The bimodality of the 10k zCOSMOS-bright galaxies up to z ~ 1: a new
  statistical and portable classification based on the optical galaxy
  properties
  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.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 the
combination of three independent classification schemes, while 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 been applied also to a lower redshift
sample (z < 0.5), exploiting the same set of data but the spectral ones,
replaced by the equivalent width of H$\alpha$. The comparison between the two
methods shows fairly good agreement on the definition on the two main clusters,
the early-type and the late-type galaxies ones. Our PCA-UFP method of
classification is robust, flexible and capable of identifying the two main
populations of galaxies as well as the intermediate population. The
intermediate galaxy population shows many of the properties of the green valley
galaxies, and constitutes a more coherent and homogeneous population. The
fairly large redshift range of the studied sample allows us to behold the
downsizing effect: galaxies with masses of the order of $3\cdot 10^{10}$ Msun
mainly are found in transition from the late type to the early type group at
$z>0.5$, while galaxies with lower masses - of the order of $10^{10}$ Msun -
are in transition at later epochs; galaxies with $M <10^{10}$ Msun did not
begin their transition yet, while galaxies with very large masses ($M > 5\cdot
10^{10}$ Msun) mostly completed their transition before $z\sim 1$.
