The ratio of molecular to atomic gas in infrared luminous galaxies

In infrared luminous galaxies the ratio of the CO(1 - 0) to H I integrated fluxes increases with the far-infrared excess, f(fir)/f(b). All infrared active galaxies with f(fir)/f(b) greater than 2 have molecular to atomic gas mass fractions greater than 0.5. Among the galaxies with the higher infrared excesses there are systems with strikingly small atomic mass fractions, where less than 15 percent of the total mass of interstellar gas is in atomic form. The optical morphology of luminous infrared galaxies indicates that the majority, if not all, of these objects are interacting systems. These observations suggest that the overall mass fraction of molecular to atomic gas, and the infrared luminosities per nucleon of interstellar gas are enhanced during galaxy-galaxy interactions.