Ultra-Red Galaxies Signpost Candidate Proto-Clusters at High Redshift

Abstract

We present images obtained with LABOCA on the APEX telescope of a sample of 22 galaxies selected via their red Herschel SPIRE 250-, 350- and $500\text{-}\hat{I}¼\text{m}$ colors. We aim to see if these luminous, rare and distant galaxies are signposting dense regions in the early Universe. Our $870\text{-}\hat{I}¼\text{m}$ survey covers an area of $\approx 0.8{\text{deg}}^2$ down to an average r.m.s. of $3.9{\text{mJy beam}}^{-1}$, with our five deepest maps going $\approx 2\times$ deeper still. We catalog 86 DSFGs around our 'signposts', detected above a significance of $3.5\ddot{I}ƒ$. This implies a $100\pm 30\%$ over-density of $S_{870}>8.5\text{mJy}$ DSFGs, excluding our signposts, when comparing our number counts to those in 'blank fields'. Thus, we are $99.93\%$ confident that our signposts are pinpointing over-dense regions in the Universe, and $\approx 95\%$ confident that these regions are over-dense by a factor of at least $\ge 1.5\times$. Using template SEDs and SPIRE/LABOCA photometry we derive a median photometric redshift of $z=3.2\pm 0.2$ for our signposts, with an interquartile range of $z=2.8\text{-}3.6$. We constrain the DSFGs likely responsible for this over-density to within $|\hat{I}”z|\le 0.65$ of their respective signposts. These 'associated' DSFGs are radially distributed within $1.6\pm 0.5\text{Mpc}$ of their signposts, have median SFRs of $\approx (1.0\pm 0.2)\times 10^3{M}_{\odot }{\text{yr}}^{-1}$ (for a Salpeter stellar IMF) and median gas reservoirs of $\sim 1.7\times 10^{11}{M}_{\odot }$. These candidate proto-clusters have average total SFRs of at least $\approx (2.3\pm 0.5)\times 10^3{M}_{\odot }{\text{yr}}^{-1}$ and space densities of $\sim 9\times 10^{-7}{\text{Mpc}}^{-3}$, consistent with the idea that their constituents may evolve to become massive ETGs in the centers of the rich galaxy clusters we see today.

24 pages, 13 figures, 5 tables, submitted for publication in ApJ