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

Abstract

We present hydrodynamic simulations of the Pegasus-Pisces Arch (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 −1 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 −1 . 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. Unified Astronomy Thesaurus concepts: Interstellar clouds (834); High-velocity clouds (735); Computational astronomy (293); Hydrodynamical simulations (767) 1. INTRODUCTION H I maps of the Pegasus-Pisces region reveal an unusually long, narrow, and straight cloud of intermediate velocity gas extending ∼ 42 o , from (l, b) ∼ (84 o , −34 o ) to (l, b) ∼ (130 o , −62 o ), with an apparent width of ∼ 3 o . It has a distinct head and tapered, bifurcated tail. It appears clearly in the map of H I with velocities between V LSR = −85 and −45 km s −1 in Figure 17 of Wakker (2001) and the maps of H I with velocities between V LSR ∼ −80 and ∼ −30 km s −1 in Figures 2 -4 in Fukui et al. (2021). Their Figure 2 is reproduced here as our Figure 1. Wakker (2001) identified this object as an intermediate velocity cloud (IVC). Wakker (2001) defined IVCs as clouds with local standard of rest velocities of V LSR =∼ 40 km s −1 to 90 km s −1 , but the upper V LSR cut-off has been placed as high as 100 km s −1 by some authors (e.g., Richter 2017). Wakker (2001) named this especially long IVC after its location, calling it the Pegasus-Pisces Arch, abbreviated as the PP Arch. Fukui et al. (2021), who concentrated on the portion that runs from (l, b) ∼ (84 o , −34 o ) to (l, b) ∼ (110 o , −55 o ), named it after the Galactic coordinates of its head, hence IVC 86-36.The Pegasus-Pisces Arch is long, but pennant-shaped, with a broader head and narrower, tapered tails. Its relatively streamline shape contrasts clearly with those of its irregularly shaped low velocity neighbors MBM 53, HLCG 92-35, MBM 54, and MBM 55 (Magnani et al. 1985;Yamamoto et al. 2003;Fukui et al. 2021), which, together, resemble a curved archipelago of small and midsized islands in maps of H I, CO, and tracers of dust.The Pegasus-Pisces Arch is also more streamli...