A blind CO detection of a distant red galaxy in the HS1700+64 protocluster

Abstract

We report the blind detection of ^(12)CO 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 ^(12)CO(3–2) from galaxies lying in the protocluster at z = 2.300 in the field HS1700+64. If indeed this line in DRG55 is ^(12)CO(3–2), its detection at 104.9 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 ^(12)CO, although the limits are ∼2 × worse towards the edge of the PB where several lie. The optical/near-IR magnitudes of DRG55 (R_AB > 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 (210 M_⊙ yr^−1), stellar mass (∼10^11 M_⊙) and ^(12)CO line luminosity (3.6 × 10^10 K km s^−1  pc^2) are comparable to other normal ^(12)CO-detected star-forming galaxies in the literature, although the galaxy is some ∼2 mag (∼6 ×) fainter in the rest-frame UV than ^(12)CO-detected galaxies at z > 2. The detection of DRG55 in ^(12)CO complements three other ^(12)CO 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 ^(12)CO in this overdense environment is warranted.

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