The Co-to-H 2 Conversion Factor From Infrared Dust Emission Across the Local Group

We estimate the conversion factor relating CO emission to H 2 mass, α CO , in five Local Group galaxies that span approximately an order of magnitude in metallicity—M 31, 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 α CO that best allows a single gas-to-dust ratio (δ GDR ) to describe each system. This approach remains sensitive to CO-dark envelopes H 2 surrounding molecular clouds. In M 31, M 33, and the LMC we find α CO ≈ 3–9 M ☉ 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 α CO . Our best estimates are α NGC6822 CO ≈ 30 M ☉ pc −2 (K km s −1 ) −1 and α SMC CO ≈ 70 M ☉ pc −2 (K km s −1 ) −1 . These results are consistent with the conversion factor becoming 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.