The evolution of submillimetre galaxies: two populations and a redshift cut-off

We explore the epoch dependence of number density and star-formation rate for submm galaxies found at 850 um. The study uses a sample of 38 submm galaxies in the GOODS-N field, for which cross-waveband identifications have been obtained for 35/38 members together with redshift measurements or estimates. A maximum-likelihood analysis is employed, along with the `single-source-survey' technique. We find a highly significant diminution in both space density and star formation rate at z > 3, closely mimicking the redshift cut-offs found for QSOs selected in different wavebands. The data further indicate that two separately-evolving populations are present, with distinct luminosity functions. These results parallel the different evolutionary behaviours of LIRGs and ULIRGs, and represent another instance of `cosmic down-sizing'.