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

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 σ 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 S 870 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) × 10 10 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