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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_{; ; [{; ; 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.

galaxies: evolution; galaxies: formation; radio lines: galaxies

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