Strong lensing of submillimetre galaxies: a tracer of foreground structure?

The steep source counts and negative K-corrections of bright submillimetre galaxies (SMGs) suggest that a significant fraction of those observed at high flux densities may be gravitationally lensed, and that the lensing objects may often lie at redshifts above 1, where clusters of galaxies are difficult to detect through other means. In this case, follow-up of bright SMGs may be used to identify dense structures along the line-of-sight. Here, we investigate the probability for SMGs to experience strong lensing, using the latest N-body simulations and observed source flux and redshift distributions. We find that almost all high-redshift sources with a flux density above 100 mJy will be lensed if they are not relatively local galaxies. We also give estimates of the fraction of sources experiencing strong lensing as a function of observed flux density. This has implications for planning follow-up observations for bright SMGs discovered in future surveys with the Submillimetre Common-User Bolometer Array 2 and other instruments. The largest uncertainty in these calculations is the maximum allowed lensing amplification, which is dominated by the presently unknown spatial extent of SMGs.