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

G. Coppa , M. Mignoli , G. Zamorani , S. Bardelli , S. J. Lilly (1) , M. Bolzonella , M. Scodeggio , D. Vergani (2) , P. Nair (3) , L. Pozzetti , Alessandro Cimatti , E. Zucca , C. M. Carollo (4) , T. Contini (5) , O. Le Fèvre (6) , A. Renzini , V. Mainieri (7) , A. Bongiorno (8) , K. Caputi (9) , O. Cucciati (10) , S. de La Torre , L. de Ravel (9) , P. Franzetti , B. Garilli , P. Memeo (9) , A. Iovino (9) , P. Kampczyk (9) , J. -P. Kneib (11) , C. Knobel (9) , A. M. Koekemoer , K. Kovac (9) , F. Lamareille (12) , J. -F. Le Borgne , V. Le Brun (13) , C. Maier (14) , R. Pellò (12) , Y. Peng (15) , E. Perez-Montero (5) , E. Ricciardelli (9) , C. Scarlata (9) , J. D. Silverman (9) , M. Tanaka , L. Tasca , L. Tresse (6) , U. Abbas (9) , D. Bottini (9) , P. Capak (16) , A. Cappi (17) , P. Cassata , M. Fumana (9) , L. Guzzo , A. Leauthaud (18) , D. Maccagni (9) , Christian Marinoni (19) , B. Meneux , P. Oesch (9) , C. Porciani , R. Scaramella , N. Scoville (18)

1 ETH Zürich - Eidgenössische Technische Hochschule - Swiss Federal Institute of Technology [Zürich]
2 OAA - INAF - Osservatorio Astrofisico di Arcetri
3 OABO - INAF - Osservatorio Astronomico di Bologna
4 University of Chicago
5 LATT - Laboratoire Astrophysique de Toulouse-Tarbes
6 LAM - Laboratoire d'Astrophysique de Marseille
7 ESO - European Southern Observatory
8 GATECH - College of Computing
9 AUTRES
10 Dipartimento di Fisica
11 EPFL - Ecole Polytechnique Fédérale de Lausanne
12 IRAP - Institut de recherche en astrophysique et planétologie
13 ISCOD-ENSMSE - Département Informatique pour les Systèmes Coopératifs Ouverts et Décentralisés
14 LLB - UMR 12 - Laboratoire Léon Brillouin
15 HNU - Hunan Normal University
16 Spitzer Science Center
17 LAGRANGE - Joseph Louis LAGRANGE
18 CALTECH - California Institute of Technology
19 CPT - Centre de Physique Théorique - UMR 6207

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 $z\sim 1$.

Domaines: Cosmologie et astrophysique extra-galactique [astro-ph.CO]

G. Coppa, M. Mignoli, G. Zamorani, S. Bardelli, S. J. Lilly, et al.. The bimodality of the 10k zCOSMOS-bright galaxies up to z ~ 1: a new statistical and portable classification based on the optical galaxy properties. Astronomy and Astrophysics - A&A, 2011, 535, pp.10. ⟨10.1051/0004-6361/201016130⟩. ⟨hal-01113766⟩