UV Absorption and Emission Lines from Highly Ionized Gas in the Galactic Halo

Abstract

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 n_e ~ 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 M_O/ 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.