Detection of molecular gas in an ALMA [CII]-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 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 $M_{\rm gas} \sim
1.7 \times 10^{10}$ ${\rm M}_\odot$. 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 $L_{\rm [CII]}/L_{\rm FIR} = 1.0 \times 10^{-3}$ observed in
ALESS65.1, combined with $L_{\rm [CII]}/L_{\rm CO} \sim 2300$, is consistent
with ALESS65.1 having more extended regions of intense star formation than
local ULIRGs.
