An extreme proto-cluster of luminous dusty starbursts in the early Universe

We report the identification of an extreme proto-cluster of galaxies in the early Universe whose core (nicknamed Distant Red Core, DRC) is formed by at least ten dusty star-forming galaxies (DSFGs), confirmed to lie at $z_{\rm spec} = 4.002$viadetectionof[CI](1-0),$^{12}$CO(6-5),$^{12}$CO(4-3),$^{12}$CO(2-1)and${\rm H_2O} (2_{11} - 2_{02})$ emission lines, detected using ALMA and ATCA. The spectroscopically-confirmed components of the proto-cluster are distributed over a $260\mathrm{k}\mathrm{p}\mathrm{c}\times 310\mathrm{k}\mathrm{p}\mathrm{c}$ region and have a collective obscured star-formation rate (SFR) of $\sim 6500 , M_\odot , {\rm yr}^{-1}$, considerably higher than has been seen before in any proto-cluster of galaxies or over-densities of DSFGs at $z\gtrsim 4$. Most of the star formation is taking place in luminous DSFGs since no Ly$\alpha$ emitters are detected in the proto-cluster core, apart from a Ly$\alpha$ blob located next to one of the DRC dusty components and extending over $60,{\rm kpc}$\mathrm{.}ThetotalobscuredSFRoftheproto-clustercouldriseto${\rm SFR} \sim 14,400 , M_\odot , {\rm yr}^{-1}$ if all the members of an over-density of bright DSFGs discovered around DRC in a wide-field LABOCA 870-$\mu$m image are part of the same structure. The total halo mass of DRC could be as high as $\sim 4.4\times {10}^{13}{M}_{\odot }$ and could be the progenitor of a Coma-like cluster at $z=0$. The relatively short gas-depletion times of the DRC components suggest either the presence of a mechanism able to trigger extreme star formation simultaneously in galaxies spread over a few hundred kpc or the presence of gas flows from the cosmic web able to sustain star formation over several hundred million years.