Detection of molecular gas in an ALMA [CII]-identified Submillimetre Galaxy at z = 4.44

Abstract

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 ALMA study of submillimetre galaxies in the Extended Chandra Deep Field South determined the redshift of this optically and near-infrared undetected source through the measurement of [CII] 157.74 $\mu$m emission. Using the luminosity of the ${}^{12}$CO(2-1) emission we estimate the gas mass to be $M_{\mathrm{gas}}\sim 1.7\times 10^{10}$ ${\mathrm{M}}_{\odot }$. The gas depletion timescale of ALESS65.1 is $\sim$ 25 Myr, similar to other high redshift submillimetre galaxies and consistent with $z>4$ SMGs being the progenitors of massive "red-and-dead" galaxies at $z>2$. The ratio of the [CII], ${}^{12}$CO and far-infrared luminosities implies a strong far-ultraviolet field of $G_0\sim 10^{3.25}$, 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 ULIRGs. The high ratio of $L_{[\mathrm{CII}]}/L_{\mathrm{FIR}}=1.0\times 10^{-3}$ observed in ALESS65.1, combined with $L_{[\mathrm{CII}]}/L_{\mathrm{CO}}\sim 2300$, is consistent with ALESS65.1 having more extended regions of intense star formation than local ULIRGs.

6 pages, 3 figures, accepted for publication in MNRAS. This article supersedes arXiv:1302.3297

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• Ionized Carbon in Galaxies: The [C II] 158 $\mu$m Line as a Total Molecular Gas Mass Tracer Revisited
• Direct detection of cool molecular gas in a star-forming galaxy at $z=7.31$