A Case Study of the Pegasus-Pisces Arch Intermediate Velocity Cloud

The Pegasus-Pisces Arch is one of the narrowest, most streamlined intermediate velocity clouds in our Galaxy. With two narrow tails trailing its head, the Pegasus-Pisces Arch subtends 42 degrees on the sky and has an aspect ratio of 14 to 1. It is traveling toward the Galactic disk at an inclined angle and with a line of sight velocity of -50 km/s and a probable total speed of ~100km/s. Here we present a case study of the cloud, including bespoke simulations of its hydrodynamicevolution and a summary of existing observational knowledge. The FLASH code is used to simulate the hydrodynamic evolution. The simulations show the cloud instigating a bow shock in the Galactic thick and thin disk interstellar gas and show the tails growing easily in the relatively benign conditions behind the bow shock. Their Reynolds numbers are in the non-turbulent regime and the lack of turbulence enables their great length. Another factor contributing to the observed narrowness of the Pegasus-Pisces Arch is its orientation somewhat crosswise to the line of sight. A head-on view would result in a blobbier appearance. The Pegasus-Pisces Arch is also known as the PP Arch and its head is independently named IVC 86-36.