Overdensity of submillimeter galaxies around the

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 (> 200 kpc) 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 450 μm 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 850 μm with respect to blank fields, likely confirming the presence of an overdensity also in obscured tracers. We find a strong detection at 850 μm 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_{\mathrm{FIR}}^{\mathrm{SF}}=2.4^{+7.4}_{-2.1}\times10^{12}\,L_{\odot} $, 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, He I