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We report results of a project to map HCN and HCO+ J = 1-0 emission toward asample of molecular clouds in the inner Galaxy, all containing dense clumpsthat are actively engaged in star formation. We compare these two molecularline tracers with millimeter continuum emission and extinction, as inferredfrom 13CO, as tracers of dense gas in molecular clouds. The fraction of theline luminosity from each tracer that comes from the dense gas, as measured byAV > 8 mag, varies substantially from cloud to cloud. In all cases, asubstantial fraction (in most cases, the majority) of the total luminosityarises in gas below the AV > 8 mag threshold and outside the region of strongmm continuum emission. Measurements of the luminosity of HCN toward othergalaxies will likely be dominated by such gas at lower surface density.Substantial, even dominant, contributions to the total line luminosity canarise in gas with densities typical of the cloud as a whole (densities about100 per cubic cm). Defining the dense clump from the HCN or HCO+ emissionitself, similarly to previous studies, leads to a wide range of clumpproperties, with some being considerably larger and less dense than in previousstudies. HCN and HCO+ have similar ability to trace dense gas for the clouds inthis sample. For the two clouds with low virial parameters, the 13CO isdefinitely a worse tracer of the dense gas, but for the other four, it isequally good (or bad) at tracing dense gas.

Star Formation Occurs in Dense Gas, but What Does "Dense" Mean?