Large-Scale Photodissociation Regions in Nearby Spiral Galaxies
Star formation in galaxies is driven by the presence of molecular gas, but detecting that gas is difficult if the density and temperature are too low. Using a method of combining high-resolution HI and Far-UV data, along with gas metallicity data, large-scale photodissociation regions (PDRs) can be identified near areas of active star formation. These data can be used to derive total hydrogen densities in the associated giant molecular clouds (GMCs).
Central to this method is the use of atomic hydrogen produced in the PDRs, combined with careful calculations of the UV radiation field. The atomic hydrogen is seen surrounding bright FUV sources, as is expected in PDRs. The size of these large scale PDRs is typically of the order of 100 parsec.
Combining results from M81, M83, and M33 provides a comprehensive overview of candidate PDRs in these nearby spiral galaxies and their properties. In M33 especially, the connection to CO emission and to PAH emission was analyzed in order to test the PDR model in more detail, at scales as small as 30 pc. Densities of the PDRs are found to be similar across the three galaxies, despite apparent local differences in gas content. There is a statistical correspondence between GMCs with high molecular density and the presence of detectable CO emission.
Date: Thursday, 17 September 2009 Time: 11:30 Where: Université de Montréal Pavillon Roger-Gaudry, Local D-460 Contact: Pierre Bergeron