Cold Molecular Gas, PDRs, and the Origin of HI in Galaxies
Abstract
Description
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.
7 pages including 2 figures. Presented at the conference "Gas and Galaxy Evolution", held 21 - 24 May 2000 at Socorro, NM, on the occasion of the 20th anniversary of the Very Large Array, eds. J.E. Hibbard, M.P. Rupen, & J.H. van Gorkom
7 pages including 2 figures. Presented at the conference "Gas and Galaxy Evolution", held 21 - 24 May 2000 at Socorro, NM, on the occasion of the 20th anniversary of the Very Large Array, eds. J.E. Hibbard, M.P. Rupen, & J.H. van Gorkom