Conclusions

These results give support to the hypothesis that the diffuse H in disc galaxies is produced by Ly continuum escaping from H II regions. This general idea agrees with work by Ferguson et al. (1996) who estimated the diffuse emission at some 40 of the total H produced by the galaxy, and showed a global geometrical association of diffuse emission and emission from the H II regions in the two galaxies they studied. Here we go further and identify the sources as the density bounded regions, showing that they emit enough Lyc to ionize the diffuse medium, and that a significant fraction of the ionizing photons produced by a disc galaxy in fact escape into the intergalactic medium. (Project support: Grant PB94-1107; DGICYT)

Table 1: Comparison between observed diffuse H and available ionizing flux predicted from the density bounded H II regions

Galaxies	Diffuse luminosity (erg s)	Available luminosity (erg s)
NGC157
NGC6764
NGC6951
NGC3631

  
Figure 1: Left: Geometrical match between the observed diffuse H and the sites of the luminous regions (density bounded). Right: Procedure for estimating the Lyman continuum flux escaping from the H II regions in 4 disc galaxies, and available to ionize the diffuse medium. The difference between the extrapolated LF (upper line) and the observed LF from the density bounded high luminosity regions gives the available ionizing component.

© Copyright Astronomical Society of Australia 1997