The bimodality of the 10k zCOSMOS-bright galaxies up to z  ~  1: a new statistical and portable classification based on optical galaxy properties^⋆

¹ Dipartimento di Astronomia, Università di Bologna, via Ranzani 1, 40138 Bologna, Italy
e-mail: graziano.coppa@studio.unibo.it
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

Abstract

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.