  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.
