# Diagnostics of Gaseous Halo of the Milky Way by a Shock Wave from the Galactic Centre

## TL;DR

This paper presents a method to diagnose the Milky Way's gaseous halo by modeling shock wave propagation from the Galactic Centre, successfully reproducing observed structures and explaining asymmetries through halo density variations.

## Contribution

It introduces a shock wave-based diagnostic approach and demonstrates that a semi-exponential halo model explains observed bipolar structures better than spherical models.

## Key findings

- Bipolar hyper-shell structure is reproduced with a semi-exponential halo model.
- Spherical halo models are inadequate to explain observed shapes.
- Halo density gradients and fluctuations account for asymmetries and irregular features.

## Abstract

A method to diagnose the gas distribution in the Galactic halo using a shock wave from the Galactic Centre (GC) is presented. Propagation of a shock wave caused by central explosion with released energy $\sim 10^{55}$ erg is calculated for various models of the gaseous halo, and the shock front morphology is compared with the observed North- (NPS) and South-Polar Spurs (SPS). The observed bipolar hyper-shell (BHS) structure of the spurs is reproduced, when a semi-exponential halo model with radially-variable scale height is adopted. A spherical halo model ($\beta$ model) is shown to be not appropriate to explain the observed BHS shape. Asymmetry of the spurs with respect to the rotation axis and to the galactic plane is explained by large-scale density gradient in the halo across the Galaxy. Such gradient may be produced by ram pressure of the IGM during motion of the Galaxy in the Local Group. A halo model having sinusoidal density fluctuations (clouds) can also explain the asymmetry, given appropriate cloud parameters are chosen. Further irregular features such as filaments and multiple curvatures superposed on the spurs are also understood as due to density fluctuations in the halo.