Hα AND FREE-FREE EMISSION FROM THE WARM IONIZED MEDIUM

Recent observations have found the ratio of Ha to free-free radio continuum to be surprisingly high in the diffuse ionized interstellar medium (the so-called warm ionized medium), corresponding to an electron temperature of only similar to 3000 K. Such low temperatures were unexpected in gas that was presumed to be photoionized. We consider a three-component model for the observed diffuse emission, consisting of a mix of (1) photoionized gas, (2) gas that is recombining and cooling, and (3) cool Hi gas. This model can successfully reproduce the observed intensities of free-free continuum, H alpha, and collisionally excited lines such as [NII]6583. To reproduce the low observed value of free-free to H alpha, the polycyclic aromatic hydrocarbon abundance in the photoionized regions must be lowered by a factor similar to 3, and similar to 20% of the diffuse Ha must be reflected from dust grains, as suggested by Wood & Reynolds.