2011ApJ...739...97H

We combine data from the Australia Telescope National Facility and Swedish ESO Submillimeter Telescope to investigate the neutral interstellar medium (ISM) in AM0644-741, a large and robustly star-forming ring galaxy. The galaxy's ISM is concentrated in the 42 kpc diameter starburst ring, but appears dominated by atomic gas, with a global molecular fraction (f_mol) of only 0.062 ± 0.005. Apart from the starburst peak, the gas ring appears stable against the growth of gravitational instabilities (Q_gas= 3-11). Including the stellar component lowers Q overall, but not enough to make Q < 1 everywhere. High star formation efficiencies (SFEs) follow from the ring's low H₂content. AM0644-741's star formation law is highly peculiar: H I obeys a Schmidt law while H₂ is uncorrelated with star formation rate density. Photodissociation models yield low volume densities in the ring, especially in the starburst quadrant (n ~ 2/cm3), implying a warm neutral medium dominated ISM. At the same time, the ring's pressure and ambient far-ultraviolet radiation field lead to the expectation of a predominantly molecular ISM. We argue that the ring's high SFE, low f_mol and n, and peculiar star formation law follow from the ISM's > ∼ 100 Myr confinement time in the starburst ring, which amplifies the destructive effects of embedded massive stars and supernovae. As a result, the ring's molecular ISM becomes dominated by small clouds, causing M_H2 to be significantly underestimated by ¹²CO line fluxes: in effect, X_CO{Gt} X_Gal despite the ring's ≥ solar metallicity. The observed H I is primarily a low-density photodissociation product, i.e., a tracer rather than a precursor of massive star formation. Such an "over-cooked" ISM may be a general characteristic of evolved starburst ring galaxies.