Do Spectroscopic Dense Gas Fractions Track Molecular Cloud Surface
  Densities?
  We use ALMA and IRAM 30-m telescope data to investigate the relationship
between the spectroscopically-traced dense gas fraction and the cloud-scale
(120 pc) molecular gas surface density in five nearby, star-forming galaxies.
We estimate the dense gas mass fraction at 650 pc and 2800 pc scales using the
ratio of HCN (1-0) to CO (1-0) emission. We then use high resolution (120 pc)
CO (2-1) maps to calculate the mass-weighted average molecular gas surface
density within 650 pc or 2770 pc beam where the dense gas fraction is
estimated. On average, the dense gas fraction correlates with the mass-weighted
average molecular gas surface density. Thus, parts of a galaxy with higher mean
cloud-scale gas surface density also appear to have a larger fraction of dense
gas. The normalization and slope of the correlation do vary from galaxy to
galaxy and with the size of the regions studied. This correlation is consistent
with a scenario where the large-scale environment sets the gas volume density
distribution, and this distribution manifests in both the cloud-scale surface
density and the dense gas mass fraction.
