2025MNRAS.536.2392C

The CO-to-H$_2$ conversion factor ($\alpha _\mathrm{CO}$) is expected to vary with dust abundance and grain size distribution through the efficiency of shielding gas from CO-dissociation radiation. We present a comprehensive analysis of $\alpha _\mathrm{CO}$ and grain size distribution for nearby galaxies, using the polycyclic aromatic hydrocarbon (PAH) fraction ($q_\mathrm{PAH}$) as an observable proxy of grain size distribution. We adopt the resolved observations at 2 kpc resolution in 42 nearby galaxies, where $\alpha _\mathrm{CO}$ is derived from measured metallicity and surface densities of dust and H I assuming a fixed dust-to-metal ratio. We use an analytical model for the evolution of H$_2$ and CO, in which the evolution of grain size distribution is controlled by the dense-gas fraction ($\eta$). We find that the observed level of $q_\mathrm{PAH}$ is consistent with the diffuse-gas-dominated model ($\eta =0.2$) where dust shattering is more efficient. Meanwhile, the slight decreasing trend of observed $q_\mathrm{PAH}$ with metallicity is more consistent with high-$\eta$ predictions, likely due to the more efficient loss of PAHs by coagulation. We discuss how grain size distribution (indicated by $q_\mathrm{PAH}$) and metallicity impact $\alpha _\mathrm{CO}$; we, however, did not obtain conclusive evidence that the grain size distribution affects $\alpha _\mathrm{CO}$. Observations and model predictions show similar anticorrelation between $\alpha _\mathrm{CO}$ and 12 + log(O/H). Meanwhile, there is a considerable difference in how resolved $\alpha _\mathrm{CO}$ behaves with $q_\mathrm{PAH}$. The observed $\alpha _\mathrm{CO}$ has a positive correlation with $q_\mathrm{PAH}$, while the model-predicted $\alpha _\mathrm{CO}$ does not have a definite correlation with $q_\mathrm{PAH}$. This difference is likely due to the limitation of one-zone treatment in the model.