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 interstellar medium, from 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)] lambda 6563 (similar toH alpha in some cases) and the fact that CS nl lambda 6717/[N (II)] lambda 6583 is almost the same (similar to0.6-0.7) in all locations and objects. Other line ratios (e.g., [O (II)] lambda 5007/H beta) vary considerably. This paper presents simple photoionization models that reproduce the observed spectra, providing extra heating beyond that supplied by photoionization is assumed. 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 degrees are fitted with no extra heating and S/H = 13 ppm, as opposed to solar (similar to 20 ppm). Local gas observed at b = -35 degrees requires extra heating of about Gamma (-25)=0.75, where Gamma (-25) is the extra heating in units of 10(-25) ergs H-1 s(-1). In the Perseus arm there are similar results: little extra heating is required at \z\ = 500 pc, and Gamma (-25) = 3.0 is needed at \z\ = 1.2 kpc. To fit the observations, the gas-phase composition in the Perseus arm must be reduced as required by the Galactic abundance gradient observed for H (II) regions. The requirements for NGC 891 (the best observed other galaxy at \z\ = 1 kpc and 2 kpc are similar to the Perseus arm: little or no extra heating near the plane (1 kpc in this case) and Gamma (-25) similar to 3 at \z\ = 2 kpc. In NGC 891 there is also an increase of lambda 5007/H alpha with \z\ that can only come about if most of the ionizing radiation is supplied by very hot stars (type O4: T* similar to 50,000 K). Either their radiation must propagate from the plane to high \z\ through very little intervening matter, or else the stars are located at high \z\. The total power requirement of the extra heating is less than or similar to 15% of the power to photoionize the WIM without extra heating. Extra heating enhances [O (II)] lambda 3727/H beta. Figure 1 shows that there is a spread in the predicted values, but the ratio can serve as a useful diagnostic of extra heating. The [S (II)] lambda lambda 9065, 9531 lines (see Fig. 2) are not useful in diagnosing extra heating.