Observational constraints on the physical nature of submillimetre source
  multiplicity: chance projections are common
  Interferometric observations have demonstrated that a significant fraction of
single-dish submillimetre (submm) sources are blends of multiple submm galaxies
(SMGs), but the nature of this multiplicity, i.e. whether the galaxies are
physically associated or chance projections, has not been determined. We
performed spectroscopy of 11 SMGs in six multi-component submm sources,
obtaining spectroscopic redshifts for nine of them. For an additional two
component SMGs, we detected continuum emission but no obvious features. We
supplement our observed sources with four sources from the literature. This
sample allows us to statistically constrain the physical nature of single-dish
submm source multiplicity for the first time. In three [3/7, or 43 (-33/+39)
per cent at 95% confidence] of the single-dish sources for which the nature of
the blending is unambiguous, the components for which spectroscopic redshifts
are available are physically associated, whereas 4/7 [57 (-39/+33) per cent]
have at least one unassociated component. When components whose spectra exhibit
continuum but no features and for which the photometric redshift is
significantly different from the spectroscopic redshift of the other component
are also considered, 6/9 [67 (-37/+26) per cent] of the single-dish sources are
comprised of at least one unassociated component. The nature of the
multiplicity of one single-dish source is ambiguous. We conclude that
physically associated systems and chance projections both contribute to the
multi-component single-dish submm source population. This result contradicts
the conventional wisdom that bright submm sources are solely a result of
merger-induced starbursts, as blending of unassociated galaxies is also
important.
