PHOTOIONIZATION OF DISK GALAXIES - AN EXPLANATION OF THE SHARP EDGES IN THE H-1 DISTRIBUTION

We have reproduced the observed radial truncation of the H I distribution in isolated spiral galaxies with a model in which extragalactic radiation photoionizes the gaseous disk. For a galactic mass distribution model that reproduces the observed rotation curves, including dark matter in the disk and halo, the vertical structure of the gas is determined self-consistently. The ionization structure and column densities of H and He ions are computed by solving the radiation transfer equation for both continuum and lines. Our model is similar to that of Maloney, and the H I structure differs by less than 10%. The radial structure of the column density of H I is found to be more sensitive to the extragalactic radiation field than to the distribution of mass. For this reason, considerable progress can be made in determining the extragalactic flux of ionizing photons, PHI(ex), with more 21 cm observations of isolated galaxies. However, owing to the uncertainty of the radial distribution of total hydrogen at large radii, inferring the extragalactic flux by comparing the observed edges to photoionization models is somewhat subjective. We find 1 x 10(4) cm-2 s-1 less than or similar to PHI(ex) less than or similar to 5 x 10(4) cm-2 s-1, corresponding to 2.1 less than or similar to I0 less than or similar to 10.5( x 10(-23)) ergs cm-2 s-1 Hz-1 sr-1 for a nu-1 spectrum. Although somewhat higher, our inferred range of I0 is consistent with the large range of values obtained by Kulkarni & Fall from the ''proximity effect'' toward QSOs at z almost-equal-to 0.5.