We have discovered a 2.5 Mpc ( projected ) long filament of infrared-bright galaxies connecting two of the three \sim 5 \times 10 ^ { 14 } M _ { \odot } 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 \mu m emission associated with a spectroscopically identified filament of galaxies spanning two X-ray bright cluster cores .
We estimate that the total ( 8–1000 \mu m ) infrared luminosity of the filament is L _ { IR } \simeq 5 \times 10 ^ { 12 } L _ { \odot } , which , if due to star formation alone , corresponds to a total SFR \simeq 900 M _ { \odot } yr ^ { -1 } .
We are witnessing the scene of the build-up of a > 10 ^ { 15 } M _ { \odot } 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 .