New Insights into the Physical Conditions and Internal Structure of a
  Candidate Proto-Globular Cluster
  We present $\sim$0.1" resolution ($\sim$10 pc) ALMA observations of a
molecular cloud identified in the merging Antennae galaxies with the potential
to form a globular cluster, nicknamed the ``Firecracker.' Since star formation
has not yet begun at an appreciable level, this cloud provides an example of
what the birth environment of a globular cluster may have looked like before
stars form and disrupt the natal conditions. Using emission from
$^{12}$CO(2-1), $^{12}$CO(3-2), $^{13}$CO(2-1), HCN(4-3), and HCO$^+$(4-3), we
are able to resolve the cloud's structure and find that it has a characteristic
radius of 22 pc and a mass of 1--9$\times10^6 M_\odot$. We also constrain the
abundance ratios of $^{12}$CO/$^{13}$CO and H$_2$/\twelveCO. Based on the
calculated mass, we determine that the commonly used CO-to-H$_2$ conversion
factor varies spatially, with average values in the range
$X_{CO}=(0.12-1.1)\times10^{20}$ cm$^{-2}$ (K km s$^{-1}$)$^{-1}$. We
demonstrate that if the cloud is bound (as is circumstantially suggested by its
bright, compact morphology), an external pressure of $P/k > 10^8$ K cm$^{-3}$
is required. This would be consistent with theoretical expectations that
globular cluster formation requires high pressure environments. The
position-velocity diagram of the cloud and its surrounding material suggests
that this pressure may be produced by the collision of filaments. The radial
profile of the column density can be fit with both a Gaussian and Bonnor-Ebert
profile. The relative line strengths of HCN and HCO$^+$ in this region suggest
that these molecular lines can be used as tracers for the evolutionary stage of
a cluster.
