A Disc-Dominated and Clumpy Circumgalactic Medium of the Milky Way Seen in X-Ray Emission

The Milky Way galaxy is surrounded by a circumgalactic medium (CGM) that may play a
key role in galaxy evolution as the source of gas for star formation and a repository of metals
and energy produced by star formation and nuclear activity. The CGM may also be a repository
for baryons seen in the early universe, but undetected locally. The CGM has an ionized
component at temperatures near 2 x 10^6 K studied primarily in the soft X-ray band. Here
we report a survey of the southern Galactic sky with a soft X-ray spectrometer optimized
to study diffuse soft X-ray emission. The X-ray emission is best fit with a disc-like model
based on the radial profile of the surface density of molecular hydrogen, a tracer of star formation,
suggesting that the X-ray emission is predominantly from hot plasma produced via
stellar feedback. Strong variations in the X-ray emission on angular scales of ~10 degrees indicate
that the CGM is clumpy. Addition of an extended, and possibly massive, halo component is
needed to match the halo density inferred from other observations.