The CO-to-H2 Conversion Factor From Infrared Dust Emission Across the Local Group
Abstract
We estimate the conversion factor relating CO emission to H2 mass, alphaCO, in five Local Group galaxies that span approximately an order of magnitude in metallicity - M31, M 33, the Large Magellanic Cloud (LMC), NGC 6822, and the Small Magellanic Cloud (SMC). We model the dust mass along the line of sight from infrared (IR) emission and then solve for the alphaCO that best allows a single gas-to-dust ratio (deltaGDR) to describe each system. This approach remains sensitive to CO-dark envelopes of H2 surrounding molecular clouds. In M 31, M 33, and the LMC we find alphaCO ≈ 3-9 Msun pc-2 (K km s-1)-1, consistent with the Milky Way value within the uncertainties. The two lowest metallicity galaxies in our sample, NGC 6822 and the SMC (12 + log(O/H) ≈ 8.2 and 8.0), exhibit a much higher alphaCO. Our best estimates are αNGC6822 ≈ 30 Msun/pc-2 (K km s-1)-1 and αSMC ≈ 70 Msun/pc-2 (K km s-1)-1. These results are consistent with the conversion factor becoming CO a strong function of metallicity around 12 + log(O/H) 8.4 - 8.2. We favor an interpretation where decreased dust-shielding leads to the dominance of CO-free envelopes around molecular clouds below this metallicity.