The Molecular Gas Component of Galaxy Disks

The molecular gas in galaxy disks shows much more galaxy to galaxy variation than does the atomic gas. Detailed studies show that this variation can be attributed to differences in hydrostatic pressure in the disks due largely to variations in the stellar surface density and the total gas surface density. One prediction of pressure modulated H₂ formation is that the location where HI and H₂ have equal surface densities occurs at a constant value of the stellar surface density in the disk. Observations confirm this constancy to 40%.