Probing the Outflowing Multiphase Gas ∼1 kpc below the Galactic Center

Comparison of interstellar medium (ISM) absorption in the UV spectrum of LS 4825, a B1 Ib−II star d  = 21 ± 5 kpc from the Sun toward l  = 1.°67 and b  = −6.°63, with ISM absorption toward an aligned foreground star at d  < 7.0 ± 1.7 kpc, allows us to isolate and study gas associated with the Milky Way nuclear wind. Spectra from the Space Telescope Imaging Spectrograph show low-ionization absorption out to d  < 7 kpc (e.g., O i, C ii, Mg ii, Si ii, Fe ii, S ii) only between 0 and 40 km s⁻¹, while absorption at d  > 7 kpc, ∼1 kpc below the Galactic plane, is complex and spans −290 to +94 km s⁻¹. The intermediate and high ions Si iii, C iv, Si iv, and N v show extremely strong absorption with multiple components from −283 to 107 km s⁻¹, implying that the ISM ∼1 kpc below the Galactic center has a substantial reservoir of plasma and more gas containing C iv and N v than in the Carina OB1 association at z  = 0 kpc. Abundances and physical conditions are presented for many absorption components. The high ion absorption traces cooling transition temperature plasma probably driven by the outflowing hot gas, while the extraordinarily large thermal pressure, p / k  ∼ 10⁵ cm⁻³ K⁻¹, in an absorption component at −114 km s⁻¹ probably arises from the ram pressure of the outflowing hot gas. The observations are consistent with a flow whose ionization structure in the high ions can be understood through a combination of nonequilibrium radiative cooling and turbulent mixing.