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J.S. Heiner (STScI, Kapteyn Astronomical Institute Groningen), R.J. Allen (STScI), P.C. van der Kruit (Kapteyn Astronomical Institute Groningen)
We report our findings on the volume densities of H2 in Giant Molecular Clouds (GMCs) associated with photodissociation regions (PDRs) in the nearby galaxies M81 and M83. Assuming this HI is a photodissociation product of star formation, it can be used independently to complement the popular CO(1-0) tracer. In many instances, patches of HI are found close to bright FUV sources, as is expected for PDRs. The detected PDRs, in which photodissociating photons dominate the radiation field, are larger than presently-well-studied PDRs and have sizes of roughly 100 parsec. The complexes of young, hot stars giving rise to these PDRs create a 'blanket' of photodissociated HI around them.
The balance equation governing the photodissociation process needs the incident ultraviolet flux G0 and the local dust-to-gas ratio \delta together with the HI column density to calculate the volume density of molecular hydrogen. Previous work by Smith et al. (2001) on M101 has shown that the GMCs in that galaxy have volume densities of roughly 102 cm-3, similar to GMCs our own Galaxy.
As PAHs arguably trace star formation, and their MIR emission is also thought to come mostly from PDRs, we use this property to determine the instances where PAH emission is linked to the occurrance of HI. We present the statistics of this comparison and our findings as to what extent we are really seeing large scale PDRs in nearby galaxies.
We used recent radio data kindly provided by the THINGS group, Galex UV data and Spitzer IRAC data.
This work is funded by STScI's Director's Discretionary Research Fund.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.