H-ATLAS: A Candidate High Redshift Cluster/Protocluster of Star-Forming
  Galaxies
  We investigate the region around the Planck-detected z=3.26 gravitationally
lensed galaxy HATLAS J114637.9-001132 (hereinafter HATLAS12-00) using both
archival Herschel data from the H-ATLAS survey and using submm data obtained
with both LABOCA and SCUBA2. The lensed source is found to be surrounded by a
strong overdensity of both Herschel-SPIRE sources and submm sources. We detect
17 bright (S_870 >~7 mJy) sources at >4sigma closer than 5 arcmin to the lensed
object at 850/870 microns. Ten of these sources have good cross-identifications
with objects detected by Herschel-SPIRE which have redder colours than other
sources in the field, with 350 micron flux > 250 micron flux, suggesting that
they lie at high redshift. Submillimeter Array (SMA) observations localise one
of these companions to ~1 arcsecond, allowing unambiguous cross identification
with a 3.6 and 4.5 micron Spitzer source. The optical/near-IR spectral energy
distribution (SED) of this source is measured by further observations and found
to be consistent with z>2, but incompatible with lower redshifts. We conclude
that this system may be a galaxy cluster/protocluster or larger scale structure
that contains a number of galaxies undergoing starbursts at the same time.
