Colour matters: the effects of lensing on the positional offsets between optical and submillimetre galaxies in Herschel-ATLAS

摘要

We report an unexpected variation in the positional offset distributions between Herschel-ATLAS sub-millimetre (submm) sources and their optical associations, depend-ing on both 250-μm signal-to-noise ratio and 250/350-μm colour.We show that redderand brighter submm sources have optical associations with a broader distribution ofpositional offsets than would be expected if these offsets were due to random positionalerrors in the source extraction. The observation can be explained by two possible ef-fects: either red submm sources trace a more clustered population than blue ones, andtheir positional errors are increased by confusion; or red submm sources are generallyat high redshifts and are frequently associated with low-redshift lensing structureswhich are identified as false counterparts. We perform various analyses of the data,including the multiplicity of optical associations, the redshift and magnitude distribu-tions in H-ATLAS in comparison to HerMES, and simulations of weak lensing, and weconclude that the effects are most likely to be explained by widespread weak lensing ofHerschel -SPIRE sources by foreground structures. This has important consequencesfor counterpart identification and derived redshift distributions and luminosity func-tions of submm surveys.