Characterizing Transition Temperature Gas in the Galactic Corona
  We present a study of the properties of the transition temperature (T~10^5 K)
gas in the Milky Way corona, based on measurements of OVI, NV, CIV, SiIV and
FeIII absorption lines seen in the far ultraviolet spectra of 58 sightlines to
extragalactic targets, obtained with Far-Ultraviolet Spectroscopic Explorer
(FUSE) and Space Telescope Imaging Spectrograph. In many sightlines the
Galactic absorption profiles show multiple components, which are analyzed
separately. We find that the highly-ionized atoms are distributed irregularly
in a layer with a scaleheight of about 3 kpc, which rotates along with the gas
in the disk, without an obvious gradient in the rotation velocity away from the
Galactic plane. Within this layer the gas has randomly oriented velocities with
a dispersion of 40-60 km/s. On average the integrated column densities are log
N(OVI)=14.3, log N(NV)=13.5, log N(CIV)=14.2, log N(SiIV)=13.6 and log
N(FeIII)=14.2, with a dispersion of just 0.2 dex in each case. In sightlines
around the Galactic Center and Galactic North Pole all column densities are
enhanced by a factor ~2, while at intermediate latitudes in the southern sky
there is a deficit in N(OVI) of about a factor 2, but no deficit for the other
ions. We compare the column densities and ionic ratios to a series of
theoretical predictions: collisional ionization equilibrium, shock ionization,
conductive interfaces, turbulent mixing, thick disk supernovae, static
non-equilibrium ionization (NIE) radiative cooling and an NIE radiative cooling
model in which the gas flows through the cooling zone. None of these models can
fully reproduce the data, but it is clear that non-equilibrium ionization
radiative cooling is important in generating the transition temperature gas.
