The Metallicity Dependence of the CO-to-H2 Conversion Factor from Observations of Local Group Galaxies

Six additional molecular clouds have been mapped in the nearby metal-poor dwarf irregular galaxy IC 10 using the Owens Valley Millimeter-Wave Interferometer. Since a good distance measurement is now available for IC 10, it is possible to measure accurately the CO-to-H2 conversion factor in this galaxy. The result for IC 10 is combined with published data for four other Local Group galaxies (M31, M33, NGC 6822, and the SMC) to trace the dependence of the CO-to-H2 conversion factor on the oxygen abundance using a well-defined sample of extragalactic giant molecular clouds. These data show conclusively that the CO-to-H2 conversion factor increases as the metallicity of the host galaxy decreases, with the conversion factor increasing by a factor of 4.6 for a factor of 10 decrease in metallicity. This new calibration of the effect of metallicity on the CO-to-H2 conversion factor will allow more accurate molecular gas masses to be obtained in galaxies and regions of galaxies with moderately low metallicity.