Far-infrared constraints on the contamination by dust-obscured galaxies of high-z dropout searches

¹ Université de Toulouse, UPS-OMP, IRAP, Toulouse, France
e-mail: frederic.boone@irap.omp.eu
² CNRS, IRAP, 9 Av. colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
³ Geneva Observatory, Université de Genève, 51 chemin des Maillettes, 1290 Versoix, Switzerland
⁴ Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany
⁵ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁶ Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA
⁷ Institute for Computational Cosmology, Durham University, South Road, Durham DH1 3LE, UK
⁸ UK Astronomy Technology Centre, Science and Technology Facilities Council, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
⁹ Institute for Astronomy, University of Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
¹⁰ Institut d’Astrophysique Spatiale, Bât. 121, Université Paris Sud 11 & CNRS (UMR8617), 91405 Orsay Cedex, France
¹¹ LERMA, Observatoire de Paris, 61 avenue de l’Observatoire, 75014 Paris, France
¹² California Institute of Technology, 1200 East California Boulevard, Pasadena, California 91125, USA
¹³ Centre de Recherche Astrophysique de Lyon, Université Lyon 1, 9 avenue Charles André, 69561 Saint Genis Laval, France
¹⁴ Laboratoire d’Astrophysique de Marseille, CNRS- Université Aix-Marseille, 38 rue F. Joliot-Curie, 13388 Marseille Cedex 13, France
¹⁵ Herschel Science Centre, European Space Astronomy Centre, ESA, Villanueva de la Caada, 28691 Madrid, Spain
¹⁶ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
¹⁷ Departamento de Astrofísica, Facultad de CC. Físicas, Universidad Complutense de Madrid, 28040 Madrid, Spain

Received: 25 May 2011
Accepted: 11 August 2011

Abstract

The spectral energy distributions (SED) of dusty galaxies at intermediate redshift may look similar to very high-redshift galaxies in the optical/near infrared (NIR) domain. This can lead to the contamination of high-redshift galaxy searches based on broad-band optical/NIR photometry by lower redshift dusty galaxies because both kind of galaxies cannot be distinguished. The contamination rate could be as high as 50%. This work shows how the far-infrared (FIR) domain can help to recognize likely low-z interlopers in an optical/NIR search for high-z galaxies. We analyze the FIR SEDs of two galaxies that are proposed to be very high-redshift (z > 7) dropout candidates based on deep Hawk-I/VLT observations. The FIR SEDs are sampled with PACS/Herschel at 100 and 160 μm, with SPIRE/Herschel at 250, 350 and 500 μm and with LABOCA/APEX at 870 μm. We find that redshifts  > 7 would imply extreme FIR SEDs (with dust temperatures  >100 K and FIR luminosities  >10¹³ L_⊙). At z ~ 2, instead, the SEDs of both sources would be compatible with those of typical ultra luminous infrared galaxies or submillimeter galaxies. Considering all available data for these sources from visible to FIR we re-estimate the redshifts and find z ~ 1.6–2.5. Owing to the strong spectral breaks observed in these galaxies, standard templates from the literature fail to reproduce the visible-to-near-IR part of the SEDs even when additional extinction is included. These sources strongly resemble dust-obscured galaxies selected in Spitzer observations with extreme visible-to-FIR colors, and the galaxy GN10 at z = 4. Galaxies with similar SEDs could contaminate other high-redshift surveys.