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.

Tidal disruption events (TDEs) in active galactic nuclei (AGNs) have been overlooked for a long time but tentatively been investigated recently. We report the discovery of a long-lasting luminous mid-infrared (mid-IR) flare in PS1-10adi, which is a newly-identified highly energetic transient event occurred in AGN. The IR luminosity of PS1-10adi, as well as other analogous events, are at least one order of magnitude higher than all known supernova, but can be well interpreted as the dust echoes of TDEs, whose ultra-high IR energy is reprocessed from the dusty torus around the black hole. The torus dust is sublimating during the early stage of the outburst and probably lead to the observed rapid emergence of Fe II lines. Moreover, the UV-optical rebrightening and contemporaneous X-ray onset after ∼ 1500 rest-frame days since the optical peak is also an intriguing feature of PS1-10adi, which could be attributed to the interaction between the high-velocity outflow and torus. We suggest that the luminous IR echo is a very typical phenomenon of TDEs in AGNs and may provide us an ideal opportunity to explore the torus properties.

Quasar outflows may play a crucial role in regulating the host galaxy, although the spatial scale of quasar outflows remain a major enigma, with their acceleration mechanism poorly understood. The kinematic information of outflow is the key to understanding its origin and acceleration mechanism. Here, we report the galactocentric distances of different outflow components for both a sample and an individual quasar. We find that the outflow distance increases with velocity, with a typical value from several parsecs to more than one hundred parsecs, providing direct evidence for an acceleration happening at a scale of the order of 10 parsecs. These outflows carry ∼1% of the total quasar energy, while their kinematics are consistent with a dust-driven model with a launching radius comparable to the scale of a dusty torus, indicating that the coupling between dust and quasar radiation may produce powerful feedback that is crucial to galaxy evolution.

The origin of narrow line region (NLR) outflows remains unknown. In this paper, we explore the scenario in which these outflows are circumnuclear clouds driven by energetic accretion disk winds. We choose the well-studied nearby Seyfert galaxy NGC 4151 as an example. By performing 3D hydrodynamical simulations, we are able to reproduce the radial distributions of velocity, mass outflow rate and kinetic luminosity of NLR outflows in the inner 100 pc deduced from spatial resolved spectroscopic observations. The demanded kinetic luminosity of disk winds is about two orders of magnitude higher than that inferred from the NLR outflows, but is close to the ultrafast outflows (UFO) detected in X-ray spectrum and a few times lower than the bolometric luminosity of the Seyfert. Our simulations imply that the scenario is viable for NGC 4151. The existence of the underlying disk winds can be confirmed by their impacts on higher density ISM, e.g., shock excitation signs, and the pressure in NLR.

In this paper, we report the peculiar HI morphology of the cluster spiral galaxy NGC 6145, which has a 150 kpc HI filament on one side that is nearly parallel to its major axis. This filament is made up of several HI clouds and the diffuse HI gas between them, with no optical counterparts. We compare its HI distribution with other one-sided HI distributions in the literature, and find that the overall HI distribution is very different from the typical tidal and ram-pressure stripped HI shape, and its morphology is inconsistent with being a pure accretion event. Only ∼30% of the total HI gas is anchored on the stellar disk, while most of HI gas forms the filament in the west. At a projected distance of 122 kpc, we find a massive elliptical companion (NGC 6146) with extended radio emission, whose axis points to an HI gap in NGC 6145. The velocity of the HI filament shows an overall light-ofsight motion of 80 to 180 km s −1 with respect to NGC 6145. Using the long-slit spectra of NGC 6145 along its major stellar axis, we find that some outer regions show enhanced star formation, while in contrast, almost no star formation activities are found in its center (<2 kpc). Pure accretion, tidal or ram-pressure stripping is not likely to produce the observed HI filament. An alternative explanation is the jet-stripping from NGC 6146, although direct evidence for a jet-cold gas interaction has not been found.

Abstract. By performing three-dimensional hydrodynamical simulations, we show that the Fermi bubbles could be inflated by winds launched from the "past" hot accretion flow in Sgr A*. The parameters of the accretion flow required in the model are consistent with those obtained independently from other observational constraints. The wind parameters are taken from small scale MHD numerical simulations of hot accretion flows. Keywords. accretion, black hole physics, jets, hydrodynamicsA pair of giant gamma-ray bubbles which extend 50 degrees above and below the Galactic plane with a width of 40 degrees are revealed by the Fermi Gamma-ray Space Telescope (Su et al. 2010). The formation mechanism of the bubbles is still under debate.Many observations have strongly indicated that the activity of the supermassive black hole located in the Galactic center, Sgr A*, is likely much stronger than the present time (Totani 2006), and the Fermi bubbles may be the result of this activity. Specifically, the previous independent quantitative studies to the past activity show that while Sgr A* was also in a hot accretion regime, the accretion rate should be 3 ∼ 4 orders of magnitude higher than the present value and last for 10 7 yr. One of the most important features of hot accretion flow is the existence of strong wind (Yuan et al. 2012; see review in Yuan & Narayan 2014). The main properties of winds such as mass flux and velocity have been obtained from the analysis to the MHD numerical simulation data (Yuan et al. 2012).Based on these knowledge and constraints, we have performed three-dimensional hydrodynamical numerical simulations to study the formation of the Fermi bubbles. The properties of wind such as the mass flux and velocity are not free parameters but are obtained from previous works on MHD numerical simulations of hot accretion flows once the mass accretion rate is given. The power of wind is P w = 2 × 10 41 erg s −1 . The supersonic winds blow into the ISM in the galactic halo, and produce shock. The contact discontinuity separating the shocked ISM and the shocked winds is the boundary of the Fermi bubbles. We find that this process can well explain the observed morphology of the Fermi bubbles. The active phases is required to last for about 10 million years and the later quiescent state should last for no more than 0.2 million years. Disc-like and massive Central Molecular Zone (CMZ), which are located along the galactic plane around Sgr A*, changes the wind orientation to be approximately towards Galactic poles. This is the reason why we obtain a narrow waist of the Fermi bubbles near the galactic plane. Viscosity suppresses the Rayleigh-Taylor (RT) instability and Kelvin-Helmholtz (KH) instability, inducing a smooth edge of the bubble. The observed ROSAT X-ray features can be interpreted by the shocked interstellar medium (ISM) and the interaction region between wind and CMZ gas. We find that the temperature in the shocked ISM at high 137 available at https://www.cambridge.org/core/terms. https://doi