Hα and free-free emission from the warm ionized medium

Recent observations have found the ratio of Hα 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 ∼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, Hα, and collisionally excited lines such as [N ii]6583. To reproduce the low observed value of free-free to Hα, the polycyclic aromatic hydrocarbon abundance in the photoionized regions must be lowered by a factor ∼3, and ∼20% of the diffuse Hα must be reflected from dust grains, as suggested byWood &Reynolds.