The Warm Ionized Medium in the Milky Way and Other Galaxies

Observations of the "Warm Ionized Medium" (or, equivalently, the "DiffuseIonized Gas") of the local ISM, the Perseus arm in the Milky Way, and also inseveral other galaxies show strong [NII]6563 (~H-alpha in some cases) and[SII]6717/[NII]6583 = 0.6 - 0.7 in all locations and objects. Other line ratios(e.g., [O III]5007/H-beta) vary considerably. Simple photoionization modelsreproduce the observed spectra, providing extra heating beyond that supplied byphotoionization is assumed (Reynolds, Haffner, & Tufte 1999). With observedgas-phase abundances (not solar), the line ratios in the local arm at b = 0 degare fitted with no extra heating and (S/H) = 13 ppm (solar is 20 ppm). Localgas observed at b = -35 deg requires extra heating of about gamma = 0.75, wheregamma is the extra heating in units of 10^{-25} erg H^{-1} s^{-1}. In thePerseus arm, there are similar results, with a domposition consistent with theGalactic abundance gradient. The requirements for NGC 891 are similar to thePerseus arm: little or no extra heating at |z| = 1 kpc and gamma 3 at 2 kpc. InNGC 891 there is also an increase of 5007/H-alpha with |z| that can only comeabout if most of the ionizing radiation is supplied by stars with T~50000 K.Either their radiation must propagate from the plane to high |z| through verylittle intervening matter, or else the stars are located at high |z|. The totalpower requirement of the extra heating is <15% of the photoionization power.[O~II]3727/H-beta can serve as a useful diagnostic of extra heating, but[S~III] 9065,9531/H-alpha is not useful in this regard.Comment: 32 pages, including 2 figures. To appear in November 20 Ap