2008ApJ...673..832M

With a goal toward deriving the physical conditions in external galaxies, we present a survey of the formaldehyde emission in a sample of starburst systems. By extending a technique used to derive the spatial density in star formation regions in our own Galaxy, we show how the relative intensity of the 1₁₀-1₁₁ and 2₁₁-2₁₂K-doublet transitions of H₂CO can provide an accurate densitometer for the active star formation environments found in starburst galaxies. Relying on an assumed kinetic temperature and cospatial emission and absorption from both H₂ CO transitions, our technique is applied to a sample of 19 infrared-bright galaxies which exhibit various forms of starburst activity. In the five galaxies of our sample where both H₂ CO transitions were detected, we have derived spatial densities. We also use H₂ CO to estimate the dense gas mass in our starburst galaxy sample, finding similar mass estimates for the dense gas-forming stars in these objects as derived using other dense gas tracers. A related trend can be seen when one compares L_IR to our derived n(H₂) for the five galaxies within which we have derived spatial densities. Even though our number statistics are small, there appears to be a trend toward higher spatial density for galaxies with higher infrared luminosity. This is likely another representation of the L_IR-M_densecorrelation.