The CO-to-H2 Conversion Factor From Infrared Dust Emission Across the
  Local Group
  We estimate the conversion factor relating CO emission to H2 mass, alpha_CO,
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 alpha_CO that best allows a
single gas-to-dust ratio (delta_GDR) 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 alpha_CO \approx 3-9 M_sun 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) \approx
8.2 and 8.0), exhibit a much higher alpha_CO. Our best estimates are
\alpha_NGC6822 \approx 30 M_sun/pc^-2 (K km s^-1)^-1 and \alpha_SMC \approx 70
M_sun/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) \sim
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.
