Volume 534, October 2011
|Number of page(s)||13|
|Published online||22 September 2011|
GOODS-Herschel: a population of 24 μm dropout sources at z < 2⋆
Laboratoire AIM-Paris-Saclay, CEA/DSM/Irfu - CNRS - Université Paris Diderot, CE-Saclay, 91191 Gif-sur-Yvette, France
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
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.
Key words: galaxies: active / galaxies: evolution / galaxies: formation / galaxies: groups: general / infrared: galaxies
© ESO, 2011
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