PHOTOIONIZATION OF THE DIFFUSE INTERSTELLAR-MEDIUM AND GALACTIC HALO BY OB ASSOCIATIONS

Assuming smoothly varying H i distributions in the Galactic disk, we have calculated the geometry fuse H II regions due to OB associations in the Galactic plane. Near the solar circle, OB associations with a Lyman continuum (Lyc) photon luminosity S0 greater-than-or-similar-to (3-4) x 10(49) s-1 produce H II regions that are density-bounded in the vertical direction (''H II chimneys'') allowing Lyc to escape the gaseous disk and penetrate into the Galactic halo. We provide analytic formulae for the Lyc escape fraction as functions of S0. From the O-star catalog of Garmany and a new Lyc stellar flux calibration, we find a production rate of Lyc photons by OB associations within 2.5 kpc of PSI(Lyc) = 3.3 x 10(7) cm-2 s-1. Integrating the fraction of Lyc photons that escape the disk over our adopted luminosity function of OB associations, we estimate that approximately 7% of the ionizing photons, Or PHI(Lyc) = 2.3 x 10(6) cm-2 s-1, escape each side of the H I disk layer and penetrate the diffuse ionized medium (''Reynolds layer''). This flux is sufficient to explain the photoionization of this layer, although we have not constructed a model for the observed Halpha emission and pulsar dispersion measures that is fully consistent with the absorption rate of Lyc in the H II layer. Since our quiescent model does not account for the effects of dynamic chimneys and superbubbles, which should enhance Lyc escape, we conclude that O stars are the probable source of ionizing radiation for the Reynolds layer. For a random distribution of OB associations throughout the disk, the Lyc flux is nearly uniform for heights Z greater-than-or-similar-to 0.8 kpc above the midplane.