Photodissociation and the HI Content of Galaxies: Atomic Hydrogen as a Tracer for Molecular Gas

Ron Allen

Space Telescope Science Institute

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 unavoidable "back-reaction", atomic hydrogen is arguably the easiest to observe through its radio 21-cm hyperfine line emission. In this talk, I briefly review the physics of the process by which H2 is photodissociated into HI, and describe a simple model which has been used to estimate the column density N(HI) of photodissociated HI which appears on the surfaces of molecular clouds. Several HI features observed in the Galaxy are ascribed to this process. Recent applications of the model using FUV and HI images of several nearby galaxies yield new ways of studying global star formation and the properties of giant molecular clouds over large areas of the galaxy disks.

Date:	Thursday, 8 April 2010
Time:	11:30
Where:	Université de Montréal
	Pavillon Roger-Gaudry, Local D-460
Contact:	Nicole St-Louis