A pair of giant gamma-ray Bubbles has been revealed by Fermi-LAT. In this paper we investigate their formation mechanism. Observations have indicated that the activity of the supermassive black hole located at the Galactic center, Sgr A*, was much stronger than at the present time. Specifically, one possibility is that while Sgr A* was also in the hot accretion regime, the accretion rate should be 10 3-10 4 times higher during the past ∼10 7 yr. On the other hand, recent magnetohydrodynamic numerical simulations of hot accretion flows have unambiguously shown the existence and obtained the properties of strong winds. Based on this knowledge, by performing threedimensional hydrodynamical simulations, we show in this paper that the Fermi Bubbles could be inflated by winds launched from the "past" hot accretion flow in Sgr A*. In our model, the active phase of Sgr A* is required to last for about 10 million years and it was quenched no more than 0.2 million years ago. The central molecular zone (CMZ) is included and it collimates the wind orientation toward the Galactic poles. Viscosity suppresses the Rayleigh-Taylor and Kelvin-Helmholtz instabilities and results in the smoothness of the Bubbles edge. The main observational features of the Bubbles can be well explained. Specifically, the ROSAT X-ray features are interpreted by the shocked interstellar medium and the interaction region between the wind and CMZ gas. The thermal pressure and temperature obtained in our model are consistent with recent Suzaku observations.

Quasar outflows carry mass, momentum and energy into the surrounding environment, and have long been considered a potential key factor in regulating the growth of supermassive black holes and the evolution of their host galaxies [1][2][3][4] . A crucial parameter for understanding the origin of these outflows and measuring their influence on their host galaxies is the distance (R) between the outflow gas and the galaxy center 5, 6 . While R has been measured in a number of individual galaxies 7-15 , its distribution remains unknown. Here we report the distributions of R and the kinetic luminosities of quasars outflows, using the statistical properties of broad absorption line variability in a sample of 915 quasars from the Sloan Digital Sky Surveys. The mean and standard deviation of the distribution of R are 10 1.4±0.5 parsecs. The typical outflow distance in this sample is tens of parsec, which is beyond the theoretically predicted location (0.01 ∼ 0.1 parsecs) where the accretion disc line-driven wind is launched 16,17 , but is smaller than the scales of most outflows that are derived using the excited state absorption lines 7-14 . The typical value of the mass-flow rate is of tens to a hundred solar masses per year, or several times the accretion rate. The typical kinetic-to-bolometric luminosity ratio is a few per cent, indicating that outflows are energetic enough to influence the evolution of their host galaxies.Nowadays, theoretical models for galaxy formation and evolution routinely invoke the concept of "quasar feedback"-the strong effect that the active supermassive black hole's (SMBH) energy output exerts on its host galaxy-to keep massive galaxies from forming many stars and becoming overly massive. In 10-40% of the quasars in which the central source and outflowing gas are both in the line of sight, outflows may manifest themselves as broad absorption lines (BALs) 18,19 and BAL outflows are therefore a candidate agent of quasar feedback. The importance of outflows to active galactic nucleus (AGN) feedback can be quantified using the mass-flow rate (Ṁout) and the kinetic luminosity (Ė k ) of the outflowing material. TheṀout and theĖ k of a BAL outflow can be estimated from the distance (R) between the out- * flowing gas and the galaxy center, the total hydrogen column density NH and the fraction Ω of the solid angle subtended by the outflowing gas. Because the ionization parameter UH of the plasma is inversely proportional to the product of hydrogen number density nH and R 2 , i.e., UH ∝ 1/(nHR 2 ), R can be obtained by measuring UH and nH. In general, nH can be determined from the absorption lines of the excited states of ions (e.g., Fe II*, Si II*, S IV*), but this method is hindered by line blending and is therefore only applicable to quasars with relatively narrow absorption lines. During the last decade or so, outflow distances have been measured for only about a dozen of individual quasars using this method 7-14 , while the distributions of primary properties of BAL outflows remain in unknown. ...

In a previous work, we have shown that the formation of the Fermi bubbles can be due to the interaction between winds launched from the hot accretion flow in Sgr A* and the interstellar medium (ISM). In that work, we focus only on the morphology. In this paper we continue our study by calculating the gamma-ray radiation. Some cosmic ray protons (CRp) and electrons must be contained in the winds, which are likely formed by physical processes such as magnetic reconnection. We have performed MHD simulations to study the spatial distribution of CRp, considering the advection and diffusion of CRp in the presence of magnetic field. We find that a permeated zone is formed just outside of the contact discontinuity between winds and ISM, where the collisions between CRp and thermal nuclei mainly occur. The decay of neutral pions generated in the collisions, combined with the inverse Compton scattering of background soft photons by the secondary leptons generated in the collisions and primary CR electrons can well explain the observed gamma-ray spectral energy distribution. Other features such as the uniform surface brightness along the latitude and the boundary width of the bubbles are also explained. The advantage of this "accretion wind" model is that the adopted wind properties come from the detailed small scale MHD numerical simulation of accretion flows and the value of mass accretion rate has independent observational evidences. The success of the model suggests that we may seriously consider the possibility that cavities and bubbles observed in other contexts such as galaxy clusters may be formed by winds rather than jets.