The Large Magellanic Cloud's $\sim30$ Kiloparsec Bow Shock and its Impact on the Circumgalactic Medium

TL;DR

This paper uses hydrodynamic simulations to predict a large bow shock caused by the LMC's motion, affecting the Milky Way's circumgalactic medium and providing observational evidence of this shock.

Contribution

It is the first to simulate and predict the extent of the LMC's bow shock and its impact on the MW CGM, supported by observational data.

Findings

Predicted a 6.7 kpc stand-off radius and 30 kpc diameter shock.

Estimated 8% of MW's CGM interacted with the shock over 500 Myr.

Identified observational evidence of the shock in H-alpha maps.

Abstract

The interaction between the supersonic motion of the Large Magellanic Cloud (LMC) and the Circumgalactic Medium (CGM) is expected to result in a bow shock that leads the LMC's gaseous disk. In this letter, we use hydrodynamic simulations of the LMC's recent infall to predict the extent of this shock and its effect on the Milky Way's (MW) CGM. The simulations clearly predict the existence of an asymmetric shock with a present day stand-off radius of $\sim6.7$ kpc and a transverse diameter of $\sim30$ kpc. Over the past 500 Myr, $\sim8\%$ of the MW's CGM in the southern hemisphere should have interacted with the shock front. This interaction may have had the effect of smoothing over inhomogeneities and increasing mixing in the MW CGM. We find observational evidence of the existence of the bow shock in recent $H\alpha$ maps of the LMC, providing a potential explanation for the envelope of ionized gas surrounding the LMC. Furthermore, the interaction of the bow shock with the MW CGM may also explain observations of ionized gas surrounding the Magellanic Stream. Using recent orbital histories of MW satellites, we find that many satellites have likely interacted with the LMC shock. Additionally, the dwarf galaxy Ret2 is currently sitting inside the shock, which may impact the interpretation of reported gamma ray excess in Ret2. This work highlights bow shocks associated with infalling satellites are an under-explored, yet potentially very important dynamical mixing process in the circumgalactic and intracluster media.