Photodissociation and the Morphology of HI in Galaxies

Abstract

Young massive stars produce Far-UV photons which dissociate the molecular gas on the surfaces of their parent molecular clouds. Of the many dissociation products which result from this ``back-reaction'', atomic hydrogen \HI is one of the easiest to observe through its radio 21-cm hyperfine line emission. In this paper I first review the physics of this process and describe a simplified model which has been developed to permit an approximate computation of the column density of photodissociated \HI which appears on the surfaces of molecular clouds. I then review several features of the \HI morphology of galaxies on a variety of length scales and describe how photodissociation might account for some of these observations. Finally, I discuss several consequences which follow if this view of the origin of HI in galaxies continues to be successful.

18 pages, 7 figures in 8 files, invited review paper for the conference "Penetrating Bars Through Masks of Cosmic Dust: The Hubble Tuning Fork Strikes a New Note", South Africa, June 2004. Proceedings to be published by Kluwer, eds. D.L. Block, K.C. Freeman, I. Puerari, R. Groess, & E.K. Block

References in corpus (3)

• The Neutral Atomic Phases of the ISM in the Galaxy
• The Formation of Molecular Clouds
• The Production of HI in Photodissociation Regions and A Comparison with CO(1-0) Emission

Cited by in corpus (7)

• The volume densities of GMCs in M81
• The volume densities of giant molecular clouds in M83
• The Bosma effect revisited - I. HI and stellar disc scaling models
• A photodissociation model for the morphology of the HI near OB associations in M33
• A new method for obtaining the star formation law in galaxies
• Hydrogen volume densities in nearby galaxies I - an automated approach
• Atomic Hydrogen produced in M33 Photodissociation Regions