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 Hi, CO, and optical IFU data from the MaNGA survey together with new FAST Hi observations. In agreement with previous results, our sample shows that the global molecular-to-atomic gas ratio ( R_mol≡log M_H_2/M_H_1 ) is correlated with the global stellar mass surface density μ* with a Kendall’s τ coefficient 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 Hi and CO global profile asymmetry and shape do not significantly rely on Rmol. Thanks to the availability of kpc-scale observations of MaNGA, we decompose galaxies into Hii, composite, and AGN-dominated regions by using the BPT diagrams. With increasing Rmol, the fraction of Hii 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 Rmol tend to have high oxygen abundance, both at one effective radius with a Kendall’s τ coefficient 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 Rmol. 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 Rmol indicates that the gas conversion is efficient in regions of high metallicity (density).