Ultra-red Galaxies Signpost Candidate Protoclusters at High Redshift

Abstract

We present images obtained with LABOCA of a sample of 22 galaxies selected via their red Herschel SPIRE colors. We aim to see if these luminous, rare, and distant galaxies are signposting dense regions in the early universe. Our 870 μ m survey covers an area of ≈1 deg 2 down to an average rms of , with our five deepest maps going ≈2× deeper still. We catalog 86 dusty star-forming galaxies (DSFGs) around our “signposts,” detected above a significance of 3.5 σ . This implies a overdensity of (or ) 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 overdense regions in the universe, and ≈95% [50%] confident that these regions are overdense by a factor of at least ≥1.5 × [2×]. Using template spectral energy distributions (SEDs) and SPIRE/LABOCA photometry, we derive a median photometric redshift of z  = 3.2 ± 0.2 for our signposts, with an inter-quartile range of z  = 2.8–3.6, somewhat higher than expected for ∼850 μ m selected galaxies. We constrain the DSFGs that are likely responsible for this overdensity to within of their respective signposts. These “associated” DSFGs are radially distributed within (physical) distances of 1.6 ± 0.5 Mpc from their signposts, have median star formation rates (SFRs) of (for a Salpeter stellar inital mass function) and median gas reservoirs of . These candidate protoclusters have average total SFRs of at least and space densities of ∼9 × 10 −7 Mpc −3 , consistent with the idea that their constituents may evolve to become massive early-type galaxies in the centers of the rich galaxy clusters we see today.