2012AJ....143...75K

We present the results of H I 21 cm line observations to explore the nature of the high-velocity (HV) H I gas at ℓ ∼ 173°. In low-resolution H I surveys this HV gas appears as faint, wing-like, H I emission that extends to velocities beyond those allowed by Galactic rotation. We designate this feature as Forbidden Velocity Wing (FVW) 172.8+1.5. Our high-resolution (3'.4) Arecibo H I observations show that FVW 172.8+1.5 is composed of knots, filaments, and ring-like structures distributed over an area of a few degrees in extent. These HV H I emission features are confined within the limits of the H II complex G173+1.5, which is composed of five Sharpless H II regions distributed along a radio continuum loop of size 4°.4x3°.4, or ∼138 pcx107 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 H I 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), 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 an SNR. We also detect hot X-ray-emitting gas inside the H II complex by analyzing the ROSAT all-sky X-ray background survey data. This also supports the SNR interpretation. We conclude that the HV H I gas and the X-rays are most likely the products of a supernova explosion(s) within the H II complex, possibly in a cluster that triggered the formation of these H II regions.