2019MNRAS.484.2954S

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 ∼10⁵⁵ erg is calculated for various models of the gaseous halo, and the shock front morphology is compared with the observed North-Polar Spurs (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 (β 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 intergalactic gas 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.