Herschel-Astrophysical Terahertz Large Area Survey: 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 within similar to 1Mpc of the centres of 66 optically selected clusters of galaxies in the Sloan Digital Sky Survey with < z > similar to 0.25. From the analysis of the multiwavelength properties of their counterparts we conclude that the far-infrared emission is associated with dust-obscured star formation and/or active galactic nuclei (AGN) within galaxies in the clusters themselves. The excess reaches a maximum at a radius of similar to 0.8Mpc, where we find 1.0 +/- 0.3 S-250 > 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 similar to 7M(circle dot) yr(-1) per cluster in sources with L-IR > 5 x 10(10) L-circle dot. 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.