The Role of Pressure in GMC Formation II: The H₂-Pressure Relation

We show that the ratio of molecular to atomic gas in galaxies is determined by hydrostatic pressure and that the relation between the two is nearly linear. The pressure relation is shown to be good over 3 orders of magnitude for 14 galaxies, including dwarfs, H I-rich, and H₂-rich galaxies, as well as the Milky Way. The sample spans a factor of 5 in mean metallicity. The rms scatter of individual points of the relation is only about a factor of 2 for all the galaxies, although some show much more scatter than others. Using these results, we propose a modified star formation prescription based on pressure determining the degree to which the ISM is molecular. The formulation is different in high- and low-pressure regimes, defined by whether the gas is primarily atomic or primarily molecular. This formulation can be implemented in simulations and provides a more appropriate treatment of the outer regions of spiral galaxies and molecule-poor systems, such as dwarf irregulars and damped Lyα systems.