Two Large H I Shells in the Outer Galaxy near l = 279°

摘要

As part of a survey of H I λ21 cm emission in the southern Milky Way, we have detected two large shells in the interstellar neutral hydrogen near l = 279°. The center velocities are +36 and +59 km s-1, which puts the shells at kinematic distances of 7 and 10 kpc. The larger shell is about 610 pc in diameter and very empty, with density contrast of at least 15 between the middle and the shell walls. It has expansion velocity of about 20 km s-1 and swept-up mass of several million solar masses. The energy indicated by the expansion may be as high as 2.4 × 1053 ergs. We estimate its age to be 15 to 20 million years. The smaller shell has diameter of about 400 pc, expansion velocity about 10 km s-1, and swept-up mass of about 106 M⊙. Morphologically both regions appear to be shells, with high-density regions mostly surrounding the voids, although the first appears to have channels of low density that connect with the halo above and below the H I layer. They lie on the edge of the Carina arm, which suggests that they may be expanding horizontally into the interarm region, as well as vertically out of the disk. If this interpretation is correct, this is the first detection of an H I chimney which has blown out of both sides of the disk.