Phase transition between atomic and molecular hydrogen in nearby spiral galaxies

We compared theoretical and observational molecular mass fractions (f mol: ratio of molecular gas density to total gas density) using observational data of ten nearby spiral galaxies. For determination of f mol, the three parameters–interstellar pressure P, UV radiation U, and metallicity Z–were obtained from the spectral line data of 12CO(J = 1–0), H i, Hα, [O iii], and [O ii]. Interstellar pressure was calculated with the sum of the hydrogen gas densities and the stellar potential based on the K s -band data. For most data other than metallicity, we used archived NRO CO Atlas, THINGS, SINGS, and 2MASS data. For comparison, we also investigated the dependence of the CO-to-H2 conversion factor X CO. It was found that the theoretical f mol agreed with the observational f mol only when the interstellar pressure is calculated with both the gas density and stellar disk potential. To fit observations more accurately, either the metallicity or the UV radiation needs to be adjusted. It was also found that, in UV radiation scaling, scaling factor γ has a correlation with the diffuse fraction of the Hα emission line data, f DIG. As for X CO, it was shown that the difference between both values of f mol becomes the least when X CO is 1.0 × 1020 cm−2 (K km s−1)−1.