Detection of molecular gas in an ALMA [C II]-identified submillimetre galaxy at z = 4.44

We present the detection of 12 CO(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 12 CO(2–1) emission, we estimatethegasmasstobeMgas ∼1.7 ×10 10 M� .Thegasdepletiontime-scaleofALESS65.1 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 [C II], 12 CO and farinfraredluminositiesimpliesastrongfar-ultravioletfieldofG0 ∼10 3.25 ,whichisatthehighend 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]/LFIR = 1.0 × 10 −3 observed in ALESS65.1, combined with L[C II]/LCO ∼ 2300, is consistent with ALESS65.1 having more extended regions of intense star formation than local ULIRGs.