SHARP EDGES TO NEUTRAL HYDROGEN DISKS IN GALAXIES AND THE EXTRAGALACTIC RADIATION-FIELD

Recent observations of the late-type spiral galaxy NGC 3198 show that the extended atomic hydrogen distribution in this galaxy is sharply truncated at a neutral hydrogen column density of N(H I) almost-equal-to 5 x 10(19) cm-2 (van Gorkom et al. 1993). In this paper I show that this cutoff can be reasonably explained as the result of photoionization of the gas by the extragalactic radiation field. I treat the gaseous disk in plane-parallel geometry, with the vertical equilibrium explicitly calculated for a composite disk/halo model; the gas is assumed to be smoothly distributed and isothermal, with T is similar to 10(4) K. The extragalactic radiation field is taken to be a power law between the Lyman limit and the observed value at 1.5 keV; the actual intensity at the Lyman limit is a free parameter. Models calculated for a range of velocity dispersions and ionizing photon fluxes show similar behavior, with a characteristic column density N(cr) is similar to a few times 10(19) cm-2 at which the ionization fraction sharply increases. This result is not significantly affected by halo eccentricity, but does depend on the clumpiness of the gas. Specific application to NGC 3198 shows that the total hydrogen distribution may be nearly axisymmetric, and that the atomic hydrogen at large radii is not significantly clumped into clouds. The column density at which this transition occurs is not very sensitive to galaxy mass or halo parameters, changing by only a factor of approximately 3 as the halo surface density SIGMA(h) is varied by a factor of approximately 30. The numerical results are in good agreement with an analytic estimate of the critical column density; the analytic result shows that N(cr) is-proportional-to [phi(i,ex)sigma(zz) V(A)/SIGMA(h)]1/2, here V(A) is the halo asymptotic velocity, sigma(zz) is the gas vertical velocity dispersion, and phi(i,ex) is the extragalactic ionizing photon flux. Because of the insensitivity to galaxy parameters, this model predicts that all disk galaxies will show sharp edges to their neutral hydrogen distributions at similar column densities, as first discussed by Bochkarev & Sunyaev (1977). It also predicts that objects with atomic hydrogen surface densities below this critical value will be mostly ionized and hence undetectable in the 21 cm line. If the observed H I cutoffs are due to photoionization, the extragalactic ionizing photon flux in the energy range hv = 13.6 approximately 200 eV must be 5 x 10(3) less than or similar to phi(i,ex) less than or similar to 5 x 10(4) photons cm-2 s-1. The emission measure EM of ionized gas at large radii is generally too small to be detectable by emission in Halpha at present. Strict limits on or eventual measurement of EM(R) and determination of the neutral gas velocity dispersion will allow a more precise determination of phi(i,ex).