The bimodality of the 10k zCOSMOS-bright galaxies up to z ~ 1: a new statistical and portable classification based on optical galaxy properties⋆
Dipartimento di Astronomia, Università di Bologna, via Ranzani 1, 40138 Bologna, Italy
2 INAF – Osservatorio Astronomico di Bologna, Bologna, Italy
3 INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica, Milano, Italy
4 Institute of Astronomy, ETH Zürich, Zürich, Switzerland
5 Laboratoire d’Astrophysique de Toulouse-Tarbes, Université de Toulouse, CNRS Toulouse, France
6 Laboratoire d’Astrophysique de Marseille, Marseille, France
7 Dipartimento di Astronomia, Università di Padova, Padova, Italy
8 European Southern Observatory, Garching, Germany
9 Max Planck Institut für Extraterrestrische Physik, Garching, Germany
10 INAF – Osservatorio Astronomico di Brera, Milano, Italy
11 California Institute of Technology, Pasadena CA, USA
12 Spitzer Science Center, Pasadena CA, USA
13 Centre de Physique Theorique, Marseille, France
14 Universitäts-Sternwarte, München, Germany
15 INAF – Osservatorio Astronomico di Arcetri, Firenze, Italy
16 INAF – Osservatorio Astronomico di Roma, Monte Porzio Catone, Italy
17 Space Telescope Science Institute, Baltimore, USA
18 INAF – Osservatorio Astronomico di Trieste, Italy
19 Max Planck Institut für Astronomie, Garching, Germany
20 Institute for the Physics and Mathematics of the Universe, University of Tokyo, Kashiwa-shi, Chiba, Japan
21 Instituto de Astrofísica de Andalucía, CSIC, Granada, Spain
22 SUPA – Institute for Astronomy, The University of Edinburgh, Royal Observatory, Edinburgh, UK
Received: 11 November 2010
Accepted: 3 August 2011
Aims. Our goal is to develop a new and reliable statistical method to classify galaxies from large surveys. We probe the reliability of the method by comparing it with a three-dimensional classification cube, using the same set of spectral, photometric and morphological parameters.
Methods. We applied two different methods of classification to a sample of galaxies extracted from the zCOSMOS redshift survey, in the redshift range 0.5 ≲ z ≲ 1.3. The first method is a combination of three independent classification schemes – a spectroscopic one based on the strength of the continuum break at 4000 Å and the rest-frame equivalent width of the [O ii] emission line, a photometric one based on the observed B − z colours, and a morphological one. The second method exploits an entirely new approach based on statistical analyses like principal component analysis (PCA) and unsupervised fuzzy partition (UFP) clustering method. The PCA+UFP method has also been applied to a lower redshift sample (z ≲ 0.5), exploiting the same set of data but replacing the spectroscopic indicators with the equivalent width of Hα.
Results. The comparison between the two methods shows fairly good agreement on the definition on the two main populations, the early-type and the late-type galaxies. Our PCA+UFP method of classification is robust, flexible and capable of identifying the two main populations of galaxies as well as an intermediate population. The intermediate galaxy population shows many of the properties of “green valley” galaxies, and constitutes a more coherent and homogeneous population. The large redshift range of the studied sample allows us to characterize downsizing: galaxies with masses of the order of 3 × 1010 M⊙ are predominantly found in the transition from the late-type to the early-type group at z > 0.5, while galaxies with lower masses, of the order of 1010 M⊙, are in transition at later epochs. Galaxies with M < 1010 M⊙ have not yet begun to transition, while galaxies with very large masses (M > 5 × 1010 M⊙) have mostly completed their transition to the early-type regime before 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, 2011