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

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 ∼ 1.7 × 1010 M⊙. The gas depletion time-scale of ALESS65.1 is ∼ 25 Myr, 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], 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[CII]/LFIR=1.0×10−3 observed in ALESS65.1, combined with L[CII]/LCO∼2300, is consistent with ALESS65.1 having more extended regions of intense star formation than local ULIRGs