The effect of lensing on the identification of bright SCUBA galaxies

Abstract

Spectroscopic surveys of luminous submillimetre-selected sources have uncovered optically-bright galaxies at z<1 close to the positions of several submillimetre (submm) sources. Naive statistical analyses suggest that these galaxies are associated with the submm emission. However, in some cases, it is difficult to understand this association given the relatively modest redshifts and unpreposessing spectral characteristics of the galaxies. These are in stark constrast to those expected from the massive dust-enshrouded starbursts and AGN thought to power the bulk of the bright submm population. We present new observations of optically-bright counterparts to two luminous submm sources, along with a compilation of previously proposed optically-bright counterparts with z<1. We suggest that the majority of these associations between bright galaxies and submm sources may be due to the action of the foreground galaxies as gravitational lenses on the much fainter and more distant submm sources. We discuss the implications of this conclusion for our understanding of the SCUBA population.