PHANGS-MeerKAT and MHONGOOSE HI observations of nearby spiral galaxies: physical drivers of the molecular gas fraction, $R_{\mathrm{mol}}$

Abstract

The molecular-to-atomic gas ratio is crucial to the evolution of the interstellar medium in galaxies. We investigate the balance between the atomic (${\Sigma }_{\mathrm{HI}}$) and molecular gas (${\Sigma }_{H2}$) 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 $R_{\mathrm{mol}}={\Sigma }_{H2}/{\Sigma }_{\mathrm{HI}}$ 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 $>3\sigma$ out to 20-120 kpc in galactocentric radius ($r_{\mathrm{gal}}$). The typical radius at which ${\Sigma }_{\mathrm{HI}}$ reaches 1~${\mathrm{M}}_{\odot }\mathrm{p}{\mathrm{c}}^{-2}$ is $r_{\mathrm{HI}}\approx 22$~kpc, which corresponds to 1-3 times the optical radius ($r_{25}$). $R_{\mathrm{mol}}$ correlates best with the dynamical equilibrium pressure, P${}_{\mathrm{DE}}$, among potential drivers studied, with a median correlation coefficient of $<\rho >=0.89$. Correlations between $R_{\mathrm{mol}}$ and star formation rate, total gas and stellar surface density, metallicity, and ${\Sigma }_{\mathrm{SFR}}$/P${}_{\mathrm{DE}}$ are present but somewhat weaker. Our results also show a direct correlation between P${}_{\mathrm{DE}}$ and ${\Sigma }_{\mathrm{SFR}}$, 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 $r_{\mathrm{gal}}\gtrsim 0.4r_{25}$, 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 ${\Sigma }_{\ast }>5{\Sigma }_{\mathrm{gas}}$. 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.

accepted for publication in A&A; 20 pages, 12 Figures (+4 appendix pages)

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