Dense molecular gas properties on 100 pc scales across the disc of NGC
  3627
  It is still poorly constrained how the densest phase of the interstellar
medium varies across galactic environment. A large observing time is required
to recover significant emission from dense molecular gas at high spatial
resolution, and to cover a large dynamic range of extragalactic disc
environments. We present new NOrthern Extended Millimeter Array (NOEMA)
observations of a range of high critical density molecular tracers (HCN, HNC,
HCO+) and CO isotopologues (13CO, C18O) towards the nearby (11.3 Mpc), strongly
barred galaxy NGC 3627. These observations represent the current highest
angular resolution (1.85"; 100 pc) map of dense gas tracers across a disc of a
nearby spiral galaxy, which we use here to assess the properties of the dense
molecular gas, and their variation as a function of galactocentric radius,
molecular gas, and star formation. We find that the HCN(1-0)/CO(2-1) integrated
intensity ratio does not correlate with the amount of recent star formation.
Instead, the HCN(1-0)/CO(2-1) ratio depends on the galactic environment, with
differences between the galaxy centre, bar, and bar end regions. The dense gas
in the central 600 pc appears to produce stars less efficiently despite
containing a higher fraction of dense molecular gas than the bar ends where the
star formation is enhanced. In assessing the dynamics of the dense gas, we find
the HCN(1-0) and HCO+(1-0) emission lines showing multiple components towards
regions in the bar ends that correspond to previously identified features in CO
emission. These features are co-spatial with peaks of Halpha emission, which
highlights that the complex dynamics of this bar end region could be linked to
local enhancements in the star formation.
