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.