We have been studying extra-planar neutral hydrogen (HI) in the disk-halo transition of the inner Galaxy with the Green Bank Telescope (GBT) and the Very Large Array (VLA) of the National Radio Astronomy Observatory (NRAO). The main results of this study are 1) the discovery of a huge superbubble, whose top lags behind the rotation of the Milky Way, and 2) increased understanding of the nature of halo clouds.
The study revealed a huge superbubble centered around longitude ≈ 30°, extending above the Galactic plane to latitudes >25° at a distance of about 7 kpc. It is detected in both HI and Hα. Using the GBT, we have measured more than 220,000 HI spectra at 9' angular resolution in and around this structure. The total HI mass in the system is about a million M Sunand it has an equal mass in H +. From a Kompaneets model we estimate that the age of this system is ≈ 30 Myr and its total energy content ~ 10 53ergs. Both observational and theoretical evidence suggests that the shell is now undergoing significant instabilities.
The top of the superbubble (located at z ≈ 3.4 kpc) contains 3 x 10 4M Sunof HI and appears to be dominated by Galactic rotation, but with a lag of 27 km s -1from corotation. We offer a model that may explain this lag.
Much of extra-planar HI in the inner Milky Way is organized into cloud-like structures not related to the classic high-velocity clouds. They are found to distances >2 kpc from the plane and can contain hundreds of M Sunof HI. Many of these clouds are parts of the superbubble possibly revealing their origin: turbulent decomposition of supershells. Twenty clouds were observed in 21cm emission with the VLA at 1' resolution, revealing their internal structure. Several of the clouds were also observed in HI absorption against background continuum sources with the VLA and the Giant Metrewave Radio Telescope (GMRT) to measure their opacity and spin temperature. These results indicate that the clouds may contain little turbulence.