On the large escape of ionizing radiation from giant extragalactic HII regions

A thorough analysis of well-studied disk giant H II regions, for which we know the ionizing stellar population, gas metallicity, and Wolf-Rayet population, leads to photoionization models that can only match all observed line intensity ratios ([O III], [O II], [N II], [S II], and [S III] with respect to the intensity of Hbeta), as well as the Hbeta luminosity and equivalent width if one allows for an important escape of energetic ionizing radiation. For the three regions presented here, the fractions of escaping Lyman continuum photons amount to 10%-73%, and in all cases, the larger fraction of escaping photons has energies of between 13.6 and 24.4 eV. These escaping photons clearly must have an important impact as sources of ionization of the diffuse ionized gas found surrounding many galaxies, as well as of the intergalactic medium.