MAMMOTH-1 450 and 850um images

These observations unveil 4.0+/-1.3 times higher source counts at 850um with respect to blank fields, likely confirming the presence of an overdensity also in obscured tracers. We find a strong detection at 850um 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^SF^_FIR_=2.4^+7.4^+_-2.1_x10^12^L_{sun}_, and hosting an obscured AGN.

Such a source is thus able to power a hard photoionization plus outflow scenario to explain the extended Lyman-, HeII1640; and CIV1549 emission, and their kinematics. In addition, the two brightest detections at 850um (f_850_>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.