Multiwavelength characterisation of an ACT-selected, lensed dusty
  star-forming galaxy at z=2.64
  We present \ci\,(2--1) and multi-transition $^{12}$CO observations of a dusty
star-forming galaxy, ACT\,J2029+0120, which we spectroscopically confirm to lie
at $z$\,=\,2.64. We detect CO(3--2), CO(5--4), CO(7--6), CO(8--7), and
\ci\,(2--1) at high significance, tentatively detect HCO$^{+}$(4--3), and place
strong upper limits on the integrated strength of dense gas tracers (HCN(4--3)
and CS(7--6)). Multi-transition CO observations and dense gas tracers can
provide valuable constraints on the molecular gas content and excitation
conditions in high-redshift galaxies. We therefore use this unique data set to
construct a CO spectral line energy distribution (SLED) of the source, which is
most consistent with that of a ULIRG/Seyfert or QSO host object in the taxonomy
of the \textit{Herschel} Comprehensive ULIRG Emission Survey. We employ RADEX
models to fit the peak of the CO SLED, inferring a temperature of T$\sim$117 K
and $n_{\text{H}_2}\sim10^5$ cm$^{-3}$, most consistent with a ULIRG/QSO object
and the presence of high density tracers. We also find that the velocity width
of the \ci\ line is potentially larger than seen in all CO transitions for this
object, and that the $L'_{\rm C\,I(2-1)}/L'_{\rm CO(3-2)}$ ratio is also larger
than seen in other lensed and unlensed submillimeter galaxies and QSO hosts; if
confirmed, this anomaly could be an effect of differential lensing of a shocked
molecular outflow.
