2012ApJ...759...97H

We present direct measures of the ionization fractions of several sulfur ions in the Galactic warm ionized medium (WIM). We obtained high-resolution ultraviolet absorption-line spectroscopy of post-asymptotic giant branch stars in the globular clusters Messier 3 [(l, b) = (42°.2, +78°.7), d = 10.2 kpc, and z = 10.0 kpc] and Messier 5 [(l, b) = (3°.9, +46°.8), d = 7.5 kpc, and z = +5.3 kpc] with the Hubble Space Telescope and Far Ultraviolet Spectroscopic Explorer to measure, or place limits on, the column densities of S I, S II, S III, S IV, S VI, and H I. These clusters also house millisecond pulsars, whose dispersion measures give an electron column density from which we infer the H II column in these directions. We find fractions of S⁺² in the WIM for the M 3 and M 5 sight lines x(S⁺²) ≡ N(S⁺²)/N(S) = 0.33±0.07 and 0.47±0.09, respectively, with variations perhaps related to location. With negligible quantities of the higher ionization states, we conclude that S⁺ and S⁺² account for all of the S in the WIM. We extend the methodology to study the ion fractions in the warm and hot ionized gas of the Milky Way, including the high ions Si⁺³, C⁺³, N⁺⁴, and O⁺⁵. The vast majority of the Galactic ionized gas is warm (T ∼ 10⁴ K) and photoionized (the WIM) or very hot (T > 4x10⁵ K) and collisionally ionized. The common tracer of ionized gas beyond the Milky Way, O⁺⁵, traces <1% of the total ionized gas mass of the Milky Way.