The Role of Pressure in GMC Formation

We examine the hypothesis that hydrostatic pressure alone determines the ratio of atomic to molecular gas averaged over a particular radius in disk galaxies. The hypothesis implies that the transition radius, the location where the ratio is unity, should always occur at the same value of stellar surface density in all galaxies. We examine data for 28 galaxies and find that the stellar surface density at the transition radius is indeed constant to 40% at a value of 120 M_sun/pc^2. If the hypothesis can be confirmed at all radii within a large range of galaxy types and metallicities, combining it with the observed constancy of the star formation rate with H_2 surface density may enable a physically motivated star formation prescription with wide applicability.