Investigating the relationship between AGN activity and stellar mass in zCOSMOS galaxies at 0 < z < 1 using emission-line diagnostic diagrams^⋆

¹ I. Physikalisches Institut, Universität zu Köln, Zülpicher Strasse 77, 50937 Köln, Germany
e-mail: vitale@ph1.uni-koeln.de
² Max-Planck Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³ Dipartimento di Astronomia, Università di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁴ INAF – Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁵ Institute for Astronomy, ETH Zurich, 8093 Zurich, Switzerland
⁶ Institut de Recherche en Astrophysique et Planétologie, CNRS, 14 avenue Édouard Belin, 31400 Toulouse, France
⁷ IRAP, Université de Toulouse, UPS-OMP, 31400 Toulouse, France
⁸ Aix-Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France
⁹ European Southern Observatory, 85748 Garching, Germany
¹⁰ INAF – Osservatorio Astronomico di Padova, Vicolo dell Osservatorio 5, 35122 Padova, Italy
¹¹ INAF – IASF Milano, 20133 Milano, Italy
¹² Max-Planck Institut für Extraterrestrische Physik, 85748 Garching, Germany
¹³ INAF Osservatorio Astronomico di Brera, 20133 Milan, Italy
¹⁴ Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh, EH93 HJ, UK
¹⁵ Instituto de Astrofisica de Andalucia, CSIC, Apartado de Correos 3004, 18080 Granada, Spain
¹⁶ Kavli Institute for the Physics and Mathematics of the Universe (WPI), Todai Institutes for Advanced Study, The University of Tokyo, 5-1-5 Kashiwanoha, 277-8583 Kashiwa, Japan
¹⁷ Space Telescope Science Institute, Baltimore, MD 21218, USA
¹⁸ Institut d’Astrophysique de Paris, UMR 7095 CNRS, Université Pierre & Marie Curie, 75014 Paris, France
¹⁹ Institut d’Astrophysique Spatiale, Bâtiment 121, Université Paris-Sud XI & CNRS, 91405 Orsay Cedex, France
²⁰ Department of Astronomy, University of Vienna, Tuerkenschanzstrasse 17, 1180 Vienna, Austria
²¹ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
²² UC Santa Cruz/UCO Lick Observatory, 1156 High Street, Santa Cruz, CA 95064, USA
²³ INAF – IASF Bologna, via P. Gobetti 101, 40129 Bologna, Italy

Received: 19 August 2012
Accepted: 13 February 2013

Abstract

Context. Active galactic nuclei (AGN) are thought to play an important role in galaxy evolution. It has been suggested that AGN feedback could be partly responsible for quenching star-formation in the hosts, leading to transition from the blue cloud to the red sequence. The transition seems to occur faster for the most massive galaxies, where traces of AGN activity have been found as early as at z < 0.1. The correlation betweenAGN activity, aging of the stellar populations, and stellar mass still needs to be fully understood, especially at high redshifts.

Aims. Our aim is to investigate the link between AGN activity, star-formation, and stellar mass of the host galaxy at 0 < z < 1, looking for spectroscopic traces of AGN and aging of the host. This work provides an extension of the existing studies at z < 0.1 and contributes to shed light on galaxy evolution at intermediate redshifts.

Methods. We used the zCOSMOS 20k data to create a sample of galaxies at z < 1. We divided the sample into several mass-redshift bins to obtain stacked galaxy spectra with an improved signal-to-noise ratio (S/N). We exploited emission-line diagnostic diagrams to separate AGN from star-forming galaxies.

Results. We found an indication of a role for the total galaxy stellar mass in leading galaxy classification. Stacked spectra show AGN signatures above the log M_∗/M_⊙ > 10.2 threshold. Moreover, the stellar populations of AGN hosts are found to be older than star-forming and composite galaxies. This could be due to the the tendency of AGN to reside in massive hosts.

Conclusions. The dependence of the AGN classification on the stellar mass agrees with what has been found in previous research. Together with the evidence of older stellar populations inhabiting the AGN-like galaxies, it is consistent with the downsizing scenario. In particular, our evidence points to an evolutionary scenario where the AGN-feedback is capable of quenching the star formation in the most massive galaxies. Therefore, the AGN-feedback is the best candidate for initiating the passive evolutionary phase of galaxies.