The Inception of Star Cluster Formation Revealed by [CII] Emission
  Around an Infrared Dark Cloud
  We present SOFIA-upGREAT observations of [CII] emission of Infrared Dark
Cloud (IRDC) G035.39-00.33, designed to trace its atomic gas envelope and thus
test models of the origins of such clouds. Several velocity components of [CII]
emission are detected, tracing structures that are at a wide range of distances
in the Galactic plane. We find a main component that is likely associated with
the IRDC and its immediate surroundings. This strongest emission component has
a velocity similar to that of the $^{13}$CO(2-1) emission of the IRDC, but
offset by $\sim3\:{\rm km\:s}^{-1}$ and with a larger velocity width of
$\sim9\:{\rm km\:s}^{-1}$. The spatial distribution of the [CII] emission of
this component is also offset predominantly to one side of the dense
filamentary structure of the IRDC. The CII column density is estimated to be of
the order of $\sim10^{17}-10^{18}\,{\rm cm}^{-2}$. We compare these results to
the [CII] emission from numerical simulations of magnetized, dense gas
filaments formed from giant molecular cloud (GMC) collisions, finding similar
spatial and kinematic offsets. These observations and modeling of [CII] add
further to the evidence that IRDC G035.39-00.33 has been formed by a process of
GMC-GMC collision, which may thus be an important mechanism for initiating star
cluster formation.
