Two large H (I) shells in the outer galaxy near l=279 degrees

As part of a survey of H I lambda 21 cm emission in the southern Milky Way, we have detected two large shells in the interstellar neutral hydrogen nea r l = 279 degrees. The center velocities are +36 and +59 km s(-1) which put s the shells at kinematic distances of 7 and 10 kpc. The larger shell is ab out 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 x 10(53) ergs. We estimate its age to be 15 to 20 million years. The smaller shell has diameter of ab out 400 pc, expansion velocity about 10 km s(-1), and swept-up mass of abou t 10(6) M-circle dot. Morphologically both regions appear to be shells, wit h high-density regions mostly surrounding the voids, although the first app ears to have channels of low density that connect with the halo above and b elow 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 a s vertically out of the disk. If this interpretation is correct, this is th e first detection of an H I chimney which has blown out of both sides of th e disk.