Volume 546, October 2012
|Number of page(s)||16|
|Published online||04 October 2012|
INAF-Osservatorio Astronomico di Bologna, via Ranzani 1, 40127
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
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⋆ ≈ 1010−1011.5 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 Lx ≃ 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 Lx > 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.
Key words: galaxies: active / galaxies: Seyfert / galaxies: statistics / galaxies: star formation / X-rays: galaxies / infrared: galaxies
Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.
© ESO, 2012
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