Properties and environment of radio-emitting galaxies in the VLA-zCOSMOS survey^*

¹ INAF – Osservatorio Astronomico di Bologna – via Ranzani 1, 40127 Bologna, Italy e-mail: sandro.bardelli@oabo.inaf.it
² Max Planck Institut für Astronomie, Königstuhl 17, Heidelberg 69117, Germany
³ California Institute of Technology, MC 105-24, 1200 East California Boulevard, Pasadena, CA 91125, USA
⁴ INAF – Osservatorio Astronomico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁵ Institute of Astronomy, Swiss Federal Institute of Technology (ETH Hönggerberg), 8093 Zürich, Switzerland
⁶ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁷ Max-Planck-Institut für extraterrestrische Physik, 84571 Garching, Germany
⁸ INAF-Istituto di Radioastronomia, via Gobetti 101, 40129 Bologna, Italy
⁹ Laboratoire d'Astrophysique de Toulouse-Tarbes, Université de Toulouse, CNRS, 14 avenue Édouard Belin, 31400 Toulouse, France
¹⁰ Laboratoire d'Astrophysique de Marseille, Université d'Aix-Marseille, CNRS, 38 rue Frederic Joliot-Curie, 13388 Marseille, France
¹¹ European Southern Observatory, Karl-Schwarzschild-Strasse 2, Garching 85748, Germany
¹² Dipartimento di Astronomia, Università di Padova, Vicolo Osservatorio 5, 35122 Padova, Italy
¹³ INAF – IASF Milano, via Bassini 15, 20133 Milan, Italy
¹⁴ INAF – Osservatorio Astronomico di Brera, via Brera 28, 20121 Milan, Italy
¹⁵ Instituto de Astrofisica de Andalucia, CSIC, Apdo. 3004, 18080 Granada, Spain
¹⁶ INAF – Osservatorio Astronomico di Torino, strada Osservatorio 20, 10025 Pino Torinese, Italy
¹⁷ Departement of Astronomy, University of Massachussetts, 710 North Pleasent Street, Amherst, MA 01003, USA
¹⁸ Dipartimento di Astronomia, Università di Bologna, via Ranzani 1, 40127 Bologna, Italy
¹⁹ LBNL & BCCP, University of California, Berkeley, CA, 94720, USA
²⁰ Centre de Physique Theorique, Marseille, France
²¹ Institut d'Astrophysique de Paris, UMR 7095 CNRS, Université Pierre et Marie Curie, 98 bis Boulevard Arago, 75014 Paris, France
²² Universitats-Sternwarte, Scheinerstrasse 1, 81679 Muenchen, Germany
²³ Argelander Institut für Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany
²⁴ INAF – Osservatorio di Roma, via di Frascati 33, 00040 Monteporzio Catone (RM), Italy
²⁵ Observatories of the Carnegie Institute of Washington, Santa Barbara Street, Pasadena, CA 91101, USA
²⁶ Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, HI 96822, USA
²⁷ Research Center for Space and Cosmic Evolution, Ehime University, Bunkyo-cho 2-5, Matsuyama 790-8577, Japan
²⁸ Max-Planck-Institut für Astrophysics, 84571 Garching, Germany

Received: 1 July 2009
Accepted: 22 October 2009

Abstract

Aims. We investigate the properties and the environment of radio sources with optical counterparts from the combined VLA-COSMOS and zCOSMOS samples. The advantage of this sample is the availability of optical spectroscopic informations, high quality redshifts, and accurate density determination.

Methods. By comparing the star formation rates estimated from the optical spectral energy distribution with those based on the radio luminosity, we divide the radio sources in to three families passive AGN, non-passive AGN, and star-forming galaxies. These families occupy specific regions of the 8.0-4.5 μm infrared color- specific star-formation plane, from which we extract the corresponding control samples.

Results. Only the passive AGN have a significantly different environmental distribution from their control sample. The fraction of radio-loud passive AGN increases from ~2% in underdense regions to ~15% for overdensities (1+δ) greater than 10. This trend is also present as a function of richness of the groups hosting the radio sources. Passive AGN in overdensities tend to have higher radio luminosities than those in lower density environments. Since the black hole mass distribution is similar in both environments, we speculate that, for low radio luminosities, the radio emission is controlled (by fuel availability or confinement of the radio jet by local gas pressure) by the interstellar medium of the host galaxy, while in other cases it is determined by the structure (group or cluster) in which the galaxy resides.