Line of sight projections tend to dominate detections of dusty galaxies
In previous work, we found that a single-dish detection of a far-infrared (FIR) source, such as those seen by the Herschel Space Observatory, is often best reproduced by a combination of multiple sources, rather than by a single, extremely luminous galaxy. This result specifically is true when the Herschel detection has both a 24 micron and a 3.6 micron detection beneath it.
In this work, we aimed to determine whether those multiple bright counterparts are physically associated with each other, or whether they are simply line-of-sight projections. If physically associated, this bolsters the historical assumption that many of these bright, dusty galaxies are analogues to the local ULIRG population, which is almost entirely comprised of interacting galaxies.
We therefore matched all of our bright counterparts to a photometric redshift (or a spectroscopic redshift if available) for all available sources in the COSMOS field, using the L'aigle+2016 catalogue of photometric redshifts, and the GAMA team's catalogue of spectroscopic redshifts in the COSMOS field.
For each FIR detection, we plot the photometric redshift solutions, and calculate the likelihood that the two galaxies are likely to be at the same redshift. If the two solutions have no overlap, it is impossible for the two galaxies to coexist at the same redshift. We calculate the likelihood of coexistence for all possible pair permutations, and summarize in the Figure below.
We find that ~70% of the sample of dusty, bright galaxies are inconsistent with existing at the same redshift. This means that they are largely line-of-sight projection effects, and not galaxy interactions. Multiple unrelated galaxies are lining up to inflate the luminosity of the source detected by Herschel. Each galaxy can therefore be a less extreme version of what had been originally suggested by the amount of star formation required to cause such a luminous FIR detection.