WHICH PHASE OF THE INTERSTELLAR MEDIUM CORRELATES WITH THE STAR FORMATION RATE?

Abstract

Nearby spiral galaxies show an extremely tight correlation between tracers of molecular hydrogen (H 2 ) in the interstellar medium 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, H 2 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 H 2 break down, and it is unclear if the star formation rate (SFR) will then correlate with H 2 or with some other quantity. Here, we show that star formation will continue to follow H 2 independent of metallicity. This is not because H 2 is directly important for cooling, but instead because the transition from predominantly atomic hydrogen (H i ) to H 2 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 SFR will correlate with total gas mass, with mass of gas where the hydrogen is H 2 , 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.