Tracing the Milky Way Nuclear Wind with 21cm Atomic Hydrogen Emission

Abstract

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 $10^{\circ }\le |b|\le 20^{\circ }$ 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 $z\approx 0$, 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 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.

22 pages, 7 figures. To appear in ApJ

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• The large-scale ionization cones in the Galaxy
• Filament formation in wind-cloud interactions. II. Clouds with turbulent density, velocity, and magnetic fields
• On the dynamics and survival of fractal clouds in galactic winds
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• The history of dynamics and stellar feedback revealed by the HI filamentary structure in the disk of the Milky Way
• Observation of Acceleration of HI Clouds Within the Fermi Bubbles
• Mapping Outflowing Gas in the Fermi Bubbles: a UV Absorption Survey of the Galactic Nuclear Wind
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• Direct observations of the atomic-molecular phase transition in the Milky Way's nuclear wind
• Interaction of the Galactic-Centre Super Bubbles with the Gaseous Disc
• Giant HI Hole inside the 3-kpc Ring and the North Polar Spur - The Galactic Crater -
• Diverse metallicities of Fermi bubble clouds indicate dual origins in the disk and halo
• Revealing Gas Inflows towards the Galactic Central Molecular Zone
• CO Emission Delineating the Interface between the Milky Way Nuclear Wind Cavity and the Gaseous Disk
• Morphological Evidence for the eROSITA Bubbles Being Giant and Distant Structures
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• One-kpc Expanding Cylinder of HI-Gas around the Galactic Center
• Diffuse Hot Plasma in the Interstellar Medium and Galactic Outflows
• How is Cold Gas Loaded into Galactic Nuclear Outflows?