Ratio of the HCN and CO conversion factors (given in Eq. (19)) as a function of the ratio of the HCN and CO intensities, where we consider α_HCN as a conversion factor for the total mass above n > 10^3.5 cm⁻³ (left) and n > 10^4.5 cm⁻³ (right). For comparison, we plot α_HCN/α_CO = 3.2 (solid black line), which is the ratio of the Gao & Solomon (2004a,b) conversion factor and Milky Way CO conversion factor. The formatting is the same as in Fig. 3. This figure demonstrates how emissivity is sensitive to the intensity per mass traced by a particular transition, whereas luminosity-to-mass conversion factors account for additional factors that allow us to estimate specific masses (e.g., total gas mass and dense gas mass) that may not be fully reflected in the molecular emissivity. We find that due to the subthermal excitation of HCN J = 1–0, this transition is a poor tracer of the of the gas mass above n > 10^4.5 cm⁻³ and is a better tracer of moderate gas densities (n > 10^3.5 cm⁻³), as found in previous observational studies.