PHANGS-MeerKAT and MHONGOOSE HI observations of nearby spiral galaxies: physical drivers of the molecular gas fraction, R₌₎₋

The molecular-to-atomic gas ratio is crucial to the evolution of the interstellar medium in galaxies. We investigate the balance between the atomic (₇₈) and molecular gas (₇₂) 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 R ₌₎₋ = ₇₂ / ₇₈ 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 >3 out to 20-120 kpc in galactocentric radius (r ₆₀₋). The typical radius at which ₇₈ reaches 1~ M_~pc^-2 is r ₇₈22~kpc, which corresponds to 1-3 times the optical radius (r₂₅). R ₌₎₋ correlates best with the dynamical equilibrium pressure, P ₃₄, among potential drivers studied, with a median correlation coefficient of =0. 89. Correlations between R ₌₎₋ and star formation rate, total gas and stellar surface density, metallicity, and ₒ₅ₑ/P ₃₄ are present but somewhat weaker. Our results also show a direct correlation between P ₃₄ and ₒ₅ₑ, 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 r ₆₀₋ 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 * > 5~ ₆₀ₒ. 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.