Spatially unassociated galaxies contribute significantly to the blended submillimetre galaxy population: predictions for follow-up observations of ALMA sources

There is anecdotal evidence that spatially and physically unassociated galaxies blended into a single submillimetre (submm) source contribute to the submm galaxy (SMG) population. This work is the first to theoretically predict the number counts of such sources. We generate mock SMG catalogues using lightcones derived from the Bolshoi cosmological simulation; to assign submm flux densities to the mock galaxies, we use a fitting function previously derived from the results of dust radiative transfer performed on hydrodynamical simulations of isolated disc and merging galaxies. We then calculate submm number counts for different beam sizes and without blending. We predict that > ~50 per cent of blended SMGs have at least one spatially unassociated component with S_850 > 1 mJy. For a 15-arcsec beam, blends of >2 galaxies in which at least one component is spatially unassociated dominate the blended sources with total S_850 > ~3 mJy. The distribution of the redshift separations amongst the components is strongly bimodal. The typical redshift separation of spatially unassociated blended sources is ~1. Our predictions for the contributions of spatially unassociated components and the distribution of redshift separations are not testable with currently available data, but they will be easily tested once sufficiently accurate redshifts for the individual subcomponents (resolved by, e.g., ALMA) of a sufficient number of single-dish-detected blended SMGs are available.