Photoionization in elliptical galaxies by old stars.

In an effort to explain the ionization of gas in elliptical galaxies, we have computed the Lyman continuum output from an ageing starburst, using isochrone synthesis spectral evolution techniques. We have shown that post-AGB stars provide sufficient ionizing radiation to account for the observed Hα luminosity and equivalent width in very early type galaxies. Photoionization models using the radiation field from these stars are shown to easily reproduce the excitation level of the ionized gas. Thus, we argue that the ionization observed in normal elliptical galaxies is provided by their old stellar populations.