The H to H₂ transition in galaxies: Totally molecular galaxies

Abstract

The transition from H to H2 in the general interstellar medium is shown to depend sensitively on the pressure and radiation field, so even a slight increase in the ambient pressure can convert a whole cloud population into H2 molecules without necessarily converting the diffuse clouds into self-gravitating clouds. Similarly, a slight increase in radiation can convert the clouds back to H. The calculation includes self-shielding and mutual shielding of line radiation between clouds, as well as dust extinction for diffuse and self-gravitating clouds with an M-1.5 mass distribution. The diffuse clouds are taken to have a constant internal density that scales with ambient pressure, and the self-gravitating clouds are assumed to have internal densities that vary as 1/r2 and to satisfy the pressure-dependent virial-theorem correlations between mass, average density, velocity dispersion, and radius. The results imply that large regions of galaxies can spontaneously convert to molecular form following an interaction or other event that triggers mass accretion or an increase in the gas surface density. Spiral density waves can also convert H to H2 in the arms because of the pressure increase and then quickly convert much of this gas back into H near the bright stars that form. Such H to H2 conversions occur in both the diffuse and self-gravitating clouds and have no relation to the formation of new self-gravitating clouds or to the onset of star formation. The fraction of the interstellar medium in diffuse form can be the same in regions of high and low molecular abundance, which implies that the H2 fraction is a poor indicator of the star formation properties of galaxies.