Cold Molecular Gas, PDRs, and the Origin of HI in Galaxies

In the currently-accepted model for star formation out of the interstellar gas in galaxies, the basic construction material is assumed to be large clouds of atomic hydrogen (HI). These clouds are thought to form higher-density complexes of gas and dust, and turn molecular (H2). Stars then form out of this molecular gas. In this paper arguments are advanced for a contrary view, in which the basic construction material is cold molecular gas out of which the stars form directly. HI appears in the region when the leftover H2 is illuminated with UV photons from nearby young stars. The physics of photodissociation regions provides a natural and quantitative explanation for the appearance of HI envelopes around the clouds, and for CO(1-0) emission from the higher-density parts of their surfaces. In this picture, much of the HI in a galaxy is a product of the star formation process, not a precursor to it.