2017MNRAS.471.2453S

Submillimetre galaxies (SMGs) are some of the most luminous star-forming galaxies in the Universe; however, their properties remain hard to determine due to the difficulty of identifying their optical/near-infrared counterparts. One of the key steps to determining the nature of SMGs is measuring a redshift distribution representative of the whole population. We do this by applying statistical techniques to a sample of 761 850 µm sources from the Submillimetre Common User Bolometer Array 2 Cosmology Legacy Survey observations of the UKIDSS Ultra-Deep Survey (UDS) field. We detect excess galaxies around >98.4 per cent of the 850 µm positions in the deep UDS catalogue, giving us the first 850 µm selected sample to have virtually complete optical/near-infrared redshift information. Under the reasonable assumption that the redshifts of the excess galaxies are representative of the SMGs themselves, we derive a median SMG redshift of z = 2.05 ± 0.03, with 68 per cent of SMGs residing between 1.07 < z < 3.06. We find an average of 1.52 ± 0.09 excess K-band galaxies within 12 arcsec of an 850 µm position, with an average stellar mass of 2.2 ± 0.1 x 10¹⁰ M_☉. Whilst the vast majority of excess galaxies are star forming, 8.0 ± 2.1 per cent have passive rest-frame colours and are therefore unlikely to be detected at submillimetre wavelengths even in deep interferometry. We show that brighter SMGs lie at higher redshifts, and use our SMG redshift distribution - along with the assumption of a universal far-infrared Spectral Energy Distribution (SED) - to estimate that SMGs contribute around 30 per cent of the cosmic star formation rate density between 0.5 < z < 5.0.