2007ApJ...656..928P

While studying extraplanar neutral hydrogen in the disk-halo transition of the inner Galaxy, we have discovered what appears to be a huge superbubble centered around l~30°, whose top extends to latitudes >25° at a distance of about 7 kpc. It is detected in both H I and Hα. Using GBT, we have measured more than 220,000 H I spectra at 9' angular resolution in and around this structure. The total H I mass in the system is ~10⁶ M_☉, and it has an equal mass in H⁺. The plume of H I capping its top is 1.2x0.6 kpc in l and b and contains 3x10⁴ M_☉ of H I. Despite its location (the main section is 3.4 kpc above the Galactic plane), the kinematics of the plume appears to be dominated by Galactic rotation, but with a lag of 27 km/s from corotation. At the base of this structure there are ``whiskers'' of H I several hundreds of parsecs wide, reaching more than 1 kpc into the halo; they have a vertical density structure suggesting that they are the bubble walls and have been created by sideways rather than upward motion. They resemble the vertical dust lanes seen in NGC 891. From a Kompaneets model of an expanding bubble, we estimate that the age of this system is ~30 Myr and its total energy content ∼10⁵³ ergs. It may just now be at the stage where its expansion has ceased and the shell is beginning to undergo significant instabilities. This system offers an unprecedented opportunity to study a number of important phenomena at close range, including superbubble evolution, turbulence in an H I shell, and the magnitude of the ionizing flux above the Galactic disk.