GOODS-Herschel: ultra-deep XMM-Newton observations reveal AGN/star-formation connection^⋆,⋆⋆

¹ INAF-Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
e-mail: emmanouil.rovilos@durham.ac.uk
² Department of Physics, Durham University, South Road, Durham, DH1, 3 LE, UK
³ Institute of Astronomy & Astrophysics, National Observatory of Athens, Palaia Penteli, 15236 Athens, Greece
⁴ Dipartimento di Astronomia, Università di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁵ Max Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁶ Laboratoire AIM, CEA/DSM-CNRS-Université Paris Diderot, IRFU/Service d’Astrophysique, Bât. 709, CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France
⁷ National Optical Astronomy Observatory, 950 North Cherry Avenue, Tucson, AZ 85719, USA
⁸ Smithsonian Astrophysical Observatory, 60 Garden Street, Cambridge, MA 02138, USA
⁹ Department of Physics and Institute of Theoretical & Computational Physics, University of Crete, 71003 Heraklion, Greece
¹⁰ IESL/Foundation for Research & Technology-Hellas, 71110 Heraklion, Greece
¹¹ Chercheur Associé, Observatoire de Paris, 75014 Paris, France
¹² UK Astronomy Technology Centre, Science and Technology Facilities Council, Royal Observatory, Blackford Hill, Edinburgh, EH9 3HJ, UK
¹³ Institute for Astronomy, University of Edinburgh, Blackford Hill, Edinburgh, EH9 3HJ, UK
¹⁴ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, 85471 Garching bei München, Germany
¹⁵ Instituto de Física de Cantabria (CSIC-Universidad de Cantabria), Avenida de los Castros, 39005 Santander, Spain
¹⁶ Department of Astronomy and Astrophysics, Pennsylvania State University, 525 Davey Laboratory, University Park, PA 16802, USA
¹⁷ Department of Physics and Astronomy, University of British Columbia, Vancouver, BC V6T 1Z1, Canada
¹⁸ Institute for Astronomy, University of Hawaii, Manoa, HI 96822, USA
¹⁹ Canada-France-Hawaii Telescope Corp., Kamuela, HI 96743, USA
²⁰ Observatories of the Carnegie Institution for Science, 813 Santa Barbara Street, Pasadena, CA 91101, USA
²¹ Herschel Science Centre, European Space Astronomy Centre, Villanueva de la Cañada, 28691 Madrid, Spain
²² Universität Wien, Institut für Astronomie, Türkenschanzstraße 17, 1180 Wien, Austria
²³ Astronomy Department, Universidad de Concepción, Concepción, Chile
²⁴ Department of Physics, University of Oxford, Keble Road, Oxford, OX1 3RH, UK

Received: 2 February 2012
Accepted: 30 July 2012

Abstract

Models of galaxy evolution assume some connection between the AGN and star formation activity in galaxies. We use the multi-wavelength information of the CDFS to assess this issue. We select the AGNs from the 3 Ms XMM-Newton survey and measure the star-formation rates of their hosts using data that probe rest-frame wavelengths longward of 20   μm, predominantly from deep 100   μm and 160   μm Herschel observations, but also from Spitzer-MIPS-70   μm. Star-formation rates are obtained from spectral energy distribution fits, identifying and subtracting an AGN component. Our sample consists of sources in the z ≈ 0.5−4 redshift range, with star-formation rates SFR ≈ 10¹−10³   M_⊙   yr^-1 and stellar masses M_⋆ ≈ 10¹⁰−10^11.5   M_⊙. We divide the star-formation rates by the stellar masses of the hosts to derive specific star-formation rates (sSFR) and find evidence for a positive correlation between the AGN activity (proxied by the X-ray luminosity) and the sSFR for themost active systems with X-ray luminosities exceeding L_x ≃ 10⁴³   erg   s^-1 and redshifts z ≳ 1. We do not find evidence for such a correlation for lower luminosity systems or those at lower redshifts, consistent with previous studies. We do not find any correlation between the SFR (or the sSFR) and the X-ray absorption derived from high-quality XMM-Newton spectra either, showing that the absorption is likely to be linked to the nuclear region rather than the host, while the star-formation is not nuclear. Comparing the sSFR of the hosts to the characteristic sSFR of star-forming galaxies at the same redshift (the so-called “main sequence”) we find that the AGNs reside mostly in main-sequence and starburst hosts, reflecting the AGN-sSFR connection; however the infrared selection might bias this result. Limiting our analysis to the highest X-ray luminosity AGNs (X-ray QSOs with L_x > 10⁴⁴   erg   s^-1), we find that the highest-redshift QSOs (with z ≳ 2) reside predominantly in starburst hosts, with an average sSFR more than double that of the “main sequence”, and we find a few cases of QSOs at z ≈ 1.5 with specific star-formation rates compatible with the main-sequence, or even in the “quiescent” region. Finally, we test the reliability of the colour–magnitude diagram (plotting the rest-frame optical colours against the stellar mass) in assessing host properties, and find a significant correlation between rest-frame colour (without any correction for AGN contribution or dust extinction) and sSFR excess relative to the “main sequence” at a given redshift. This means that the most “starbursty” objects have the bluest rest-frame colours.