The Long Tails of the Pegasus-Pisces Arch Intermediate Velocity Cloud

TL;DR

This paper uses hydrodynamic simulations to study the Pegasus-Pisces Intermediate Velocity Cloud, revealing how its long tails form and predicting its eventual merging with the Galactic disk.

Contribution

It provides the first detailed hydrodynamic model of the PP Arch, explaining its unique tail structure and predicting its future evolution.

Findings

Simulated cloud reproduces observed HI 21cm intensity and velocity.

Bow shock formation protects the cloud from shear and aids tail formation.

Future evolution involves deformation, dissipation, and merging with the Galactic disk.

Abstract

We present hydrodynamic simulations of the Pegasus-Pisces (PP Arch), an intermediate velocity cloud in our Galaxy. The PP Arch, also known as IVC 86-36, is unique among intermediate and high velocity clouds, because its twin tails are unusually long and narrow. Its -50 km/s line-of-sight velocity qualifies it as an intermediate velocity cloud, but the tails' orientations indicate that the cloud's total three-dimensional speed is at least ~100 km/s. This speed is supersonic in the Reynold's Layer and thick disk. We simulated the cloud as it travels supersonically through the Galactic thick and thin disks at an oblique angle relative to the midplane. Our simulated clouds grow long double tails and reasonably reproduce the H I 21~cm intensity and velocity of the head of the PP Arch. A bow shock protects each simulated cloud from excessive shear and lowers its Reynolds number. These factors may similarly protect the PP Arch and enable the survival of its unusually long tails. The simulations predict the future hydrodynamic behavior of the cloud when it collides with denser gas nearer to the Galactic midplane. It appears that the PP Arch's fate is to deform, dissipate, and merge with the Galactic disk.