TRACING THE MILKY WAY NUCLEAR WIND WITH 21 cm ATOMIC HYDROGEN EMISSION

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 less than or similar to 2.4 kpc from the extended neutral gas layer characteristic of much of the Galactic disk, to a thin Gaussian layer with FWHM similar to 125 pc. An anti-correlation between H I and gamma-ray emission at latitudes 10 degrees <= vertical bar b vertical bar <= 20 degrees 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 vertical bar z vertical bar > 2 kpc down to z approximate to 0, where they have a radius similar to 2 kpc. The extended HI layer likely results from star formation in the disk, which is limited largely to R greater than or similar to 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.