Spiderweb galaxy 870um and 1.4GHz images

We also present evidence that another SMG in this field, detected earlier at 850{mu}m, has a counterpart that exhibits H{alpha} and CO(1-0) emission at z=2.15. Including the radio galaxy and two SMGs with FIR photometric redshifts at z=2.2, we conclude that at least eight submm sources are part of the protocluster at z=2.16 associated with the radio galaxy MRC1138-262. We measure a star formation rate density SFRD~1500M_{sun}_/yr/Mpc^3^, four magnitudes higher than the global SFRD of blank fields at this redshift.

Strikingly, these eight sources are concentrated within a region of 2Mpc (the typical size of clusters in the local universe) and are distributed within the filaments traced by the HAEs at z=~2.2. This concentration of massive, dusty starbursts is not centered on the submillimeter-bright radio galaxy which could support the infalling of these sources into the cluster center. Approximately half (6/11) of the SMGs that are covered by the H{alpha} imaging data are associated with HAEs, demonstrating the potential of tracing SMG counterparts with this population.

To summarize, our results demonstrate that submillimeter observations may enable us to study (proto)clusters of massive, dusty starbursts.