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