H I Shells behind the Coalsack

We report the discovery of two new large H i shells in the direction of the Coalsack nebula. Both shells were observed with the Parkes Radio Telescope as part of the Southern Galactic Plane Survey. The largest shell, GSH 304-00-12, is at a distance of ∼ 1.2 kpc and has derived physical dimensions of 280×200 pc. The second shell, GSH 305+01-24, is at a distance of ∼ 2.2 kpc and has derived dimensions of 280× 440 pc. We present a simple numerical model to show that GSH 305+01-24 most likely formed from stellar winds in the Centaurus OB1 stellar association. There is associated radio, infrared and Hα continuum emission. Both shells are situated in the Sagittarius-Carina arm, with GSH 305+01-24 more distant. The far edge of GSH 304-00-12 is at the near side of the arm and opens into the interarm region. We find no evidence for closure at the near side of the shell and therefore describe the geometry as conical. Emission from the near side of the shell may be lost in absorption by the Coalsack Nebula. Subject headings: ISM: structure, bubbles — Galaxy: structure, kinematics and dynamics — radio lines: ISM