Temperature Variations and Non-photoionization Heating in the Warm Ionized Medium of Galaxies

Optical emission lines observed in the gaseous halo of the Milky Way and other galaxies suggest the existence of a supplemental heat source—in addition to photoionization—that increases the electron temperature in regions of low density. Such heat sources are in fact predicted to exit in the interstellar medium. For example, both the dissipation of turbulence through ion-neutral dampening and photoelectric heating by small grains could heat the warm ionized medium in the Milky Way at the rate of about 10 ne ergs cm −3 s. If such a source were present, it would dominate over photoionization heating in regions where ne ∼< 0.1 cm, producing the observed increases in the [S II]/Hα and [N II]/Hα intensity ratios at large distances from the Galactic midplane. Magnetic reconnection and Coulomb collisions by cosmic rays are other potential sources. This heating rate would also account for the emission line ratio variations observed in NGC 891, including the increase in [O III] λ5007/Hβ with distance from the midplane, which cannot be explained by pure photoionization models.