2010A&A...518L..19H

We present wide-field Herschel/PACS observations of A1689, a massive galaxy cluster at z=0.1832, from our open time key programme. We detect 39 spectroscopically confirmed 100µm-selected cluster members down to 1.5x10¹⁰L_☉. These galaxies are forming stars at rates in the range 1-10M_☉/yr, and appear to comprise two distinct populations: two-thirds are unremarkable blue, late-type spirals found throughout the cluster; the remainder are dusty red sequence galaxies whose star formation is heavily obscured with A(Hα)∼2mag and are found only in the cluster outskirts. The specific-SFRs of these dusty red galaxies are lower than the blue late-types, suggesting that the former are in the process of being quenched, perhaps via pre-processing, the unobscured star formation being terminated first. We also detect an excess of 100µm-selected galaxies extending ∼6Mpc in length along an axis that runs NE-SW through the cluster center at > 95% confidence. Qualitatively this structure is consistent with previous reports of substructure in X-ray, lensing, and near-infrared maps of this cluster, further supporting the view that this cluster is a dynamically active, merging system.