Evidence for an additional heat source in the warm ionized medium of galaxies

Spatial variations of the [S II]/H alpha and [N II]/H alpha line intensity ratios observed in the gaseous halo of the Milky Way and other galaxies are inconsistent with pure photoionization models. They appear to require a supplemental heating mechanism that increases the electron temperature at low densities, n(e). This would imply that in addition to photoionization, which has a heating rate per unit volume proportional to n(e)(2), there is another source of heat with a rate per unit volume proportional to a lower power of n(e). One possible mechanism is the dissipation of interstellar plasma turbulence, which, according to Minter & Spangler, heats the ionized interstellar medium in the Milky Way at a rate of similar to 1 x 10(-25)n(e) ergs cm(-3) s(-1). If such a source were present, it would dominate over photoionization heating in regions where n(e) less than or similar to 0.1 cm(-3), producing the observed increases in the [S II]/H alpha and [N II]/H alpha intensity ratios at large distances from the galactic midplane as well as accounting for the constancy of [S II]/[N II], which is not explained by pure photoionization. Other supplemental heating sources, such as magnetic reconnection, cosmic rays, or photoelectric emission from small grains, could also account for these observations, provided they supply similar to 10(-5) ergs s(-1) per square centimeter of the Galactic disk to the warm ionized medium.