THE METALLICITY DEPENDENCE OF THE CO-TO-H-2 CONVERSION FACTOR FROM OBSERVATIONS OF LOCAL GROUP GALAXIES

Six additional molecular clouds have been mapped in the nearby metal-p oor dwarf irregular galaxy IC 10 using the Owens Valley Millimeter-Wav e Interferometer. Since a good distance measurement is now available f or IC 10, it is possible to measure accurately the CO-to-H-2 conversio n factor in this galaxy. The result for IC 10 is combined with publish ed data for four other Local Group galaxies (M31, M33, NGC 6822, and t he SMC) to trace the dependence of the CO-to-H-2 conversion factor on the oxygen abundance using a well-defined sample of extragalactic gian t molecular clouds. These data show conclusively that the CO-to-H-2 co nversion factor increases as the metallicity of the host galaxy decrea ses, with the conversion factor increasing by a factor of 4.6 for a fa ctor of 10 decrease in metallicity. This new calibration of the effect of metallicity on the CO-to-H-2 conversion factor will allow more acc urate molecular gas masses to be obtained in galaxies and regions of g alaxies with moderately low metallicity.