CO-to-H2 conversion factor and grain size distribution through the analysis of αCO–qPAH relation

The CO-to-H2 conversion factor (αCO) 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 αCO and grain size distribution for nearby galaxies, using the PAH fraction (qPAH) as an observable proxy of grain size distribution. We adopt the resolved observations at 2 kpc resolution in 42 nearby galaxies, where αCO is derived from measured metallicity and surface densities of dust and H i assuming a fixed dust-to-metals ratio. We use an analytical model for the evolution of H2 and CO, in which the evolution of grain size distribution is controlled by the dense gas fraction (η). We find that the observed level of qPAH is consistent with the diffuse-gas-dominated model (η = 0.2) where dust shattering is more efficient. Meanwhile, the slight decreasing trend of observed qPAH with metallicity is more consistent with high-η predictions, likely due to the more efficient loss of PAHs by coagulation. We discuss how grain size distribution (indicated by qPAH) and metallicity impact αCO; we however did not obtain conclusive evidence that the grain size distribution affects αCO. Observations and model predictions show similar anti-correlation between αCO and 12+log(O/H). Meanwhile, there is a considerable difference in how resolved αCO behaves with qPAH. The observed αCO has a positive correlation with qPAH, while the model-predicted αCO does not have a definite correlation with qPAH. This difference is likely due to the limitation of one-zone treatment in the model.