In the hierarchical model of structure formation , giant elliptical galaxies form through merging processes within the highest density peaks known as protoclusters .
While high-redshift radio galaxies usually pinpoint the location of these environments , we have recently discovered at z \sim 2 - 3 three Enormous ( > 200 kpc ) Lyman-Alpha Nebulae ( ELANe ) that host multiple Active Galactic Nuclei and that are surrounded by overdensities of Lyman- \alpha Emitters ( LAE ) .
These regions are prime candidates of massive protoclusters in the early stages of assembly .
To characterize the star-forming activity within these rare structures – both on ELAN and protocluster scales – we have initiated an observational campaign with the James Clerk Maxwell Telescope ( JCMT ) and the Atacama Pathfinder EXperiment ( APEX ) telescopes .
In this paper we report on sensitive SCUBA-2/JCMT 850 and 450 \mu m observations of a \approx 128 arcmin ^ { 2 } field comprising the ELAN MAMMOTH-1 , together with the peak of the hosting BOSS1441 LAE overdensity at z = 2.32 .
These observations unveil 4.0 \pm 1.3 times higher source counts at 850 \mu m with respect to blank fields , likely confirming the presence of an overdensity also in obscured tracers .
We find a strong detection at 850 \mu m associated with the continuum source embedded within the ELAN MAMMOTH-1 , which – together with the available data from the literature – allow us to constrain the spectral energy distribution of this source to be of an Ultra-Luminous Infrared Galaxy ( ULIRG ) with a far-infrared luminosity of L _ { FIR } ^ { SF } = 2.4 ^ { +7.4 } _ { -2.1 } \times 10 ^ { 12 } L _ { \odot } , and hosting an obscured AGN .
Such a source is thus able to power the hard photoionization plus outflow scenario depicted in \citet Cai2016 to explain the extended Lyman- \alpha , He ii \lambda 1640 and C iv \lambda 1549 emission , and their kinematics .
In addition , the two brightest detections at 850 \mu m ( f _ { 850 } > 18 mJy ) sit at the density peak of the LAEs overdensity , likely pinpointing the core of the protocluster .
Future multiwavelength and spectroscopic datasets targeting the full extent of the BOSS1441 overdensity have the potential of firmly characterizing a cosmic nursery of giant elliptical galaxies , and ultimately of a massive cluster .