The BLAST 250 mum-selected galaxy population in GOODS-South

International audienceWe identify and investigate the nature of the 20 brightest 250mum sources detected by the Balloon-borne Large Aperture Submillimetre Telescope (BLAST) within the central 150arcmin2 of the Great Observatories Origins Deep Survey (GOODS)-South field. Aided by the available deep VLA 1.4GHz radio imaging, reaching S1.4 ~= 40muJy (4sigma), we have identified radio counterparts for 17/20 of the 250mum sources. The resulting enhanced positional accuracy of ~=1arcsec has then allowed us to exploit the deep optical (Hubble Space Telescope), near-infrared (VLT) and mid-infrared (Spitzer) imaging of GOODS-South to establish secure galaxy counterparts for the 17 radio-identified sources, and plausible galaxy candidates for the three radio-unidentified sources. Confusion is a serious issue for this deep BLAST 250mum survey, due to the large size of the beam. Nevertheless, we argue that our chosen counterparts are significant, and often dominant contributors to the measured BLAST flux densities. For all of these 20 galaxies we have been able to determine spectroscopic (eight) or photometric (12) redshifts. The result is the first near-complete redshift distribution for a deep 250mum-selected galaxy sample. This reveals that 250mum surveys reaching detection limits of ~=40mJy have a median redshift z ~= 1, and contain not only low-redshift spirals/LIRGs, but also the extreme z ~= 2 dust-enshrouded starburst galaxies previously discovered at sub-millimetre wavelengths. Inspection of the LABOCA 870mum imaging of GOODS-South yields detections of ~=1/3 of the proposed BLAST sources (all at z > 1.5), and reveals 250/870mum flux-density ratios consistent with a standard 40K modified blackbody fit with a dust emissivity index beta = 1.5. Based on their Infrared Array Camera (IRAC) colours, we find that virtually all of the BLAST galaxy identifications appear better described as analogues of the M82 starburst galaxy, or Sc star-forming discs rather than highly obscured ULIRGs. This is perhaps as expected at low redshift, where the 250mum BLAST selection function is biased towards spectral energy distributions which peak longward of lambdarest = 100mum. However, it also appears largely true at z ~= 2