The zCOSMOS redshift survey: the three-dimensional classification cube and bimodality in galaxy physical properties *
INAF – Osservatorio Astronomico di Bologna, Bologna, Italy e-mail: firstname.lastname@example.org
2 INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica, Milano, Italy
3 Dipartimento di Astronomia, Università degli Studi di Bologna, Bologna, Italy
4 INAF – Osservatorio Astronomico di Arcetri, Firenze, Italy
5 Institute of Astronomy, ETH Zurich, Zurich, Switzerland
6 Laboratoire d'Astrophysique de Toulouse-Tarbes, Université de Toulouse, CNRS Toulouse, France
7 Laboratoire d'Astrophysique de Marseille, Marseille, France
8 European Southern Observatory, Garching, Germany
9 Dipartimento di Astronomia, Università di Padova, Padova, Italy
10 Max Planck Institut für Extraterrestrische Physik, Garching, Germany
11 INAF – Osservatorio Astronomico di Brera, Milano, Italy
12 California Institute of Technology, Pasadena, CA, USA
13 Centre de Physique Theorique, Marseille, Marseille, France
14 Institut d'Astrophysique de Paris, Université Pierre & Marie Curie, Paris, France
15 INAF – Osservatorio Astronomico di Roma, Monte Porzio Catone, Italy
Accepted: 6 October 2008
Aims. We investigate the relationships between three main optical galaxy observables (spectral properties, colors, and morphology), exploiting the data set provided by the COSMOS/zCOSMOS survey. The purpose of this paper is to define a simple galaxy classification cube, with a carefully selected sample of ≈1000 galaxies.
Methods. Using medium resolution spectra of the first zCOSMOS-bright sample, optical photometry from the Subaru/COSMOS observations, and morphological measurements derived from ACS imaging, we analyze the properties of the galaxy population out to z ~ 1. Applying three straightforward classification schemes (spectral, photometric, and morphological), we identify two main galaxy types, which appear to be linked to the bimodality of galaxy population. The three parametric classifications constitute the axes of a “classification cube”.
Results. A very good agreement exists between the classification from spectral data (quiescent/star-forming galaxies) and the one based on colors (red/blue galaxies). The third parameter (morphology) is not as well correlated with the first two; in fact, a good correlation between the spectral classification and the classification based on morphological analysis (early-/late-type galaxies) is achieved only after partially complementing the morphological classification with additional color information. Finally, analyzing the 3D-distribution of all galaxies in the sample, we find that about 85% of the galaxies show a fully concordant classification, being either quiescent, red, bulge-dominated galaxies (~20%) or star-forming, blue, disk-dominated galaxies (~65%). These results imply that the galaxy bimodality is a consistent behavior both in morphology, color, and dominant stellar population, at least out to z ~ 1.
Key words: galaxies: general / galaxies: evolution / galaxies: fundamental parameters
Based on observations undertaken at the European Southern Observatory (ESO) Very Large Telescope (VLT) under Large Program 175.A-0839. Also based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, operated by AURA Inc., under NASA contract NAS 5-26555, with the Subaru Telescope, operated by the National Astronomical Observatory of Japan, with the telescopes of the National Optical Astronomy Observatory, operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under cooperative agreement with the National Science Foundation, and with the Canada-France-Hawaii Telescope, operated by the National Research Council of Canada, the Centre National de la Recherche Scientifique de France, and the University of Hawaii.
© ESO, 2008