Highly ionized gas in the galactic halo has been detected through UV absorption and emission lines. In absorption the species studied include Si IV, C IV and N V. The UV emission studies have recorded C IV and O III]. Absorption measurements toward galactic stars reveal that the |z| distribution of the gas is roughly exponential with a scale height of approximately 3 kpc and has column densities perpendicular to the galactic plane of N ~ 2×1013, 1×1014 and 3×1013 atoms cm−2, for Si IV, C IV and NV, respectively. Similar absorption line profiles for these species suggests a common process for their origin. The presence of N V absorption implies the existence of some gas with a temperature near T ~ 2×105 K. The highly ionized absorbing gas toward distant stars in direction b < −50° has simple and relatively narrow line profiles (FWHM ~ 45 to 70 km−1) and small average LSR velocities while the gas in the direction b > 50° reveals a complex pattern of motions with substantial inflow and outflow velocities. Galactic rotation has an appreciable effect on the absorption line profiles to very distant stars located in the low halo. C IV emission has been seen at greater than a 3σ level of significance in 4 of 8 directions. The emission brightens toward the galactic poles and has a polar intensity I(C IV) ~ 5000 photons cm−2s−1ster−1. If the emitting and absorbing gas coincide in space the measurements imply ne ~ 0.01 cm−3 and P/k ~ 2000 cm−3 K for gas with T ~ 105 K. This phase of the gas fills only a small volume of the space (f ~ 0.03) and accounts for only a small fraction of the total column density of gas perpendicular to the galactic plane [~3×1018 atoms cm −2 vs 3.5×1020 atoms cm −2 for H I and 1×1020 atoms cm −2 for H+]. However, the gas provides a large EUV/UV emission line flux (~1×10−5erg. cm−2 s−1) which corresponds to a H I ionizing flux of ~2×105 ionizations cm−2 s−1. Gas with T near 2×105 K cools very rapidly. Its origin may be associated with the cooling gas of a galactic fountain flow or with thermal condensations in cosmic ray driven fountains. In the nonequilbrium cooling of a Galactic fountain, a flow rate of 4 MO/ year to each side of the Galaxy is required to produce the amount of N V absorption found in the halo while a flow rate 5x larger is required to produce the observed level of C IV emission.
The Milky Way halo as a QSO absorption-line system