Which Phase of the Interstellar Medium Correlates With the Star Formation Rate?

Nearby spiral galaxies show an extremely tight correlation between tracers of molecular hydrogen (H₂) in the interstellar medium (ISM) and tracers of recent star formation, but it is unclear whether this correlation is fundamental or accidental. In the galaxies that have been surveyed to date, H₂ resides predominantly in gravitationally bound clouds cooled by carbon monoxide (CO) molecules, but in galaxies of low metal content the correlations between bound clouds, CO, and H₂ break down, and it is unclear if the star formation rate will then correlate with H₂ or with some other quantity. Here we show that star formation will continue to follow H₂ independent of metallicity. This is not because H₂ is directly important for cooling, but instead because the transition from predominantly atomic hydrogen (HI) to H₂ occurs under the same conditions as a dramatic drop in gas temperature and Bonnor-Ebert mass that destabilizes clouds and initiates collapse. We use this model to compute how star formation rate will correlate with total gas mass, with mass of gas where the hydrogen is H₂, and with mass of gas where the carbon is CO in galaxies of varying metallicity, and show that preliminary observations match the trend we predict.