A blind CO detection of a Distant Red Galaxy in the HS1700+64
  proto-cluster
  We report the blind detection of 12CO emission from a Distant Red Galaxy,
HS1700.DRG55. We have used the IRAM PdBI-WIDEX, with its 3.6GHz of
instantaneous dual-polarization bandwidth, to target 12CO(3--2) from galaxies
lying in the proto-cluster at z=2.300 in the field HS1700+64. If indeed this
line in DRG55 is 12CO(3--2), it's detection at 104.9GHz indicates a z_CO=2.296.
None of the other eight known z~2.30 proto-cluster galaxies lying within the
primary beam (PB) are detected in 12CO, although the limits are ~2x worse
towards the edge of the PB where several lie. The optical/near-IR magnitudes of
DRG55 (R_AB>27, K_AB=22.3) mean that optical spectroscopic redshifts are
difficult with 10m-class telescopes, but near-IR redshifts would be feasible.
The 24um-implied SFR (210 M_odot yr-1), stellar mass (~10^11 M-odot) and 12CO
line luminosity (3.6x10^10 K km s-1 pc^2) are comparable to other normal
12CO-detected star forming galaxies in the literature, although the galaxy is
some ~2 mag (~6x) fainter in the rest-frame UV than 12CO-detected galaxies at
z>2. The detection of DRG55 in 12CO complements three other 12CO detected
UV-bright galaxies in this proto-cluster from previous studies, and suggests
that many optically faint galaxies in the proto-cluster may host substantial
molecular gas reservoirs, and a full blind census of 12CO in this overdense
environment is warranted.
