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

Abstract

The CO-to-H 2 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 polycyclic aromatic hydrocarbon (PAH) fraction ( q PAH ) 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-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 ( η). We find that the observed level of q PAH is consistent with the diffuse-gas-dominated model ( η= 0 . 2) where dust shattering is more efficient. Meanwhile, the slight decreasing trend of observed q PAH with metallicity is more consistent with high- ηpredictions, likely due to the more efficient loss of PAHs by coagulation. We discuss how grain size distribution (indicated by q PAH ) and metallicity impact αCO ; we, ho we ver, did not obtain conclusive evidence that the grain size distribution affects αCO . Observations and model predictions show similar anticorrelation between αCO and 12 + log(O/H). Meanwhile, there is a considerable difference in how resolved αCO behaves with q PAH . The observed αCO has a positive correlation with q PAH , while the model-predicted αCO does not have a definite correlation with q PAH . This difference is likely due to the limitation of one-zone treatment in the model.