Hydrogen volume densities in nearby galaxies I - an automated approach
  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 and
metallicity data, provide an independent probe of the combined atomic and
molecular hydrogen gas in galactic disks. We present a new, flexible and fully
automated procedure using this simple model. This automated method will allow
us to take full advantage of a host of available data on galaxies in order to
calculate total hydrogen volume densities of giant molecular clouds surrounding
sites of recent star formation. So far this was only possible on a
galaxy-by-galaxy basis using by-eye analysis of candidate photodissociation
regions. We test the automated method by adopting different models for the
dust-to-gas ratio and comparing the resulting densities for M74, including a
new metallicity map of M74 produced by integral field spectroscopy. We test the
procedure against previously published M83 volume densities based on the same
method and find no significant differences. The range of total hydrogen volume
densities obtained for M74 is approximately 5-700 cm-3 . Different dust-to-gas
ratio models do not result in measurably different densities. The cloud
densities presented here add M74 to the list of galaxies analyzed using the
assumption of photodissociated atomic hydrogen occurring near sites of recent
star formation and further solidify the method. For the first time, full
metallicity maps were included in the analysis as opposed to metallicity
gradients. The results will need to be compared to other tracers of the
interstellar medium and photodissociation regions, such as CO and CII, in order
to test our basic assumptions, specifically, our assumption that the HI we
detect originates in photodissociation regions.
