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

Wardlow, J. L. and Smail, Ian and Coppin, K. E. K. and Alexander, D. M. and Brandt, W. N. and Danielson, A. L. R. and Luo, B. and Swinbank, A. M. and Walter, F. and Weiß, A. and Xue, Y. Q. and Zibetti, S. and Bertoldi, F. and Biggs, A. D. and Chapman, S. C. and Dannerbauer, H. and Dunlop, J. S. and Gawiser, E. and Ivison, R. J. and Knudsen, K. K. and Kovács, A. and Lacey, C. G. and Menten, K. M. and Padilla, N. and Rix, H.-W. and van der Werf, P. P. (2011) The LABOCA survey of the Extended Chandra Deep Field-South: a photometric redshift survey of submillimetre galaxies. Monthly Notices of the Royal Astronomical Society, 415 (2). pp. 1479-1508. ISSN 0035-8711

Other (pdf)
1006.2137
Download (2MB)

Abstract

We derive photometric redshifts from 17-band optical to mid-infrared photometry of 78 robust radio, 24-μm and Spitzer IRAC counterparts to 72 of the 126 submillimetre galaxies (SMGs) selected at 870 μm 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 σz= 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 μm > 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 β= 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 higher far-infrared luminosities and luminosity density than those at z= 1-2. This is mirrored in the evolution of the star formation rate density (SFRD) for SMGs which peaks at z˜ 2. The maximum contribution of bright SMGs to the global SFRD (˜5 per cent for SMGs with S870 μm≳ 4 mJy or ˜50 per cent extrapolated to SMGs with S870 μm > 1 mJy) also occurs at z˜ 2.