Inferring the mass of submillimetre galaxies by exploiting their gravitational magnification of background galaxies

Dust emission at submillimetre wavelengths allows us to trace the early phases of star formation in the Universe. In order to understand the physical processes involved in this mode of star formation, it is essential to gain knowledge about the dark matter structures - most importantly their masses - that submillimetre galaxies live in. Here we use the magnification effect of gravitational lensing to determine the average mass and dust content of submillimetre galaxies with 250 mu m flux densities of S-250 > 15 mJy selected using data from the Herschel Multi-tiered Extragalactic Survey. The positions of hundreds of submillimetre foreground lenses are cross-correlated with the positions of background Lyman-break galaxies at z similar to 3-5 selected using optical data from the Canada-France-Hawaii Telescope Legacy Survey. We detect a cross-correlation signal at the 7 sigma level over a sky area of 1 deg(2), with similar to 80 per cent of this signal being due to magnification, whereas the remaining similar to 20 per cent comes from dust extinction. Adopting some simple assumptions for the dark matter and dust profiles and the redshift distribution enables us to estimate the average mass of the haloes hosting the submillimetre galaxies to be log(10)[M-200/M-circle dot] = 13.17(-0.08)(+0.05)(stat.) and their average dust mass fraction (at radii of > 10 kpc) to be M-dust/M-200 approximate to 6 x 10(-5). This supports the picture that submillimetre galaxies are dusty, forming stars at a high rate, reside in massive group-sized haloes and are a crucial phase in the assembly and evolution of structure in the Universe.