Tracing the Milky Way Nuclear Wind with 21cm Atomic Hydrogen Emission
  There is evidence in 21cm HI emission for voids several kpc in size centered
approximately on the Galactic centre, both above and below the Galactic plane.
These appear to map the boundaries of the Galactic nuclear wind. An analysis of
HI at the tangent points, where the distance to the gas can be estimated with
reasonable accuracy, shows a sharp transition at Galactic radii $R\lesssim 2.4$
kpc from the extended neutral gas layer characteristic of much of the Galactic
disk, to a thin Gaussian layer with FWHM $\sim 125$ pc. An anti-correlation
between HI and $\gamma$-ray emission at latitudes $10^{\circ} \leq |b| \leq
20^{\circ}$ suggests that the boundary of the extended HI layer marks the walls
of the Fermi Bubbles. With HI we are able to trace the edges of the voids from
$|z| > 2$ kpc down to $z\approx0$, where they have a radius $\sim 2$ kpc. The
extended HI layer likely results from star formation in the disk, which is
limited largely to $R \gtrsim 3$ kpc, so the wind may be expanding into an area
of relatively little HI. Because the HI kinematics can discriminate between gas
in the Galactic center and foreground material, 21cm HI emission may be the
best probe of the extent of the nuclear wind near the Galactic plane.
