  We present the results of HI 21-cm line observations to explore the nature of
the high-velocity (HV) HI gas at l ~ 173{\circ} . In low-resolution Hi surveys
this HV gas appears as faint, wing-like, HI emission that extends to velocities
beyond those allowed by Galactic rotation. We designate this feature FVW
(Forbidden Velocity Wing) 172.8+1.5. Our high-resolution (3.'4) Arecibo HI
observations show that FVW 172.8+1.5 is composed of knots, filaments, and
ring-like structures distributed over an area a few degrees in extent. These HV
HI emission features are confined within the limits of the HII complex
G173+1.5, which is composed of five Sharpless HII regions distributed along a
radio continuum loop of size 4{\circ}.4 {\times} 3{\circ}.4, or ~ 138 pc
{\times} 107 pc, at a distance of 1.8 kpc. G173+1.5 is one of the largest
star-forming regions in the outer Galaxy. We demonstrate that the HV HI gas is
well correlated with the radio continuum loop and that the two seem to trace an
expanding shell. The expansion velocity of the shell is large (55 km s-1)
suggesting that it represents a supernova-remnant (SNR). We derive physical
parameters for the shell and show these to be consistent with the object being
a SNR. We also detect hot X-ray emitting gas inside the HII complex by
analyzing the ROSAT all-sky X-ray background survey data. This also supports
the SNR interpretation. We conclude that the HV HI gas and the X-rays are most
likely the products of a supernova explosion(s) within the HII complex,
possibly in a cluster that triggered the formation of these HII regions.

Astrophysics of Galaxies