Galactic Center Shells and a Recurrent Starburst Model

By applying filtering techniques to remove straight filaments in the 20-cm VLA radio image of the Galactic Center Arc region, we have shown that numerous concentric radio shells of radii 5 to 20 pc are surrounding the Pistol and Sickle region, which we call Galactic Center Shells (GCS). Each shell has thermal energy of the order of $10^{49-50}$ erg. Several CO-line shells are associated, whose kinetic energies are of the order of $10^{49-50}$ erg. Summing up the energies of recognized GCSs, the total energy amounts to $\sim 10^{51}$ erg. The GCSs show an excellent correlation with the FIR shells observed at 16--26 microns with the MSX. We propose a model in which GCSs were produced by recurrent and/or intermittent starbursts in the Pistol area during the last million yr. The most recent burst occurred some $10^5$ years ago, producing an inner round-shaped shell (GCS I); earlier ones a million years ago produced outer shells (GCS II and III), which are more deformed by interactions with the surrounding ISM and Sgr A halo. We argue that recurrent starbursts had also occurred in the past, which produced larger scale hyper shell structures as well. A burst some million years ago produced the Galactic Center Lobe, and a much stronger one 15 million years ago produced the North Polar Spur.