For its proximity, our Galactic center (GC) is the best laboratory to study feeding and
feedback processes of nuclear activities. Recent high-energy observations have actually
revealed relics of past GC activities. X-ray studies discovered many X-ray reflection
nebulae illuminated by past flares (100-1000 years ago) of the supermassive black hole,
Sagittarius A* (Sgr A*). Subsequently, the GeV gamma-ray survey performed by Fermi/LAT
discovered the bubble-like structures extending up to 10 kpc from the GC (the so-called
“Fermi bubbles”), origin of which is considered to be a Sgr A* flare or a starburst
activity ~10 million years ago.

In this talk, I will present the new relic of a past GC activity discovered by the
Suzaku and ASCA observations. We found outflow-like plasmas extending up to ~300 pc from
the GC. On the basis of its large thermal energy and the peculiar ionization state, we
concluded that the plasmas were formed by a GC activity ~100,000 years ago. It might be
related to the other past activities.