Giant molecular clouds and the CO-to-H2 conversion factor in IC 10

Abstract

Three molecular clouds have been mapped in the nearby metal-poor dwarf irregular galaxy IC 10 using the Owens Valley Millimeter-Wave Interferometer. The clouds have velocity widths, diameters, temperatures, and masses that are very similar to those of Galactic giant molecular clouds. However, the virial masses are substantially larger than the molecular masses. Two of these clouds obey the size-line width relation established for molecular clouds in the Galaxy and, thus, are probably gravitationally bound. Assuming these two clouds are self-gravitating and adopting a distance of 1 Mpc, the CO-to-H2 conversion factor of IC 10 is 6 x 10 to the 20th/sq cm per (K km/s), twice as large as the Galactic value. Thus, for a given mass of molecular gas, IC 10 is underluminous in CO by only a factor of 2 relative to the Galaxy. This observational result disagrees with recent theoretical predictions that the CO-to-H2 conversion factor should be a strong function of metallicity. 21 refs.