Abstract
We report the detection of a ringlike H I structure toward l = 900, b = 28, with a velocity of vLSR = -99 km s-1. This velocity implies a distance of d = 13 kpc, corresponding to a Galactocentric radius of Rgal = 15 kpc. The l-vLSR diagram implies an expansion velocity of vexp 15 km s-1 for the shell. The structure has an oblate, irregular shell-like appearance, which surrounds weak infrared emission as seen in the 60 μm Infrared Astronomical Satellite data. At a distance of 13 kpc the size of the object is about 110 × 220 pc and placed 500 pc above the Galactic plane, with a mass of 105 M☉. An expanding shell with such a high mass and diameter cannot be explained by a single supernova explosion or by a single stellar wind bubble. We interpret the structure as a relic of a distant stellar activity region powered by the joint action of strong stellar winds from early-type stars and supernova explosions.
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