GOODS–Herschel: an infrared main sequence for star-forming galaxies^⋆

¹ Laboratoire AIM-Paris-Saclay, CEA/DSM/Irfu, CNRS, Université Paris Diderot, Saclay, pt courrier 131, 91191 Gif-sur-Yvette, France
e-mail: delbaz@cea.fr
² National Optical Astronomy Observatory, 950 North Cherry Avenue, Tucson, AZ 85719, USA
³ Department of Physics and Institute of Theoretical & Computational Physics, University of Crete, 71003 Heraklion, Greece
⁴ Max-Planck-Institut für Extraterrestrische Physik (MPE), Postfach 1312, 85741, Garching, Germany
⁵ Institut d’Astrophysique de Paris, UMR 7095, CNRS, UPMC Univ. Paris 06, 98bis boulevard Arago, 75014 Paris, France
⁶ Spitzer Science Center, California Institute of Technology, Pasadena, CA 91125, USA
⁷ IESL/Foundation for Research and Technology, Hellas, 71110, Heraklion, Greece and Chercheur Associé, Observatoire de Paris, 75014 Paris, France
⁸ Herschel Science Centre, European Space Astronomy Centre, Villanueva de la Cañada, 28691 Madrid, Spain
⁹ Astronomy Department, Universidad de Concepción, Casilla 160-C, Concepción, Chile
¹⁰ IPAC, California Institute of Technology, Pasadena, CA, 91125, USA
¹¹ Department of Physics and Astronomy, Durham University, South Road, Durham, DH1 3LE, UK
¹² Laboratoire d’Astrophysique de Marseille, OAMP, Université Aix-Marseille, CNRS, 38 rue Frédéric Joliot-Curie, 13388 Marseille Cedex 13, France
¹³ INAF – Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
¹⁴ UK Astronomy Technology Centre, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
¹⁵ Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
¹⁶ Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA
¹⁷ Institute for Astronomy, University of Hawaii, Honolulu, HI 96822, USA
¹⁸ Canada-France-Hawaii Telescope, Kamuela, HI 96743, USA
¹⁹ Department of Astronomy, University of Massachusetts, Amherst, MA 01003, USA
²⁰ Department of Physics & Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, BC V6T 1Z1, Canada
²¹ Institut d’Astrophysique Spatiale (IAS), bâtiment 121, Université Paris-Sud 11 and CNRS (UMR 8617), 91405 Orsay, France
²² Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
²³ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21228, USA
²⁴ National Radio Astronomy Observatory, PO Box 2, Green Bank, WV24944, USA
²⁵ Department of Physics and Astronomy, Texas A&M University, College Station, TX 77845-4242, USA
²⁶ George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy, Texas A&M University, College Station, TX 77845-4242, USA

Received: 11 May 2011
Accepted: 3 August 2011

Abstract

We present the deepest 100 to 500 μm far-infrared observations obtained with the Herschel Space Observatory as part of the GOODS-Herschel key program, and examine the infrared (IR) 3–500 μm spectral energy distributions (SEDs) of galaxies at 0 < z < 2.5, supplemented by a local reference sample from IRAS, ISO, Spitzer, and AKARI data. We determine the projected star formation densities of local galaxies from their radio and mid-IR continuum sizes.

We find that the ratio of total IR luminosity to rest-frame 8 μm luminosity, IR8 (≡ L_IR^tot), follows a Gaussian distribution centered on IR8 = 4 (σ = 1.6) and defines an IR main sequence for star-forming galaxies independent of redshift and luminosity. Outliers from this main sequence produce a tail skewed toward higher values of IR8. This minority population (<20%) is shown to consist of starbursts with compact projected star formation densities. IR8 can be used to separate galaxies with normal and extended modes of star formation from compact starbursts with high–IR8, high projected IR surface brightness (Σ_IR  >  3 × 10¹⁰ L_⊙ kpc^-2) and a high specific star formation rate (i.e., starbursts). The rest-frame, UV-2700 Å size of these distant starbursts is typically half that of main sequence galaxies, supporting the correlation between star formation density and starburst activity that is measured for the local sample.

Locally, luminous and ultraluminous IR galaxies, (U)LIRGs (L_IR^tot), are systematically in the starburst mode, whereas most distant (U)LIRGs form stars in the “normal” main sequence mode. This confusion between two modes of star formation is the cause of the so-called “mid-IR excess” population of galaxies found at z > 1.5 by previous studies. Main sequence galaxies have strong polycyclic aromatic hydrocarbon (PAH) emission line features, a broad far-IR bump resulting from a combination of dust temperatures (T_dust  ~ 15–50 K), and an effective T_dust  ~ 31 K, as derived from the peak wavelength of their infrared SED. Galaxies in the starburst regime instead exhibit weak PAH equivalent widths and a sharper far-IR bump with an effective T_dust  ~ 40 K. Finally, we present evidence that the mid-to-far IR emission of X-ray active galactic nuclei (AGN) is predominantly produced by star formation and that candidate dusty AGNs with a power-law emission in the mid-IR systematically occur in compact, dusty starbursts. After correcting for the effect of starbursts on IR8, we identify new candidates for extremely obscured AGNs.