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

Abstract

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 $z_{\mathrm{spec}}=4.002$ via detection of [CI](1-0), ${}^{12}$CO(6-5), ${}^{12}$CO(4-3), ${}^{12}$CO(2-1) and ${\mathrm{H}}_2\mathrm{O}(2_{11}-2_{02})$ emission lines, detected using ALMA and ATCA. The spectroscopically-confirmed components of the proto-cluster are distributed over a $260\mathrm{kpc}\times 310\mathrm{kpc}$ region and have a collective obscured star-formation rate (SFR) of $\sim 6500M_{\odot }{\mathrm{yr}}^{-1}$, 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$\hat{I}\pm$ emitters are detected in the proto-cluster core, apart from a Ly$\hat{I}\pm$ blob located next to one of the DRC dusty components and extending over $60\mathrm{kpc}$. The total obscured SFR of the proto-cluster could rise to $\mathrm{SFR}\sim 14,400M_{\odot }{\mathrm{yr}}^{-1}$ if all the members of an over-density of bright DSFGs discovered around DRC in a wide-field LABOCA 870-$\hat{I}¼$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.

Submitted to ApJ. Minor updates added, including a change of the source name. Comments welcome