Fermi Bubbles Inflated by Winds Launched from the Hot Accretion Flow in Sgr A*

摘要

A pair of giant gamma-ray bubbles have been revealed by the {\it Fermi} LAT.In this paper we investigate their formation mechanism. Observations haveindicated that the activity of the supermassive black hole located at theGalactic center, Sgr A*, was much stronger than the present time. Specifically,one possibility is that while Sgr A* was also in the hot accretion regime, theaccretion rate should be $10^3-10^4$ times higher during the past $\sim 10^7$yr. On the other hand, recent MHD numerical simulations of hot accretion flowshave unambiguously shown the existence of strong winds and obtained theirproperties. Based on these knowledge, by performing three-dimensionalhydrodynamical simulations, we show in this paper that the Fermi bubbles couldbe inflated by winds launched from the ``past' hot accretion flow in Sgr A*. Inour model, the active phase of Sgr A* is required to last for about 10 millionyears and it was quenched no more than 0.2 million years ago. The CentralMolecular Zone (CMZ) is included and it collimates the wind orientation towardsthe Galactic poles. Viscosity suppresses the Rayleigh-Taylor andKelvin-Helmholtz instabilities and results in the smoothness of the bubbleedge. The main observational features of the bubbles can be well explained.Specifically, the {\it ROSAT} X-ray features are interpreted by the shockedinterstellar medium and the interaction region between winds and CMZ gas. Thethermal pressure and temperature obtained in our model are in good consistencywith the recent {\it Suzaku} observations.