The Ophiuchus Superbubble: A Gigantic Eruption from the Inner Disk of the Milky Way

While studying extraplanar neutral hydrogen in the disk-halo transition ofthe inner Galaxy we have discovered what appears to be a huge superbubblecentered around l ~ 30 deg, whose top extends to latitudes > 25 deg at adistance of about 7 kpc. It is detected in both HI and Halpha. Using the GreenBank Telescope of the NRAO, we have measured more than 220,000 HI spectra at 9'angular resolution in and around this structure. The total HI mass in thesystem is ~ 10^6 Msol and it has an equal mass in H+. The Plume of HI cappingits top is 1.2 x 0.6 kpc in l and b and contains 3 x 10^4 Msol of HI. Despiteits location, (the main section is 3.4 kpc above the Galactic plane) thekinematics of the Plume appears to be dominated by Galactic rotation, but witha lag of 27 km/s from corotation. At the base of this structure there are``whiskers'' of HI several hundreds of pc wide, reaching more than 1 kpc intothe halo; they have a vertical density structure suggesting that they are thebubble walls and have been created by sideways rather than upwards motion. Theyresemble the vertical dust lanes seen in NGC891. From a Kompaneets model of anexpanding bubble, we estimate that the age of this system is ~ 30 Myr and itstotal energy content ~ 10^53 ergs. It may just now be at the stage where itsexpansion has ceased and the shell is beginning to undergo significantinstabilities. This system offers an unprecedented opportunity to study anumber of important phenomena at close range, including superbubble evolution,turbulence in an HI shell, and the magnitude of the ionizing flux above theGalactic disk.Comment: 44 pages, 14 figures, accepted for publication in the Astrophysical Journa