2.3. Warm Ionized Medium
Another global effect of radiative feedback is the diffuse, warm ionized medium (WIM). This 104 K component of the ISM contributes ~ 40% of the total H luminosity in star-forming galaxies for a wide variety of Hubble types (e.g., Walterbos 1998). In the Galaxy, the WIM has a scale height of ~ 1 kpc, temperature of ~ 8000 K, and mean density ~ 0.025 cm-3 (Minter & Balser 1997). While it has long been thought that massive stars dominate its ionization (e.g., Frail et al. 1991; Reynolds & Tufte 1995), contributions from other processes also appear to be necessary. Dissipation of turbulence (Minter & Spangler 1997; Minter & Balser 1997) and photoelectric heating (Reynolds & Cox 1992) are among the suggested heating candidates in our Galaxy.
The WIM is most often studied through optical nebular emission. For the Galaxy, the largest optical survey is from the Wisconsin H Mapper (WHAM) project (Reynolds et al. 1998; Figure 2). In addition to H, the WHAM Fabry-Perot data also include observations of [S II]6717, [N II] 6583, 5755, [O III]5007, He I 5876, and other nebular emission lines. The other disk galaxies in the Local Group have also been studied optically: the LMC (Kennicutt et al. 1995), M31 (Galarza et al. 2000; Greenawalt et al. 1997; Walterbos & Braun 1992, 1994), M33 (Hoopes & Walterbos 2000), and NGC 55 (Otte & Dettmar 1999; Ferguson et al. 1996).
Figure 2. WHAM H survey of the Milky Way warm ionized medium, centered at l = 120° (Reynolds 1998; http://www.astro.wisc.edu/wham/).
Other techniques, notably at radio wavelengths, are available for studying the WIM in the Milky Way. These offer additional probes of the WIM distribution and filling factor. Heiles et al. (1998) observed radio recombination lines in the Galaxy, and Frail et al. (1991) examined lines of sight through the WIM via pulsar dispersion measures. Faraday rotation obtained through radio polarimetry has been exploited by e.g., Uyaniker et al. (2003), Gray et al. (1999), and Minter & Spangler (1996); this technique is also used by Berkhuijsen et al. (2003) for M31.