Probing the Outflowing Multiphase Gas ~1 kpc Below the Galactic Center
  Comparison of 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 deg and b = -6.63 deg, 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 (STIS) show low ion 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, while absorption at d > 7 kpc, ~1 kpc below the galactic plane, is
complex and spans -290 to + 94 km/ s. 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 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 extraordinary large thermal pressure, p/k ~ 10^5
cm^-3 K^-1 in an absorption component at -114 km/s 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 non-equilibrium radiative cooling and turbulent mixing.
