AN OLD SUPERNOVA REMNANT WITHIN AN H II COMPLEX AT ℓ ≈ 173°: FVW 172.8+1.5

Abstract

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.