Two Large H [CSC]i[/CSC] Shells in the Outer Galaxy near [ITAL][CLC]l[/CLC][/ITAL] = 279°

As part of a survey of HI 21-cm emission in the Southern Milky Way, we havedetected two large shells in the interstellar neutral hydrogen near l=279 deg.The center velocities are +36 and +59 km/s, which puts the shells at kinematicdistances of 7 and 10 kpc. The larger shell is about 610 pc in diameter andvery empty, with density contrast of at least 15 between the middle and theshell walls. It has expansion velocity of about 20 km/s and swept up mass ofseveral million solar masses. The energy indicated by the expansion may be ashigh as 2.4 X 10^53 ergs. We estimate its age to be 15 to 20 million years. Thesmaller shell has diameter of about 400 pc, expansion velocity about 10 km/sand swept up mass of about 10^6 solar masses. Morphologically both regions appear to be shells, with high density regionsmostly surrounding the voids, although the first appears to have channels oflow density which connect with the halo above and below the HI layer. They lieon the edge of the Carina arm, which suggests that they may be expandinghorizontally into the interarm region as well as vertically out of the disk. Ifthis interpretation is correct, this is the first detection of an HI chimneywhich has blown out of both sides of the disk.Comment: 21 pages, 14 jpeg figures, accepted for publication in A