GOODS-Herschel: a population of 24   μm dropout sources at z  <  2^⋆

¹ Laboratoire AIM-Paris-Saclay, CEA/DSM/Irfu - CNRS - Université Paris Diderot, CE-Saclay, 91191 Gif-sur-Yvette, France
e-mail: georgios.magdis@astro.ox.ac.uk
² Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK
³ Space Science & Technology Department, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, UK
⁴ National Optical Astronomy Observatory, 950 North Cherry Avenue, Tucson, AZ 85719, USA
⁵ Department of Physics & Institute of Theoretical and Computation Physics, University of Crete, 71003 Heraklion, Greece
⁶ IESL/Foundation for Research & Technology-Hellas, 71110 Heraklion, Greece
⁷ Chercheur Associé, Observatoire de Paris, 75014 Paris, France
⁸ Spitzer Science Center, California Institute of Technology, Pasadena, CA, USA
⁹ Laboratoire d’Astrophysique de Marseille, OAMP, Universit Aix-Marseille, CNRS, 38 rue Frdric Joliot-Curie, 13388 Marseille Cedex 13, France
¹⁰ Institute for Astronomy, University of Hawaii, Manoa, HI 96822; Canada-France-Hawaii Telescope Corp., Kamuela, HI 96743, USA
¹¹ Max-Planck-Institut für Extraterrestrische Physik (MPE), Postfach 1312, 85741 Garching, Germany
¹² Department of Physics and Astronomy, University of British Columbia, Vancouver, BC V6T 1Z1, Canada
¹³ Herschel Science Centre, European Space Astronomy Centre, Villanueva de la Cañada, 28691 Madrid, Spain
¹⁴ Department of Astronomy University of Massachusetts, LGRT-B 618, 710 North Pleasant Street, Amherst, MA 01003, USA
¹⁵ Univ Paris-Sud, Laboratoire IAS, UMR8617, 91405 Orsay Cedex ; CNRS, 91405 Orsay, France

Received: 7 July 2011
Accepted: 3 August 2011

Abstract

Using extremely deep PACS 100- and 160   μm Herschel data from the GOODS-Herschel program, we identify 21 infrared bright galaxies previously missed in the deepest 24   μm surveys performed by Spitzer/MIPS. These MIPS dropouts are predominantly found in two redshift bins, centred at z ~ 0.4 and  ~1.3. Their S₁₀₀/S₂₄ flux density ratios are similar to those of local (ultra-) luminous infrared galaxies (LIRGs and ULIRGs), whose silicate absorption features at 18   μm (at z ~ 0.4) and 9.7   μm (at z ~ 1.3) are shifted into the 24   μm MIPS band at these redshifts. The high-z sub-sample consists of 11 infrared luminous sources, accounting for  ~2% of the whole GOODS-Herschel sample and putting strong upper limits on the fraction of LIRGs/ULIRGs at 1.0 < z < 1.7 that are missed by the 24   μm surveys. We find that a S₁₀₀/S₂₄  >  43 colour cut selects galaxies with a redshift distribution similar to that of the MIPS dropouts and when combined with a second colour cut, S₁₆/S₈  >  4, isolates sources at 1.0 < z < 1.7. We show that these sources have elevated specific star formation rates (sSFR) compared to main sequence galaxies at these redshifts and are likely to be compact starbursts with moderate/strong 9.7   μm silicate absorption features in their mid-IR spectra. Herschel data reveal that their infrared luminosities extrapolated from the 24   μm flux density are underestimated, on average, by a factor of  ~3. These silicate break galaxies account for 16% (8%) of the ULIRG (LIRG) population in the GOODS fields, indicating a lower limit in their space density of 2.0 × 10^-5 Mpc^-3. Finally, we provide estimates of the fraction of z < 2 MIPS dropout sources as a function of the 24-, 100-, 160-, 250- and 350   μm sensitivity limits, and conclude that previous predictions of a population of silicate break galaxies missed by the major 24   μm extragalactic surveys have beenoverestimated.