A millimetre-wave redshift search for the unlensed HyLIRG, HS1700.850.1

We report the redshift of an unlensed, highly obscured submillimetre galaxy (SMG), HS1700.850.1, the brightest SMG (S850 μm = 19.1 mJy) detected in the James Clerk Maxwell Telescope/Submillimetre Common-user Bolometer Array-2 (JCMT/SCUBA-2) Baryonic Structure Survey, based on the detection of its 12CO line emission. Using the Institute Radio Astronomie Millimetrique Plateau de Bure Interferometer with 3.6 GHz band width, we serendipitously detect an emission line at 150.6 GHz. From a search over 14.5 GHz in the 3- and 2-mm atmospheric windows, we confirm the identification of this line as 12CO(5–4) at z = 2.816, meaning that it does not reside in the z ∼ 2.30 proto-cluster in this field. Measurement of the 870 μm source size (<0.85 arcsec) from the Sub-Millimetre Array (SMA) confirms a compact emission in a S870 μm = 14.5 mJy, LIR ∼ 1013 L⊙ component, suggesting an Eddington-limited starburst. We use the double-peaked 12CO line profile measurements along with the SMA size constraints to study the gas dynamics of a HyLIRG, estimating the gas and dynamical masses of HS1700.850.1. While HS1700.850.1 is one of the most extreme galaxies known in the Universe, we find that it occupies a relative void in the Lyman-Break Galaxy distribution in this field. Comparison with other extreme objects at similar epochs (HyLIRG Quasars), and cosmological simulations, suggests such an anti-bias of bright SMGs could be relatively common, with the brightest SMGs rarely occupying the most overdense regions at z = 2–4.