Ionized Gas in the First 10 Kiloparsecs of the Interstellar Galactic Halo: Metal Ion Fractions

TL;DR

This study measures ionization fractions of sulfur ions in the Galactic warm ionized medium using ultraviolet spectroscopy, revealing that S+ and S+2 dominate and providing insights into the ionization state of the Milky Way's gas.

Contribution

It provides direct measurements of sulfur ion fractions in the Galactic WIM, extending methodology to hot ionized gas, and clarifies the dominant ionization states in different gas phases.

Findings

S+2 accounts for about 33-47% of sulfur in the WIM.

Higher ionization states like S IV and S VI are negligible in the WIM.

O+5 traces less than 1% of the total ionized gas mass.

Abstract

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 the globular clusters Messier 3 [(l,b)=(42.2, +78.7); d=10.2 kpc, z=10.0 kpc] and Messier 5 [(l,b)=(3.9, +46.8); d=7.5 kpc, 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 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 ~ 10^4 K) and photoionized (the WIM) or very hot (T > 4x10^5 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.