Molecular line signatures of cloud-cloud collisions
  Collisions between interstellar gas clouds are potentially an important
mechanism for triggering star formation. This is because they are able to
rapidly generate large masses of dense gas. Observationally, cloud collisions
are often identified in position-velocity (PV) space through bridging features
between intensity peaks, usually of CO emission. Using a combination of
hydrodynamical simulations, time-dependent chemistry, and radiative transfer,
we produce synthetic molecular line observations of overlapping clouds that are
genuinely colliding, and overlapping clouds that are just chance
superpositions. Molecules tracing denser material than CO, such as NH$_3$ and
HCN, reach peak intensity ratios of $0.5$ and $0.2$ with respect to CO in the
`bridging feature' region of PV space for genuinely colliding clouds. For
overlapping clouds that are just chance superpositions, the peak NH$_3$ and HCN
intensities are co-located with the CO intensity peaks. This represents a way
of confirming cloud collisions observationally, and distinguishing them from
chance alignments of unrelated material.
