The Metallicity Dependence of the CO-to-H[TINF]2[/TINF] Conversion Factor from Observations of Local Group Galaxies

High-resolution CO maps of 9 molecular clouds in IC 10 are combined with thenew measurement of the distance to this nearby metal poor galaxy to measureaccurately the CO-to-H2 conversion factor. The result for IC 10 is combinedwith published data for four other Local Group galaxies (M31, M33, NGC 6822,the SMC) to trace the dependence of the CO-to-H2 conversion factor on oxygenabundance. These data show conclusively that the CO-to-H$_2$ conversion factorincreases as the metallicity of the host galaxy decreases, with the conversionfactor increasing by a factor of 4.6 for a factor of 10 decrease inmetallicity.Comment: 16 pages (including 2 figures), uuencoded compressed postscript file. Accepted in ApJ Letter