Implications for the intergalactic medium

The density bounding hypothesis, and our measurements, separately, of the H luminosities from the full set of H II regions in a disc, and from the diffuse component, allow us to estimate the fraction of the ionizing continuum which escapes into the intergalactic medium. The difference between the flux escaping from the ionization bounded regions and the observed diffuse H flux gives us this estimate. It turns out to be around one third of the total ionizing flux from a galaxy, i.e. of order 10 erg s per galaxy in those observed. This fraction depends on the photons escaping from the H II regions, and the amount of diffuse gas available to soak them up. Of the four galaxies observed in Rozas et al. (1997, these proceedings), three emit over one third of their ionizing photons into the IGM, while the fourth, NGC3631, emits only some 15%. As well as offering a potential quantification of the ionization equilibrium in the IGM, these losses imply major corrections when using observed integrated H fluxes to compute overall star formation rates in disc galaxies.

© Copyright Astronomical Society of Australia 1997