The Origin of the Hot Gas in the Galactic Halo: Testing Galactic
  Fountain Models' X-ray Emission
  We test the X-ray emission predictions of galactic fountain models against
XMM-Newton measurements of the emission from the Milky Way's hot halo. These
measurements are from 110 sight lines, spanning the full range of Galactic
longitudes. We find that a magnetohydrodynamical simulation of a
supernova-driven interstellar medium, which features a flow of hot gas from the
disk to the halo, reproduces the temperature but significantly underpredicts
the 0.5-2.0 keV surface brightness of the halo (by two orders of magnitude, if
we compare the median predicted and observed values). This is true for versions
of the model with and without an interstellar magnetic field. We consider
different reasons for the discrepancy between the model predictions and the
observations. We find taking into account overionization in cooled halo plasma,
which could in principle boost the predicted X-ray emission, is unlikely in
practice to bring the predictions in line with the observations. We also find
that including thermal conduction, which would tend to increase the surface
brightnesses of interfaces between hot and cold gas, would not overcome the
surface brightness shortfall. However, charge exchange emission from such
interfaces, not included in the current model, may be significant. The
faintness of the model may also be due to the lack of cosmic ray driving,
meaning that the model may underestimate the amount of material transported
from the disk to halo. In addition, an extended hot halo of accreted material
may be important, by supplying hot electrons that could boost the emission of
the material driven out from the disk. Additional model predictions are needed
to test the relative importance of these processes in explaining the observed
halo emission.
