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Using a simple model of photodissociated atomic hydrogen on a galactic scale,it is possible to derive total hydrogen volume densities. These densities,obtained through a combination of atomic hydrogen, far-ultraviolet andmetallicity data, provide an independent probe of the combined atomic andmolecular hydrogen gas in galactic disks. We present a new, flexible and fullyautomated procedure using this simple model. This automated method will allowus to take full advantage of a host of available data on galaxies in order tocalculate total hydrogen volume densities of giant molecular clouds surroundingsites of recent star formation. So far this was only possible on agalaxy-by-galaxy basis using by-eye analysis of candidate photodissociationregions. We test the automated method by adopting different models for thedust-to-gas ratio and comparing the resulting densities for M74, including anew metallicity map of M74 produced by integral field spectroscopy. We test theprocedure against previously published M83 volume densities based on the samemethod and find no significant differences. The range of total hydrogen volumedensities obtained for M74 is approximately 5-700 cm-3 . Different dust-to-gasratio models do not result in measurably different densities. The clouddensities presented here add M74 to the list of galaxies analyzed using theassumption of photodissociated atomic hydrogen occurring near sites of recentstar formation and further solidify the method. For the first time, fullmetallicity maps were included in the analysis as opposed to metallicitygradients. The results will need to be compared to other tracers of theinterstellar medium and photodissociation regions, such as CO and CII, in orderto test our basic assumptions, specifically, our assumption that the HI wedetect originates in photodissociation regions.

Hydrogen volume densities in nearby galaxies I - an automated approach