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

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 × 10^(10) 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 10^(10) M_⊙, are in transition at later epochs. Galaxies with M < 10^(10) M_⊙ have not yet begun to transition, while galaxies with very large masses (M > 5 × 10^(10 M_⊙) have mostly completed their transition to the early-type regime before z ~ 1.

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