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 determinedby hydrostatic pressure and that the relation between the two is nearly linear.The pressure relation is shown to be good over three orders of magnitude for 14galaxies including dwarfs, HI-rich, and H_2-rich galaxies as well as the MilkyWay. The sample spans a factor of five in mean metallicity. The rms scatter ofindividual points of the relation is only about a factor of two for all thegalaxies, though some show much more scatter than others. Using these results,we propose a modified star formation prescription based on pressure determiningthe degree to which the ISM is molecular. The formulation is different in highand low pressure regimes defined by whether the gas is primarily atomic orprimarily molecular. This formulation can be implemented in simulations andprovides a more appropriate treatment of the outer regions of spiral galaxiesand molecule-poor systems such as dwarf irregulars and damped Lyman-alphasystems.Comment: 14 pages, 7 figures, Accepted to the Astrophysical Journa