In this paper, we investigate the physical associations between blended
far-infrared (FIR)-emitting galaxies, in order to identify the level of
line-of-sight projection contamination in the single-dish Herschel data.
Building on previous work, and as part of the Herschel Extragalactic
Legacy Project, we identify a sample of galaxies in the COSMOS field,
which are found to be both FIR-bright (typically ˜15 mJy) and
blended within the Herschel 250-μ m beam. We identify a spectroscopic
or photometric redshift for each FIR-bright source. We conduct a joint
probability distribution analysis on the redshift probability density
functions to determine the fraction of the FIR sources with multiple
FIR-bright counterparts that are likely to be found at consistent
(Δz <0.01) redshifts. We find that only three (0.4 per cent)
of the pair permutations between counterparts are >50 per cent likely
to be at consistent redshifts. A majority of counterparts (72 per cent)
have no overlap in their redshift probability distributions whatsoever.
This is in good agreement with the results of recent simulations, which
indicate that single-dish observations of the FIR sky should be strongly
contaminated by line-of-sight projection effects. We conclude that for
our sample of 3.6- and 24-μ m selected, FIR-bright objects in the
COSMOS field, the overwhelming majority of multicomponent FIR systems
are line-of-sight projections within the 18.1-arcsec Herschel beam,
rather than physical associations.