There is evidence in 21cm H i emission for voids several kpc in size centered approximately on the Galactic center , both above and below the Galactic plane .
These appear to map the boundaries of the Galactic nuclear wind .
An analysis of H i 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 H i and \gamma -ray emission at latitudes 10 \arcdeg \leq|b| \leq 20 \arcdeg suggests that the boundary of the extended H i layer marks the walls of the Fermi Bubbles .
With H i we are able to trace the edges of the voids from |z| > 2 kpc down to z \approx 0 , 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 H i .
Because the H i kinematics can discriminate between gas in the Galactic center and foreground material , 21cm H i emission may be the best probe of the extent of the nuclear wind near the Galactic plane .