Enhanced star formation rates in AGN hosts with respect to inactive galaxies from PEP-Herschel observations^⋆,⋆⋆

¹ Max-Planck-Institut für Extraterrestrische Physik (MPE), Postfach 1312, 85741 Garching, Germany
e-mail: paola.santini@oa-roma.inaf.it
² INAF – Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monte Porzio Catone, Italy
³ Department of Physics, Durham University, South Road, Durham, DH1 3LE, UK
⁴ European Space Astronomy Centre, Villafranca del Castillo, Spain
⁵ ESO, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
⁶ INAF – Osservatorio Astronomico di Trieste, via Tiepolo 11, 34131 Trieste, Italy
⁷ Laboratoire AIM, CEA/DSM-CNRS-Université Paris Diderot, IRFU/Service d’Astrophysique, Bât. 709, CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France
⁸ Pontificia Universidad Católica de Chile, Departamento de Astronomía y Astrofísica, Casilla 306, Santiago 22, Chile
⁹ Instituto de Astrofísica de Canarias, 38205 La Laguna, Spain
¹⁰ Departamento de Astrofísica, Universidad de La Laguna, Spain
¹¹ Department of Astronomy and Astrophysics, 525 Davey Lab, Pennsylvania State University, University Park, PA 16802, USA
¹² Dipartimento di Astronomia, Università di Bologna, via Ranzani 1, 40127 Bologna, Italy
¹³ Dipartimento di Astronomia, Università di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
¹⁴ IPP-Max-Planck-Institut für Plasmaphysik, Boltzmannstrasse 2, 85748 Garching, Germany

Received: 13 October 2011
Accepted: 8 February 2012

Abstract

We compare the average star formation (SF) activity in X-ray selected AGN hosts with a mass-matched control sample of inactive galaxies, including both star forming and quiescent sources, in the 0.5 < z < 2.5 redshift range. Recent observations carried out by PACS, the 60−210 μm photometric camera on board the Herschel Space Observatory, in GOODS-S, GOODS-N and COSMOS allow us to obtain an unbiased estimate of the far-IR luminosity, and hence of the SF properties, of the two samples. Accurate AGN host stellar mass estimates are obtained by decomposing their total emission into the stellar and the nuclear components. We report evidence of a higher average SF activity in AGN hosts with respect to the control sample of inactive galaxies. The level of SF enhancement is modest (~0.26 dex at  ~3σ confidence level) at low X-ray luminosities (L_X ≲ 10^43.5 erg s^-1) and more pronounced (0.56 dex at  > 10σ confidence level) in the hosts of luminous AGNs. However, when comparing to star forming galaxies only, AGN hosts are found broadly consistent with the locus of their “main sequence”. We investigate the relative far-IR luminosity distributions of active and inactive galaxies, and find a higher fraction of PACS detected, hence normal and highly star forming systems among AGN hosts. Although different interpretations are possible, we explain our findings as a consequence of a twofold AGN growth path: faint AGNs evolve through secular processes, with instantaneous AGNaccretion not tightly linked to the current total SF in the host galaxy, while the luminous AGNs co-evolve with their hosts through periods of enhanced AGN activity and star formation, possibly through major mergers. While an increased SF activity with respect to inactive galaxies of similar mass is expected in the latter, we interpret the modest SF offsets measured in low-L_X AGN hosts as either a) generated by non-synchronous accretion and SF histories in a merger scenario or b) due to possible connections between instantaneous SF and accretion that can be induced by smaller scale (non-major merger) mechanisms. Far-IR luminosity distributions favour the latter scenario.