The Warm Ionized Medium in the Milky Way and Other Galaxies

There are now observations of several emission lines from the "Warm Ionized Medium" (WIM) (or, equivalently, the "Diffuse Ionized Gas") of the local ISM, the Perseus Arm in the Milky Way, and also in several other galaxies. Interesting features of these observations include the great strength of [N II]6563 ( Hff in some cases) and the fact that [S II]6717/[N II]6583 is almost the same (0.6 -- 0.7) in all locations and objects. Other line ratios (e.g., [O III]5007/Hfi) vary considerably. This paper presents simple photoionization models that reproduce the observed spectra, providing extra heating beyond that supplied by photoionization is assumed (see Reynolds, Haffner, & Tufte 1999). The same extra heating was used for models of all stellar temperatures being combined together, although it could easily depend on T . With observed gas-phase abundances (not solar), the line ratios in the local arm at b = 0 ffi are fitted with no extra heating and (S/H) = 13 ppM, as opposed to solar ( ...