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. astro-ph/0103025

For a postscript version of the article, click here.


COLD MOLECULAR GAS, PDRS, AND THE ORIGIN OF HI IN GALAXIES

Ronald J. Allen


Space Telescope Science Institute 3700 San Martin Drive, Baltimore, MD 21218

``It's B stars, stupid!''


Abstract. 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 (2). 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 2 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.


Key words: molecular processes - galaxies: ISM - stars: early-type - ISM: atoms - ISM: molecules


Table of Contents

INTRODUCTION

THE ASSOCIATION OF HI WITH H2
... in the Galaxy
... and in other nearby galaxies

COLD MOLECULAR GAS IN THE INNER DISK OF M31

GAS, DUST, AND YOUNG STARS IN THE OUTER DISK OF M31

FUTURE PERSPECTIVES

REFERENCES

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