2016ApJ...826..215L

There is evidence in 21 cm H i emission for voids several kiloparsecs 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 <= 2.4 kpc from the extended neutral gas layer characteristic of much of the Galactic disk, to a thin Gaussian layer with FWHM ∼ 125 pc. An anti-correlation between H i and γ-ray emission at latitudes 10^○\leqslant | b| \leqslant 20^○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| \gt 2 kpc down to z ≃ 0, where they have a radius ∼2 kpc. The extended Hi layer likely results from star formation in the disk, which is limited largely to R >= 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, 21 cm H i emission may be the best probe of the extent of the nuclear wind near the Galactic plane.