Overdensity of submillimeter galaxies around the $z\simeq 2.3$ MAMMOTH-1
  nebula
  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-Alpha Nebulae (ELANe) that host multiple AGN 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 JCMT and the APEX telescopes. In this paper we report on sensitive
SCUBA-2/JCMT 850 and 450 $\mu$m observations of a 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 $4.0\pm1.3$ 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
ULIRG with a far-infrared luminosity of $L_{\rm FIR}^{\rm
SF}=2.4^{+7.4}_{-2.1}\times10^{12}$ L$_{\odot}$, and hosting an obscured AGN.
Such a source is thus able to power the hard photoionization plus outflow
scenario depicted in Cai et al. (2017b) to explain the extended Lyman-alpha,
HeII$\lambda1640$ and CIV$\lambda1549$ 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.
