The ALMaQUEST Survey ⅩⅤ:The dependence of the molecular-to-atomic gas ratios on resolved optical diagnostics

The atomic-to-molecular gas conversion is a critical step in the baryon cycle of galaxies,which sets the initial conditions for subsequent star formation and influences the multi-phase interstellar medium.We compiled a sample of 94 nearby galaxies with observations of multi-phase gas contents by utilizing public H_(Ⅰ),CO,and optical IFU data from the Ma NGA survey together with new FAST H_(Ⅰ)observations.In agreement with previous results,our sample shows that the global molecular-to-atomic gas ratio(R_(mol)≡log M_(H2)/M_(H_(Ⅰ)))is correlated with the global stellar mass surface densityμ_*with a Kendall'sτcoefficient of 0.25 and p<10^(-3),less tightly but still correlated with stellar mass and NUV-r color,and not related to the specific star formation rate(sSFR).The cold gas distribution and kinematics inferred from the H_(Ⅰ)and CO global profile asymmetry and shape do not significantly rely on R_(mol).Thanks to the availability of kpc-scale observations of MaNGA,we decompose galaxies into H_(Ⅱ),composite,and AGN-dominated regions by using the BPT diagrams.With increasing R_(mol),the fraction of H_(Ⅱ)regions within 1.5 effective radius decreases slightly;the density distribution in the spatially resolved BPT diagram also changes significantly,suggesting changes in metallicity and ionization states.Galaxies with high R_(mol)tend to have high oxygen abundance,both at one effective radius with a Kendall'sτcoefficient of 0.37(p<10^(-3))and their central regions.Among all parameters investigated here,the oxygen abundance at one effective radius has the strongest relation with global R_(mol).The dependence of gas conversion on gas distribution and galaxy ionization states is weak.In contrast,the observed positive relation between oxygen abundance(μ_(*))and R_(mol)indicates that the gas conversion is efficient in regions of high metallicity(density).