Ultra-Red Galaxies Signpost Candidate Proto-Clusters at High Redshift
  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
$500\textrm{-}\mu\textrm{m}$ colors. We aim to see if these luminous, rare and
distant galaxies are signposting dense regions in the early Universe. Our
$870\textrm{-}\mu\textrm{m}$ survey covers an area of
$\approx0.8\,\textrm{deg}^2$ down to an average r.m.s. of $3.9\,\textrm{mJy
beam}^{-1}$, with our five deepest maps going $\approx2\times$ deeper still. We
catalog 86 DSFGs around our 'signposts', detected above a significance of
$3.5\sigma$. This implies a $100\pm30\%$ over-density of
$S_{870}>8.5\,\textrm{mJy}$ 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
$\approx95\%$ confident that these regions are over-dense by a factor of at
least $\ge1.5\times$. Using template SEDs and SPIRE/LABOCA photometry we derive
a median photometric redshift of $z=3.2\pm0.2$ for our signposts, with an
interquartile range of $z=2.8\textrm{-}3.6$. We constrain the DSFGs likely
responsible for this over-density to within $|\Delta z|\le0.65$ of their
respective signposts. These 'associated' DSFGs are radially distributed within
$1.6\pm0.5\,\textrm{Mpc}$ of their signposts, have median SFRs of
$\approx(1.0\pm0.2)\times10^3\,M_{\odot}\,\textrm{yr}^{-1}$ (for a Salpeter
stellar IMF) and median gas reservoirs of $\sim1.7\times10^{11}\,M_{\odot}$.
These candidate proto-clusters have average total SFRs of at least $\approx
(2.3\pm0.5)\times10^3\,M_{\odot}\,\textrm{yr}^{-1}$ and space densities of
$\sim9\times10^{-7}\,\textrm{Mpc}^{-3}$, consistent with the idea that their
constituents may evolve to become massive ETGs in the centers of the rich
galaxy clusters we see today.
