A robust sample of submillimetre galaxies: constraints on the prevalence of dusty, high-redshift starbursts

We develop and apply a dual-survey extraction technique to published submm images of the Lockman Hole using the SCUBA and MAMBO bolometer arrays. Cut above 5 sigma, this catalogue of submm galaxies (SMGs) is significantly more robust than previous samples, typically selected above 3.0-3.5 sigma, with a much-reduced likelihood of real, faint SMGs (beneath and around the confusion limit) entering the catalogue via superposition with noise. Our selection technique yields 19 SMGs of which we expect at most two to be due to chance superposition of SCUBA and MAMBO noise peaks. The flux limit of the sample (~5mJy at 0.85mm), which is sensitive to luminous, dusty galaxies at extreme redshifts, is optimally matched to a deep 1.4-GHz image (4.6 uJy/beam rms) which probes starbursts in the z<~4 regime. A high fraction of these robust SMGs (~80%) have radio counterparts which, given the probable ~10% contamination by spurious sources, suggests that very distant SMGs (z>~3-4) are unlikely to make up more than ~10% of the bright SMG population. Finally, the accurate radio positions are used to test the accuracy of positions determined in the submm, with important implications for upcoming surveys with SCUBA-2.