DENSE MOLECULAR GAS, STAR FORMATION, AND THE CO TO H-2 CONVERSION FACTOR IN NEARBY GALAXIES

A search for CS J = 2 → 1 emission from 14 nearby galaxies yielded six detections and five marginal detections. The detection of CS emission confirms the presence of large quantities of dense gas near the centers of nearby galaxies. The CS and CO 1 → 0 fluxes from the centers of galaxies are linearly correlated. The ratio of CS to CO flux is independent of galaxian parameters and can be understood as a consequence of a beam average of two populations of molecular clouds, one similar to Galactic disk clouds, the other similar to Galactic center clouds. Both CS and CO emission are correlated with central C II (λ158 μm) line emission and 100 μm continuum emission, indicating that both molecules trace active star formation. We conclude that the mass of dense gas traced by CS is approximately proportional to the total molecular mass and that a standard CO-H2 conversion factor produces a reasonable estimate of the H2 mass in most galaxies.