Physical conditions of the gas in an ALMA [C II]-identified submillimetre galaxy at z = 4.44

We present 12CO(2-1) observations of the submillimetre galaxy ALESS65.1 performed with the Australia Telescope Compact Array (ATCA) at 42.3 GHz. A previous Atacama Large Millimeter Array study of submillimetre galaxies (SMGs) in the Extended Chandra Deep Field South detected [C II] 157.74 μmemission from this galaxy at a redshift of z=4.44. No 12CO(2- 1) emission was detected but we derive a firm upper limit to the cold gas mass in ALESS65.1 of Mgas >1.7 × 10¹⁰M . The estimated gas depletion time-scale is >50 Myr, which is similar to other high-redshift SMGs, and consistent with z > 4 SMGs being the likely 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, as seen in Galactic star-forming regions or local ultraluminous infrared galaxies (ULIRGs). The observed L[C II]/LFIR = 2.3 × 10^-3 is high compared to local ULIRGs and, combined withL[C II]/LCO 2700, it is consiste×nt with ALESS65.1 either having an extended (several kpc) [C II] emitting region or lower than solar metallicity.