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 μm survey covers an area of ≈1
deg2 down to an average rms of 3.9 {mJy}
{beam}}-1, 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
3.5σ. This implies a {100}-30+30 %
overdensity of {S}870> 8.5 {mJy} (or
{L}FIR}=6.7× {10}12{--}2.9×
{10}13 {L}⊙ ) 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 μm selected
galaxies. We constrain the DSFGs that are likely responsible for this
overdensity to within | {{Δ }}z| ≤slant 0.65 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 ≈ (1.0+/-
0.2)× {10}3 {M}⊙ {yr}}-1
(for a Salpeter stellar inital mass function) and median gas reservoirs
of ˜ 1.7× {10}11 {M}⊙ . These
candidate protoclusters have average total SFRs of at least ≈ (2.3+/-
0.5)× {10}3 {M}⊙ {yr}}-1
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.
