Variations in the Star Formation Efficiency of the Dense Molecular Gas
  across the Disks of Star-Forming Galaxies
  We present a new survey of HCN(1-0) emission, a tracer of dense molecular
gas, focused on the little-explored regime of normal star-forming galaxy disks.
Combining HCN, CO, and infrared (IR) emission, we investigate the role of dense
gas in Star Formation (SF), finding systematic variations in both the apparent
dense gas fraction and the apparent SF efficiency (SFE) of dense gas. The
latter may be unexpected, given the popularity of gas density threshold models
to explain SF scaling relations. We used the IRAM 30-m telescope to observe
HCN(1-0) across 29 nearby disk galaxies whose CO(2-1) emission has previously
been mapped by the HERACLES survey. Because our observations span a range of
galactocentric radii, we are able to investigate the properties of the dense
gas as a function of local conditions. We focus on how the IR/CO, HCN/CO, and
IR/HCN ratios (observational cognates of the SFE, dense gas fraction, and dense
gas SFE) depend on the stellar surface density and the molecular/atomic ratio.
The HCN/CO ratio correlates tightly with these two parameters across a range of
2.1 dex and increases in the high surface density parts of galaxies.
Simultaneously, the IR/HCN ratio decreases systematically with these same
parameters and is ~6-8 times lower near galaxy centers than in the outer
regions. For fixed line-mass conversion factors, these results are incompatible
with a simple model in which SF depends only on the gas mass above some density
threshold. Only a specific set of environment-dependent conversion factors can
render our observations compatible with such a model. Whole cloud models, such
as the theory of turbulence regulated SF, do a better job of matching our data.
We explore one such model in which variations in the Mach number and in the
mean density would respectively drive the trends within galaxy disks and the
differences between disk and merging galaxies (abridged).
