THE HERSCHEL FILAMENT: A SIGNATURE OF THE ENVIRONMENTAL DRIVERS OF GALAXY EVOLUTION DURING THE ASSEMBLY OF MASSIVE CLUSTERS AT z = 0.9

We have discovered a 2.5 Mpc (projected) long filament of infrared-bright galaxies connecting two of the three ∼5 × 1014 M☉ clusters making up the RCS 2319+00 supercluster at z = 0.9. The filament is revealed in a deep Herschel Spectral and Photometric Imaging REceiver (SPIRE) map that shows 250-500 μm emission associated with a spectroscopically identified filament of galaxies spanning two X-ray bright cluster cores. We estimate that the total (8-1000 μm) infrared luminosity of the filament is LIR ≅ 5 × 1012 L☉, which, if due to star formation alone, corresponds to a total SFR ≅ 900 M☉ yr–1. We are witnessing the scene of the buildup of a >1015 M☉ cluster of galaxies, seen prior to the merging of three massive components, each of which already contains a population of red, passive galaxies that formed at z > 2. The infrared filament demonstrates that significant stellar mass assembly is taking place in the moderate density, dynamically active circumcluster environments of the most massive clusters at high redshift, and this activity is concomitant with the hierarchical buildup of large-scale structure.