Galactic Center Shells and Recurrent Starburst Model

Yoshiaki SOFUE


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