A High-Velocity Cloud Impact Forming a Supershell in the Milky Way
  Neutral atomic hydrogen (HI) gas in interstellar space is largely organized
into filaments, loops, and shells, the most prominent of which are
"supershells". These gigantic structures requiring $\gtrsim 3 \times 10^{52}$
erg to form are generally thought to be produced by either the explosion of
multiple supernovae (SNe) in OB associations or alternatively by the impact of
high-velocity clouds (HVCs) falling to the Galactic disk. Here we report the
detection of a kiloparsec (kpc)-size supershell in the outskirts of the Milky
Way with the compact HVC 040+01$-$282 (hereafter CHVC040) at its geometrical
center using the "Inner-Galaxy Arecibo L-band Feed Array" HI 21-cm survey data.
The morphological and physical properties of both objects suggest that CHVC040,
which is either a fragment of a nearby disrupted galaxy or a cloud originated
from an intergalactic accreting flow, collided with the disk $\sim 5$ Myrs ago
to form the supershell. Our result shows that some compact HVCs can survive
their trip through the Galactic halo and inject energy and momentum into the
Milky Way disk.
