PHANGS-MeerKAT and MHONGOOSE HI observations of nearby spiral galaxies: physical drivers of the molecular gas fraction, Rₘₒₗ

The molecular-to-atomic gas ratio is crucial to the evolution of the interstellar medium in galaxies. We investigate the balance between the atomic (ΣHI) and molecular gas (ΣH2) surface densities in eight nearby star-forming galaxies using new high-quality observations from MeerKAT and ALMA (for HI and CO, respectively). We define the molecular gas ratio as Rₘₒₗ = ΣH2 / ΣHI and measure how it depends on local conditions in the galaxy disks using multi-wavelength observations. We find that, depending on the galaxy, HI is detected at >3σ out to 20-120 kpc in galactocentric radius (rgal). The typical radius at which ΣHI reaches 1~M_~pc⁻² is rHI≈22~kpc, which corresponds to 1-3 times the optical radius (r₂₅). Rₘₒₗ correlates best with the dynamical equilibrium pressure, PDE, among potential drivers studied, with a median correlation coefficient of <ρ>=0.89. Correlations between Rₘₒₗ and star formation rate, total gas and stellar surface density, metallicity, and ΣSFR/PDE are present but somewhat weaker. Our results also show a direct correlation between PDE and ΣSFR, supporting self-regulation models. Quantitatively, we measure similar scalings as previous works and attribute the modest differences that we find to the effect of varying resolution and sensitivity. At rgal 0.4~r₂₅, atomic gas dominates over molecular gas, and at the balance of these two gas phases, we find that the baryon mass is dominated by stars, with Σ* > 5~Σgas. Our study constitutes an important step in the statistical investigation of how local galaxy properties impact the conversion from atomic to molecular gas in nearby galaxies.