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 su pplemental heating mechanism that increases the electron temperature at low densities, n(e). This would imply that in addition to photoionization, whi ch has a heating rate per unit volume proportional to n(e)(2), there is ano ther source of heat with a rate per unit volume proportional to a lower pow er of n(e). One possible mechanism is the dissipation of interstellar plasm a turbulence, which, according to Minter & Spangler, heats the ionized inte rstellar medium in the Milky Way at a rate of similar to 1 x 10(-25)n(e) er gs cm(-3) s(-1). If such a source were present, it would dominate over phot oionization 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 alph a 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 p ure photoionization. Other supplemental heating sources, such as magnetic r econnection, cosmic rays, or photoelectric emission from small grains, coul d 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 ionize d medium.