  We have discovered a 2.5 Mpc (projected) long filament of infrared-bright
galaxies connecting two of the three ~5x10^14 Msun 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 250-500um
emission associated with a spectroscopically identified filament of galaxies
spanning two X-ray bright cluster cores. We estimate that the total (8-1000um)
infrared luminosity of the filament is Lir~5x10^12 Lsun, which, if due to star
formation alone, corresponds to a total SFR 900 Msun/yr. We are witnessing the
scene of the build-up of a >10^15 Msun 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 build-up of large scale structure.
