Star Formation Occurs in Dense Gas, but What Does "Dense" Mean?
  We report results of a project to map HCN and HCO+ J = 1-0 emission toward a
sample of molecular clouds in the inner Galaxy, all containing dense clumps
that are actively engaged in star formation. We compare these two molecular
line tracers with millimeter continuum emission and extinction, as inferred
from 13CO, as tracers of dense gas in molecular clouds. The fraction of the
line luminosity from each tracer that comes from the dense gas, as measured by
AV > 8 mag, varies substantially from cloud to cloud. In all cases, a
substantial fraction (in most cases, the majority) of the total luminosity
arises in gas below the AV > 8 mag threshold and outside the region of strong
mm continuum emission. Measurements of the luminosity of HCN toward other
galaxies will likely be dominated by such gas at lower surface density.
Substantial, even dominant, contributions to the total line luminosity can
arise in gas with densities typical of the cloud as a whole (densities about
100 per cubic cm). Defining the dense clump from the HCN or HCO+ emission
itself, similarly to previous studies, leads to a wide range of clump
properties, with some being considerably larger and less dense than in previous
studies. HCN and HCO+ have similar ability to trace dense gas for the clouds in
this sample. For the two clouds with low virial parameters, the 13CO is
definitely a worse tracer of the dense gas, but for the other four, it is
equally good (or bad) at tracing dense gas.
