The ALMaQUEST Survey XV: The Dependence of the Molecular-to-Atomic Gas Ratios on Resolved Optical Diagnostics

This is the authors' abstract. We don't add key points for this paper.

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 MaNGA survey together with new FAST H I observations. In agreement with previous results, our sample shows that the global molecular-to-atomic gas ratio (Rₘₒₗ ≡ log MH₂/MH\ I) 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 H I 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 H II, composite, and AGN-dominated regions by using the BPT diagrams. With increasing Rₘₒₗ, 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 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, but the dependence of gas conversion on gas distribution and galaxy ionization states is weak.