How was the mushroom-shaped GW 123.4--1.5 formed in the Galactic disk?

TL;DR

This study uses hydrodynamical simulations to demonstrate that a head-on collision between a high-velocity cloud and the Galactic disk can produce mushroom-shaped HI structures like GW 123.4--1.5, explaining their origin and morphology.

Contribution

The paper presents the first detailed hydrodynamical simulation showing how head-on HVC collisions create mushroom-shaped structures in the Galactic disk.

Findings

Reproduced GW 123.4--1.5's shape and size in simulations.

Estimated the impact parameters and timing of the collision.

Suggested such structures are rare due to infrequent head-on collisions.

Abstract

The unusual mushroom-shaped HI cloud, GW 123.4--1.5, is hundreds of parsecs in size but does not show any correlations to HI shells or chimney structures. To investigate the origin and velocity structure of GW 123.4--1.5, we perform three-dimensional hydrodynamical simulations of the collision of a high-velocity cloud with the Galactic disk. We also perform a parameter study of the density, radius, and incident angle of the impact cloud. The numerical experiments indicate that we reproduce the mushroom-shaped structure which resembles GW 123.4--1.5 in shape, size, position-velocity across the cap of the mushroom, and the density ratio between the mushroom and surrounding gas. GW 123.4--1.5 is expected to be formed by the almost head-on collision of a HVC with velocity $\sim 100 \kms$ and mass $\sim 10^5 \Msun$ about $5 \times 10^7 \yr$ ago. A mushroom-shaped structure like GW 123.4--1.5 must be infrequent on the Galactic plane, because the head-on collision which explains the mushroom structure seems rare for observed HVCs. HVC-disk collision explains not only the origin of the mushroom-shaped structure but also the formation of a variety of structures like shells, loops, and vertical structures in our Galaxy.