A High-Velocity Cloud Impact Forming a Supershell in the Milky Way

TL;DR

This paper reports the discovery of a large supershell in the Milky Way formed by a high-velocity cloud impact, demonstrating that such clouds can survive halo transit and influence galactic structure.

Contribution

It provides observational evidence linking a specific high-velocity cloud impact to the formation of a supershell in the Milky Way, highlighting a novel formation mechanism.

Findings

A kpc-sized supershell was detected in the Milky Way outskirts.

The supershell was formed by the impact of a high-velocity cloud about 5 million years ago.

High-velocity clouds can survive halo transit and impact the galactic disk.

Abstract

Neutral atomic hydrogen (HI) gas in interstellar space is largely organized into filaments, loops, and shells, the most prominent of which are "supershells". These gigantic structures requiring $\gtrsim 3 \times 10^{52}$ erg to form are generally thought to be produced by either the explosion of multiple supernovae (SNe) in OB associations or alternatively by the impact of high-velocity clouds (HVCs) falling to the Galactic disk. Here we report the detection of a kiloparsec (kpc)-size supershell in the outskirts of the Milky Way with the compact HVC 040+01$-$282 (hereafter CHVC040) at its geometrical center using the "Inner-Galaxy Arecibo L-band Feed Array" HI 21-cm survey data. The morphological and physical properties of both objects suggest that CHVC040, which is either a fragment of a nearby disrupted galaxy or a cloud originated from an intergalactic accreting flow, collided with the disk $\sim 5$ Myrs ago to form the supershell. Our result shows that some compact HVCs can survive their trip through the Galactic halo and inject energy and momentum into the Milky Way disk.