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

The modest significance of most sources detected in current ( sub) millimetre [( sub) mm] surveys can potentially compromise some analyses due to the inclusion of spurious sources in catalogues typically selected at >= 3.0 -3.5 sigma. Here, we develop and apply a dual-survey extraction technique to Submillimetre Common-User Bolometer Array ( SCUBA) and Max-Planck Millimetre Bolometer( MAMBO) images of the Lockman Hole. Cut above 5 sigma, our catalogue of submillimetre galaxies ( SMGs) is more robust than previous samples, with a reduced likelihood of real, but faint SMGs (beneath and around the confusion limit) entering via superposition with noise. Our selection technique yields 19 SMGs in an effective area of 165 arcmin(2), of which we expect at most two to be due to chance superposition of SCUBA and MAMBO noise peaks. The effective flux limit of the survey (similar to 4 mJy at similar to 1 mm) is well matched to our deep 1.4-GHz image (sigma = 4.6 mu Jy beam(-1)). The former is sensitive to luminous, dusty galaxies at extreme redshifts whilst the latter probes the z less than or similar to 3 regime. A high fraction of our robust SMGs (similar to 80 per cent) have radio counterparts which, given the similar to 10 per cent contamination by spurious sources, suggests that very distant SMGs (z >> 3) are unlikely to make up more than similar to 10 per cent of the bright SMG population. This implies that almost all of the S-1mm greater than or similar to 4 mJy SMG population is amenable to study via the deepest current radio imaging. We use these radio counterparts to provide an empirical calibration of the positional uncertainty in SMG catalogues. We then go on to outline the acquisition of redshifts for radio-identified SMGs, from sample selection in the submm, to counterpart selection in the radio and optical/infrared, to slit placement on spectrograph masks. We determine a median of z = 2.05 +/- 0.41 from a sample of six secure redshifts for unambiguous radio-identified submm sources and z = 2.14 +/- 0.27 when we include submm sources with multiple radio counterparts and/or less reliable redshifts. These figures are consistent with previous estimates, suggesting that our knowledge of the median redshift of bright SMG population has not been biased by the low significance of the source catalogues employed.