Dense Molecular Gas in Nearby Southern Starburst Galaxies

Abstract

With the Australia Telescope Compact Array, a sample of 8 galaxies have been searched for lines of ammonia, HCN, and HNC. Those molecules are known to trace high-density molecular gas, a gas phase that is closely related to associated star formation. For gas with densities ≳ 104 cm-3 we find that the temperature is around ∼ 30–50 K, independent of the surrounding starburst activity. The HCN/HNC ratio, however, becomes ∼1.5 where the starburst activity is strongest and increases by factors of a few within a few hundreds of pc. This line ratio gradient likely indicates a gradient of gas density, increasing toward starburst centers with values ∼106 cm-3 at the peaks. At a stable temperature this translates into higher pressures and pressure support may therefore play a larger role in the stability and formation of molecular clouds in starburst centers. We do not find that an AGN significantly influences the state of the dense molecular gas phase.