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  Title: Galactic Center Shells and a Recurrent Starburst Model
  Authors: Yoshiaki SOFUE (IoA, Univ. Tokyo)
  Comments: To appear in PASJ, Latex 8 pages, 5 figures,
See http://www.ioa.s.u-tokyo.ac.jp/~sofue/htdocs/2003gcs/ for original figures.
  Report no: Univ. Tokyo Astron. Preprint No. 2003-03 
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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.
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