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Stars in galaxies form in giant molecular clouds that coalesce when theatomic hydrogen is converted into molecules. There are currently two dominantmodels for what property of the galactic disk determines its molecularfraction: either hydrostatic pressure driven by the gravity of gas and stars,or a combination of gas column density and metallicity. To assess the validityof these models, we compare theoretical predictions to the observed atomic gascontent of low-metallicity dwarf galaxies with high stellar densities. Theextreme conditions found in these systems are optimal to distinguish the twomodels, otherwise degenerate in nearby spirals. Locally, on scales <100 pc, wefind that the state of the interstellar medium is mostly sensitive to the gascolumn density and metallicity rather than hydrostatic pressure. On largerscales where the average stellar density is considerably lower, both pressureand shielding models reproduce the observations, even at low metallicity. Weconclude that models based on gas and dust shielding more closely describe theprocess of molecular formation, especially at the high resolution that can beachieved in modern galaxy simulations or with future radio/millimeter arrays.

Testing models for molecular gas formation in galaxies: hydrostatic pressure or gas and dust shielding?