# Giant HI Hole inside the 3-kpc Ring and the North Polar Spur - The Galactic Crater -

## TL;DR

This paper uses a new tangent-circle method to map HI gas in the inner Galaxy, revealing a crater-shaped hole around the Galactic Center linked to the North Polar Spur and explaining its formation via halo gas shock-wave sweeping.

## Contribution

It introduces a novel tangent-circle method for detailed HI density mapping and uncovers a crater-shaped HI hole connected to the 3-kpc ring and North Polar Spur.

## Key findings

- HI hole has a crater-shaped wall coinciding with the North Polar Spur
- The crater is formed by shock-wave sweeping of halo gas from the Galactic Center
- Unperturbed HI halo follows a sech^2 density law with a 450 pc scale height

## Abstract

Applying a newly developed tangent-circle method (TCM), we derive a volume density map of HI gas in the inner Galaxy as a function of galacto-centric distance $R$ and height $Z$. The HI hole around the Galactic Center (GC) is shown to have a crater-shaped wall, which coincides with the brightest ridge of the North Polar Spur and emanates from the 3-kpc expanding ring. The crater structure is explained by sweeping of the halo gas by a shock-wave from the GC. The unperturbed HI halo outside 3 kpc is shown to be in hydrostatic equilibrium, obeying the sech$^2 Z/h$ density law with a scale height $h \sim 450$ pc.