The escape of ionizing photons from the Galaxy

The Magellanic Stream and several high-velocity clouds have now been detected in optical line emission. The observed emission measures and kinematics are most plausibly explained by photoionization due to hot, young stars in the Galactic disk. The highly favorable orientation of the Stream allows an unambiguous determination of the fraction of ionizing photons f(esc) which escape the Galactic disk. We have modeled the production and transport of ionizing photons through an opaque interstellar medium. Normalization to the Stream detections requires f(esc) approximate to 6%, which is in reasonable agreement with the flux required to ionize the Reynolds layer. Neither shock heating nor emission within a hot Galactic corona can be important in producing the observed Hat emission. If such a large escape fraction is typical oft, galaxies, star-forming systems dominate the extragalactic ionizing background. Within the context of this model, both the three-dimensional orientation of the Stream and the distances to high-velocity clouds can be determined by sensitive Ha observations.