We present APEX LABOCA 870 \mu m observations of the field around the high-redshift radio galaxy MRC1138 - 262 at z = 2.16 .
We detect 16 submillimeter galaxies in this \sim 140 arcmin ^ { 2 } bolometer map with flux densities in the range 3 - 11 mJy .
The raw number counts indicate a density of submillimeter galaxies ( SMGs ) that is up to four times that of blank field surveys .
Based on an exquisite multiwavelength database , including VLA 1.4 GHz radio and infrared observations , we investigate whether these sources are members of the protocluster structure at z \approx 2.2 .
Using Herschel PACS + SPIRE and Spitzer MIPS photometry , we derive reliable far-infrared photometric redshifts for all sources .
Follow-up VLT ISAAC and SINFONI near-infrared spectra confirm that four of these SMGs have redshifts of z \approx 2.2 .
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 far-IR 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 \sim 1500 M _ { \odot } yr ^ { -1 } 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 2 Mpc ( the typical size of clusters in the local universe ) and are distributed within the filaments traced by the H \alpha emitters at z \approx 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 H \alpha emitters , 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 .