2014MNRAS.443L..54H

We present the detection of 12CO(2-1) in the z = 4.44 submillimetre galaxy ALESS65.1 using the Australia Telescope Compact Array. A previous Atacama Large Millimeter/submillimeter Array study of submillimetre galaxies (SMGs) in the Extended Chandra Deep Field South determined the redshift of this optically and near-infrared undetected source through the measurement of [C ii] 157.74 µm emission. Using the luminosity of the 12CO(2-1) emission, we estimate the gas mass to be Mgas ∼ 1.7x10¹⁰M_☉. The gas depletion time-scale of ALESS65.1 is ∼ 25Myr, similar to other high-redshift SMGs and consistent with z > 4 SMGs being the progenitors of massive `red-and-dead' galaxies at z > 2. The ratio of the [Cii], 12CO and far-infrared luminosities implies a strong far-ultraviolet field of G0 ∼ 103.25, which is at the high end of the far-ultraviolet fields seen in local starbursts, but weaker than the far-ultraviolet fields of most nearby ultraluminous infrared galaxies (ULIRGs). The high ratio of L_[C II]/L_FIR = 1.0 ×10^-3 observed in ALESS65.1, combined with L_[C II]/L_CO ∼2300, is consistent with ALESS65.1 having more extended regions of intense star formation than local ULIRGs.