2015MNRAS.449L..68C

We report the blind detection of 12CO emission from a distant red galaxy, HS1700.DRG55. We have used the IRAM Plateau de Bure Interferometer WideX, with its 3.6 GHz of instantaneous dual-polarization bandwidth, to target 12CO(3-2) from galaxies lying in the protocluster at z = 2.300 in the field HS1700+64. If indeed this line in DRG55 is 12CO(3-2), its detection at 10^4.9 GHz indicates zCO = 2.296. None of the other eight known z ∼ 2.30 protocluster galaxies lying within the primary beam (PB) are detected in 12CO, although the limits are ∼ 2xworse towards the edge of the PB where several lie. The optical/near-IR magnitudes of DRG55 (RAB > 27, KAB = 22.3) mean that optical spectroscopic redshifts are difficult with 10-m-class telescopes, but near-IR redshifts would be feasible. The 24-µm-implied star formation rate (210M_☉/yr), stellar mass ( ∼ 10¹¹M_☉) and 12CO line luminosity (3.6x10¹⁰K km/spc2) are comparable to other normal 12CO-detected star-forming galaxies in the literature, although the galaxy is some ∼ 2 mag ( ∼ 6 x) 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 protocluster from previous studies, and suggests that many optically faint galaxies in the protocluster may host substantial molecular gas reservoirs, and a full blind census of 12CO in this overdense environment is warranted.