The LABOCA survey of the Extended Chandra Deep Field-South: a photometric redshift survey of submillimetre galaxies

J. L. Wardlow 6 7 Ian Smail 5 K. E. K. Coppin 5 D. M. Alexander 6 W. N. Brandt 12 A. L. R. Danielson 6 B. Luo 12 A. M. Swinbank 5 F. Walter 11 A. Wei 10 Y. Q. Xue 12 S. Zibetti 11 F. Bertoldi 9 A. D. Biggs 14 S. C. Chapman 13 H. Dannerbauer 11 J. S. Dunlop 8 E. Gawiser 3 R. J. Ivison 4 8 K. K. Knudsen 1 9 A. Kov acs 10 C. G. Lacey 5 K. M. Menten 10 N. Padilla 2 H.-W. Rix 11 P. P. van der Werf 0 0 Leiden Observatory, Leiden University , PO Box 9513, 2300 RA Leiden , the Netherlands 1 Onsala Space Observatory, Chalmers University of Technology , SE-43992 Onsala , Sweden 2 Departmento de Astronomia y Astrofisica, Pontificia Universidad Catolica de Chile , Santiago , Chile 3 Physics and Astronomy Department, Rutgers University , Piscataway, NJ 08854 , USA 4 UK Astronomy Technology Centre , Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ 5 Institute for Computational Cosmology, Durham University , South Road, Durham DH1 3LE 6 Department of Physics & Astronomy, Frederick Reines Hall, University of California , Irvine, CA 92697-4575 , USA 7 Department of Physics, Durham University , South Road, Durham DH1 3LE 8 SUPA (Scottish University Physics Alliance), Institute for Astronomy, University of Edinburgh , Royal Observatory, Edinburgh EH9 3HJ 9 Argelander Institut fu r Astronomie , Auf dem Hu gel 71, D-53121 Bonn , Germany 10 Max-Planck-Institut fu r Radioastronomie , Auf dem Hu gel 69, D-53121 Bonn , Germany 11 Max-Planck-Institut fu r Astronomie , Ko nigstuhl 17, D-69117 Heidelberg , Germany 12 Department of Astronomy and Astrophysics, 525 Davey Lab, Pennsylvania State University , University Park, PA 16802 , USA 13 Institute of Astronomy , Madingley Road, Cambridge CB3 0HA 14 European Southern Observatory , Karl-Schwarzschild Strasse 2, D-85748 Garching , Germany A B S T R A C T We derive photometric redshifts from 17-band optical to mid-infrared photometry of 78 robust radio, 24-m and Spitzer IRAC counterparts to 72 of the 126 submillimetre galaxies (SMGs) selected at 870 m by LABOCA observations in the Extended Chandra Deep Field-South (ECDF-S). We test the photometric redshifts of the SMGs against the extensive archival spectroscopy in the ECDF-S. The median photometric redshift of identified SMGs is z = 2.2 0.1, the standard deviation is z = 0.9 and we identify 11 (15 per cent) high-redshift (z 3) SMGs. A statistical analysis of sources in the error circles of unidentified SMGs identifies a population of possible counterparts with a redshift distribution peaking at z = 2.5 0.2, which likely comprises 60 per cent of the unidentified SMGs. This confirms that the bulk of the undetected SMGs are coeval with those detected in the radio/mid-infrared. We conclude that at most 15 per cent of all the SMGs are below the flux limits of our IRAC observations and thus may lie at z 3 and hence at most 30 per cent of all SMGs have z 3. We estimate that the full S870 m > 4 mJy SMG population has a median redshift of 2.5 0.5. In contrast to previous suggestions, we find no significant correlation between submillimetre flux and redshift. The median stellar mass of the SMGs derived from spectral energy distribution fitting is (9.1 0.5) 1010 M although we caution that the uncertainty in the star formation histories results in a factor of 5 uncertainty in these stellar masses. Using a single temperature modified blackbody fit with = 1.5, the median characteristic dust temperature of SMGs is 37.4 1.4 K. The infrared luminosity function shows that SMGs at z = 2-3 typically have 1 I N T R O D U C T I O N Observations in the millimetre and submillimetre wavebands provide a uniquely powerful route to survey the distant Universe for intense dust-obscured starbursts (Blain & Longair 1993). This is due to the negative K-correction arising from the shape of the spectral energy distribution (SED) of the dust emission in the rest-frame far-infrared (FIR), which results in an almost constant apparent flux for sources with a fixed luminosity at z 18. Over the past decade, a series of ever larger surveys in the submillimetre and millimetre wavebands have mapped out a population of sources at mJy-flux limits with a surprisingly high surface density (e.g. Smail, Ivison & Blain 1997; Barger et al. 1998; Hughes et al. 1998; Eales et al. 1999; Bertoldi et al. 2000, 2007; Coppin et al. 2006; Knudsen, van der Werf & Kneib 2008; Wei et al. 2009; Austermann et al. 2010). The mJy fluxes of these sources imply FIR luminosities of 1012 L , if the sources are at cosmological distances, z 1, classifying them as ultraluminous IR galaxies (ULIRGs; Sanders & Mirabel 1996). Their high surface density is far in excess of that expected from a no-evolution model, suggesting very strong evolution of the population: (1 + z)4 (Smail et al. 1997; Blain et al. 1999). If this results from the strong luminosity evolution of starburst galaxies [as opposed to obscured active galactic nuclei (AGNs); Alexander et al. 2005; Laird et al. 2010], then a significant fraction of the massive star formation (and metal production) at high redshift may be occurring in this population. To confirm this evolution and understand the physical processes driving it requires redshifts for the submillimetre galaxies (SMGs). Due to the coarse spatial resolution of the submillimetre and millimetre maps with which the SMGs are identified, combined with their optical faintness (in part due to their high dust obscuration), it has proved challenging to measure their spectroscopic redshift distribution (e.g. Barger, Cowie & Sanders 1999; Chapman et al. 2003a, 2005). In fact, spectroscopic redshifts are not necessary to map the broad evolution of the SMG population; cruder photometric redshifts can be sufficient, if they are shown to be reliable. Various photometric redshift techniques have therefore been applied in an attempt to trace the evolution of SMGs, using their optical/near-/mid-IR or FIR/radio SEDs (e.g. Carilli & Yun 1999; Smail et al. 2000; Ivison et al. 2004; Pope et al. 2005, 2006; Aretxaga et al. 2007; Ivison et al. 2007; Clements et al. 2008; Dye et al. 2008; Biggs et al. 2011). Both the spectroscopic and photometric analyses suggest that the bulk of the SMG population lie at z 1, with an apparent peak at z 2.2 for the subset of SMGs which can be located through their Jy radio emission (Chapman et al. 2005). Nevertheless, there are significant disagreements between the different studies (see e.g. Chapman et al. 2005; Clements et al. 2008; Dye et al. 2008), which may arise in part due to differing levels and types of incompleteness in the identifications and biases in the redshift measurements. The most serious of these is the incompleteness due to challenges in reliably S870 m z 2. Key words: galaxies: evolution galaxies: high-redshift galaxies: starburst submillimetre: galaxies. locating the correct SMG counterpart. They are typically identified through statistical arguments and physical correlations based on rad (...truncated)