we found that the best performing are the red SED and the quiescent ones, providing a percentage of contamination only slightly higher than the red & passive ETGs criterion (on average of a factor of ~2) but with absolute values of the properties of contaminants still compatible with a red, passively evolving population. We also find a strong dependence of the contamination on the stellar mass and conclude that, almost irrespective of the adopted selection criteria, a cut at log (M/M_⊙) > 10.75 provides a significantly purer sample in terms of star-forming contaminants. By studying the restframe color-mass and color−color diagrams, we provided two revised definitions of passive galaxies based on these criteria that better reproduce the observed bimodality in the properties of zCOSMOS-20k galaxies. The analysis of the number densities of the various samples shows evidences of mass-assembly “downsizing”, with galaxies at 10.25 < log  (M/M_⊙) < 10.75 increasing their number by a factor ~2−4 from z = 0.6 to z = 0.2, by a factor ~2−3 from z = 1 to z = 0.2 at 10.75 < log  (M/M_⊙) < 11, and by only ~10−50% from z = 1 to z = 0.2 at 11 < log  (M/M_⊙) < 11.5.