2018A&A...620A.202A

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∼2-3 three enormous (>200kpc) Lyman-α nebulae (ELANe) that host multiple active galactic nuclei (AGN) and that are surrounded by overdensities of Lyman-α emitters (LAE). These regions are prime candidates for 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 450µm observations of a ∼128-arcmin² 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±1.3 times higher source counts at 850µm with respect to blank fields, likely confirming the presence of an overdensity also in obscured tracers. We find a strong detection at 850µm 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_–2.1^+7.4x10¹²L_☉, and hosting an obscured AGN. Such a source is thus able to power a hard photoionization plus outflow scenario to explain the extended Lyman-α, HeIIλ1640, and CIVλ1549 emission, and their kinematics. In addition, the two brightest detections at 850µm (f₈₅₀>18mJy) 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 to firmly characterize a cosmic nursery of giant elliptical galaxies, and ultimately of a massive cluster.