Journal Article

Atomic hydrogen produced in M33 photodissociation regions

J. S. Heiner, R. J. Allen and P. C. van der Kruit

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 416, issue 1, pages 2-12
Published in print September 2011 | ISSN: 0035-8711
Published online August 2011 | e-ISSN: 1365-2966 | DOI:
Atomic hydrogen produced in M33 photodissociation regions

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We derive total (atomic + molecular) hydrogen densities in giant molecular clouds (GMCs) in the nearby spiral galaxy M33 using a method that views the atomic hydrogen near regions of recent star formation as the product of photodissociation. Far-ultraviolet (FUV) photons emanating from a nearby OB association produce a layer of atomic hydrogen on the surfaces of nearby GMCs. Our approach provides an estimate of the total hydrogen density in these GMCs from observations of the excess FUV emission that reaches the GMC from the OB association and of the excess 21-cm radio H i emission produced after these FUV photons convert H2 into H i on the GMC surface. The method provides an alternative approach to the use of CO emission as a tracer of H2 in GMCs and is especially sensitive to a range of densities well below the critical density for CO(1–0) emission.

We describe our ‘PDR method’ in more detail and apply it using GALEX FUV and Very Large Array 21-cm radio data to obtain volume densities in a selection of GMCs in the nearby spiral galaxy M33. We have also examined the sensitivity of the method to the linear resolution of the observations used; the results obtained at 20 pc are similar to those for the larger set of data at 80-pc resolution. The cloud densities we derive range from 1 to 500 cm−3, with no clear dependence on the galactocentric radius; these results are generally similar to those obtained earlier in the cases of M81, M83 and M101 using the same method.

Keywords: ISM: atoms; ISM: clouds; ISM: molecules; galaxies: individual: M33; galaxies: ISM; ultraviolet: galaxies

Journal Article.  7455 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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