We present images obtained with the Atacama Pathfinder Experiment ( APEX ) telescope ’ s Large APEX BOlometer CAmera ( LABOCA ) of a sample of 22 galaxies selected via their red Herschel SPIRE ( Spectral and Photometric Imaging Receiver ) 250- , 350- and 500 - \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 { - } \micron { } survey covers an area of \approx 0.8 \textrm { deg } ^ { 2 } down to an average r.m.s .
of 3.9 \textrm { mJy } { } \textrm { beam } { } ^ { -1 } , with our five deepest maps going \approx 2 \times deeper still .
We catalog 86 dusty star-forming galaxies ( DSFGs ) around our ‘ signposts ’ , detected above a significance of 3.5 \sigma .
This implies a 100 ^ { +30 } _ { -30 } \% 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 \approx 95 % [ 50 % ] confident that these regions are over-dense by a factor of at least \geq 1.5 \times [ 2 \times ] .
Using template spectral energy distributions 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 \textrm { - - } 3.6 , somewhat higher than expected for \sim 850 \mu \textrm { m } -selected galaxies .
We constrain the DSFGs likely responsible for this over-density to within | \Delta z| \leq 0.65 of their respective signposts ; over half of our ultra-red targets ( \approx 55 \% ) have an average of two DSFGs within | \Delta z| \leq 0.5 .
These ‘ associated ’ DSFGs are radially distributed within ( physical ) distances of 1.6 \pm 0.5 \textrm { Mpc } from their signposts , have median star-formation rates ( SFRs ) of \approx ( 1.0 \pm 0.2 ) \times 10 ^ { 3 } M _ { \odot } \textrm { yr } ^ { -1 } ( for a Salpeter stellar initial mass function ) 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 } { } \textrm { yr } ^ { -1 } and space densities of \sim 9 \times 10 ^ { -7 } \textrm { 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 .