Herschel-ATLAS: detection of a far-infrared population around galaxy clusters

We report the detection of a significant excess in the surface density of far-infrared sources from the Herschel-Astrophysical Terahertz Large Area Survey (H-ATLAS) within ∼ 1Mpc of the centres of 66 optically-selected clusters of galaxies in the SDSS with 〈z〉 ∼ 0.25. From the analysis of the multiwavelength properties of their counterparts we conclude that the farinfrared emission is associated with dust-obscured star formation and/or active galactic nuclei within galaxies in the clusters themselves. The excess reaches a maximum at a radius of ∼ 0.8Mpc, where we find 1.0± 0.3 S250 >34 mJy sources on average per cluster above what would be expected for random field locations. If the far-infrared emission is dominated by star formation (as opposed to AGN) then this corresponds to an average star formation rate of ∼7 M⊙ yr per cluster in sources with LIR > 5× 10 L⊙. Although lensed sources make a negligible contribution to the excess signal, a fraction of the sources around the clusters could be gravitationally lensed, and we have identified a sample of potential cases of cluster-lensed Herschel sources that could be targeted in follow-up studies.