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, 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.

• The paper quantifies a strong metallicity dependence of the CO-to-H2 conversion factor, with values ranging from 3–9 M⊙ pc⁻² (K km s⁻¹)⁻¹ in high-metallicity galaxies to as high as 30–70 in low-metallicity environments.
• The paper employs a methodology combining infrared dust emission with HI and CO data to accurately trace molecular hydrogen, particularly exposing CO-dark regions in metal-poor systems.
• The paper confirms a linear relationship between gas-to-dust ratios and metallicity, urging recalibration of molecular gas models for low-metallicity galaxies.

The CO-to-H$_2$ Conversion Factor from Infrared Dust Emission Across the Local Group

This paper by Leroy et al. investigates the relationship of CO emission to molecular hydrogen mass, commonly known as the CO-to-H$_2$ conversion factor, $\alpha_{\text{CO}}$, across five Local Group galaxies: M31, M33, the Large Magellanic Cloud (LMC), NGC 6822, and the Small Magellanic Cloud (SMC). These galaxies span an order of magnitude in metallicity, providing a robust test of how $\alpha_{\text{CO}}$ varies with chemical abundance.

Methodology

The authors employ an approach that leverages infrared (IR) dust emission to estimate the dust mass along the line of sight. By combining this with measurements of atomic gas (HI) and CO emission, they solve for the $\alpha_{\text{CO}}$ that allows a single gas-to-dust ratio ($\delta_{\text{GDR}}$) to characterize each system. This methodology is designed to be particularly sensitive to "CO-dark" regions—molecular clouds where the outer envelope is not CO-luminous, typically a significant concern in lower-metallicity environments.

Key Findings

1. Metallicity Dependence: There is a clear trend indicating a strong dependence of $\alpha_{\text{CO}}$ on metallicity across different galaxies. For galaxies with metallicities above $12+\log (\text{O/H}) \approx 8.4$, $\alpha_{\text{CO}}$ is approximately consistent with Milky Way values, ranging from 3 to 9 M$_\odot$ pc$^{-2}$ (K km s$^{-1}$)$^{-1}$. However, for the lower metallicity environments in NGC 6822 and the SMC, $\alpha_{\text{CO}}$ increases significantly to about 30 and 70 M$_\odot$ pc$^{-2}$ (K km s$^{-1}$)$^{-1}$ respectively.
2. IR-Derived Gas Estimation: By solving for $\alpha_{\text{CO}}$ using dust as a tracer of $\text{H}_2$, the study confirms that dust-derived gas estimates are effective even in environments where CO emission might underestimate molecular content due to CO-dark gas.
3. $\delta_{\text{GDR}}$ Consistency: The analysis found consistency in $\delta_{\text{GDR}}$ as a linear function of metallicity, confirming the theory that a fixed fraction of metals remains in dust form across different environments.

Implications and Future Directions

This study provides substantial evidence that $\alpha_{\text{CO}}$ becomes a strong function of metallicity below $12+\log ({\rm O/H}) \approx 8.4$. The authors favor an interpretation where decreased dust-shielding in low-metallicity environments leads to the predominance of CO-free $\text{H}_2$ envelopes. This shift challenges the assumption that CO reliably traces $\text{H}_2$ across all galactic environments and necessitates recalibration when dealing with metal-poor galaxies.

Practically, these results imply that models of star formation and molecular cloud evolution must incorporate metallicity-dependent conversions to be accurate. The findings compel a reevaluation of molecular gas content in the early universe, where metallicities were presumably lower.

Future advancements, particularly those utilizing data from observatories like ALMA and the Herschel Space Observatory, are likely to refine our understanding of these trends. In particular, more precise quantification of dust emissivity variations and $\delta_{\text{GDR}}$ across diverse environments would help to eliminate prevailing uncertainties and enable a comprehensive model of molecular cloud chemistry across cosmic time.

Conclusion

In sum, Leroy et al. have provided valuable insights into the behavior of the CO-to-H$_2$ conversion factor across the Local Group galaxies, highlighting the importance of metallicity in determining $\alpha_{\text{CO}}$. Their work underscores the need for adaptable methodologies in galactic studies, particularly when extrapolating findings to diverse cosmic environments. With ongoing surveys and technological advancements, the understanding of interstellar medium processes is poised for significant progress.