The CO-to-H2 Conversion Factor From Infrared Dust Emission Across the Local Group

TL;DR

This study estimates the CO-to-H2 conversion factor in five Local Group galaxies using infrared dust emission, revealing a strong dependence on metallicity and highlighting the increasing prevalence of CO-dark H2 envelopes in lower-metallicity environments.

Contribution

The paper introduces a dust-based method to estimate alpha_CO across galaxies with varying metallicity, extending understanding of molecular gas tracers in different environments.

Findings

Alpha_CO in M31, M33, and LMC is similar to the Milky Way.

Higher alpha_CO values in NGC 6822 and SMC indicate metallicity dependence.

Conversion factor increases significantly below 12+log(O/H) ~ 8.2.

Abstract

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.