The Effects of Photodissociation on the HI Content of Galaxies:
Atomic Hydrogen as a Tracer for Molecular Gas
Ron ALLEN
STScI Baltimore
Résumé :
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
developed 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 galaxy disks.
The results suggest alternate explanations for several curious
anomalies in the study of global star formation in nearby galaxies,
such as variations in the CO brightness from one galaxy to the next,
and the index in the Schmidt Law for star formation.
Modeling a high velocity LMC. The formation of the Magellanic Stream.
