Astronomy and Astrophysics – Astrophysics
Scientific paper
1999-11-03
ApJ, 1999, 525, L21
Astronomy and Astrophysics
Astrophysics
10 pages, 1 figure
Scientific paper
10.1086/312326
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 (1997) heats the ionized interstellar medium in the Milky Way at a rate ~ 1x10^-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 < 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 to the warm ionized medium ~ 10^-5 ergs s^-1 per cm^2 of Galactic disk.
Haffner Matthew L.
Reynolds Ron J.
Tufte Stephen Louis
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