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

Dust emission at submillimetrewavelengths 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 darkmatter 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 μmflux densities of S250 >15 mJy selected using data from the Herschel Multi-tiered Extragalactic Survey. The positions of hundreds of submillimetre foreground lenses are crosscorrelated with the positions of background Lyman-break galaxies at z ~ 3-5 selected using optical data from the Canada-France-Hawaii Telescope Legacy Survey. We detect a crosscorrelation signal at the 7σ level over a sky area of 1 deg², with ~80 per cent of this signal being due to magnification, whereas the remaining ~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 log₁₀[M₂₀₀/M⊙] = 13.17^+0.05 _-0.08(stat.) and their average dust mass fraction (at radii of gt;10 kpc) to be M_dust/M₂₀₀ ≈ 6 × 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.