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Sgr A lobes are a pair of 15-pc-sized bipolar bubbles with co-aligned major axes perpendicular to the Galactic plane found in X-ray and radio observations of the Galactic center (GC). Their elusive origin is a vital ingredient in understanding the ongoing high energy processes at the GC. Here we perform a suite of hydrodynamic simulations to explore the tidal disruption event (TDE) outflow model for the origin of the Sgr A lobes. By following the outflow evolution in the circumnuclear medium, we find that TDE outflows naturally produce bipolar lobes delimited by forward shocks, and the dense postshock shell contributes significantly to the lobe's X-ray emission. Our fiducial run reproduces the morphology, density, temperature, and X-ray surface brightness distribution of the observed Sgr A lobes reasonably well. The current lobe age is ~3300 yr. Our model further predicts that the uplifted wake flow breaks through the ejecta-induced shock front, producing a shock-enclosed head region which is relatively dim in X-rays compared to other lobe regions. Both the dim head region and the predicted limb-brightening feature of the lobes are slightly inconsistent with current X-ray observations, and may be used to further test our model with more sensitive future X-ray observations. While light narrow jets and massive wide winds from TDE events usually do not reproduce the observed oval-shaped morphology of the lobes, the TDE outflow in our fiducial run is massive and yet narrow. Whether it is a jet or wind remains uncertain and future simulations covering both the outflow acceleration region and its pc-scale evolution would be very helpful in determining whether the Sgr A lobes indeed originate from a pair of TDE jets or winds., Comment: 10 pages, 10 figures. Comments welcome

We report the discovery of the 10 kilo-parsec (kpc) scale radio lobes in the Sombrero galaxy (NGC 4594), using data from the Continuum Halos in Nearby Galaxies - an Expanded Very Large Array (VLA) Survey (CHANG-ES) project. We further examine the balance between the magnetic pressure inside the lobes and the thermal pressure of the ambient hot gas. At the radii $r$ of ~(1-10) kpc, the magnetic pressure inside the lobes and the thermal pressure of the ambient hot gas are generally in balance. This implies that the jets could expand into the surroundings at least to r ~ 10 kpc. The feedback from the active galactic nucleus (AGN) jet responsible for the large-scale lobes may help to explain the unusually high X-ray luminosity of this massive quiescent isolated disk galaxy, although more theoretical work is needed to further examine this possibility., Comment: 8 pages, 3 figures, 2 tables, accepted for publication in ApJ

Multi-wavelength observations indicate that the intracluster medium in some galaxy clusters contains cold filaments, while their formation mechanism remains debated. Using hydrodynamic simulations, we show that cold filaments could naturally condense out of hot gaseous wake flows uplifted by the jet-inflated active galactic nucleus (AGN) bubbles. Consistent with observations, the simulated filaments extend to tens of kiloparsecs from the cluster center, with a representative mass of $\rm 10^{8}- 10^{9}\ M_{\odot}$ for a typical AGN outburst energy of $10^{60}~ \rm erg$. They show smooth velocity gradients, stretching typically from inner inflows to outer outflows with velocity dispersions of several hundred kilometers per second. The properties of cold filaments are affected substantially by jet properties. Compared to kinetic-energy-dominated jets, thermal-energy-dominated jets are easier to produce long cold filaments with large masses as observed. AGN jets with an early turn-on time, a low jet base, or a very high power tend to overheat the cluster center, and produce short cold filaments that take a relatively long time to condense out., Comment: 15 pages, 8 figures, revised version, published in ApJ

The Hot Universe Baryon Surveyor (HUBS) is a proposed space-based X-ray telescope for detecting X-ray emissions from the hot gas content in our universe. With its unprecedented spatially-resolved high-resolution spectroscopy and large field of view, the HUBS mission will be uniquely qualified to measure the physical and chemical properties of the hot gas in the interstellar medium, the circumgalactic medium, the intergalactic medium, and the intracluster medium. These measurements will be valuable for two key scientific goals of HUBS, namely to unravel the AGN and stellar feedback physics that governs the formation and evolution of galaxies, and to probe the baryon budget and multi-phase states from galactic to cosmological scales. In addition to these two goals, the HUBS mission will also help us solve some problems in the fields of galaxy clusters, AGNs, diffuse X-ray backgrounds, supernova remnants, and compact objects. This paper discusses the perspective of advancing these fields using the HUBS telescope., Comment: 52 pages, 22 figures. Accepted for publication in Science China: Physics, Mechanics and Astronomy

Inflows at the dusty torus and smaller scales is crucial to investigate the process of supermassive black hole accretion. However, only few cases of inflowing gas at small scales have been reported through redshifted broad absorption lines so far. Here we report 9 redshifted narrow absorption lines (NALs) of $\rm Mg^+$ ions with inflowing speeds of 1071 -- 1979 km/s, which are likely along the directions close to the axes of accretion disks. The quasars showing inflowing Mg II NALs have on average slightly smaller Eddington ratios when compared to the sources with outflow Mg II NALs. The upper limits of locations of the detected NALs are at parsec scale, around the distances of dusty tori to central SMBHs. The one possible origin of these infalling NALs is from dusty tori. Of course, these infalling NALs can also be naturally explained by chaotic cold accretion resulted from the nonlinear interaction of active galactic nucleus (AGN) jets with the interstellar medium, and these cold gaseous blobs may originally precipitate in metal-rich trailing outflows uplifted by AGN jet ejecta. The infalling NALs may thus provide direct evidence for cold gas precipitation and accretion in AGN feedback processes, and provide the direct evidence of inflowing gas along the directions close to quasar jets and at parsec scale. It does not matter whether these infalling NALs are from the dusty tori or the interaction of AGN jets with the ISM, the infalling NALs cannot provide sufficient fuels to power the quasars., Comment: 12 page, 7 figures, Accepted by A&A

The hot halo gas distribution in the inner Milky Way (MW) contains key fossil records of the past energetic feedback processes in the Galactic center. Here we adopt a variety of spherical and disk-like MW halo gas models as initial conditions in a series of simulations to investigate the formation of the Fermi bubbles in the jet-shock scenario. The simulation results are compared directly with relevant X-ray and gamma-ray observations of the Fermi bubbles to constrain the halo gas distribution in the inner Galaxy before the Fermi bubble event. Our best-fit gas density distribution can be described by a power law in radius $n_{\rm e}(r)=0.01(r/1 \text{~kpc})^{-1.5}$ cm$^{-3}$. Our study can not determine if there is an inner density core, which if exists, should be very small with size $r_{c} \lesssim 0.5$ kpc. When extrapolating to large radii $r\sim 50-90$ kpc, our derived density distribution lies appreciably below the recently estimated gas densities from ram-pressure stripping calculations, suggesting that the halo gas density profile either flattens out or has one or more discontinuities within $10 \lesssim r \lesssim 50$ kpc. Some of these discontinuities may be related to the eROSITA bubbles, and our derived gas density profile may correspond to the hot gas distribution in the inner eROSITA bubbles about $5$ Myr ago., Comment: 16 pages, 12 figures. Accepted for publication ApJ

Tidal disruption events (TDEs) occurred in active galactic nuclei (AGNs) are a special class of sources with outstanding scientific significance. TDEs can generate ultrafast winds, which should almost inevitably collide with the preexisting AGN dusty tori. We perform analytical calculations and simulations on the wind-torus interactions and find such a process can generate considerable X-ray afterglow radiation several years or decades later after the TDE outburst. This provides a new origin for the years delayed X-rays in TDEs. The X-ray luminosity can reach 10^{41-42} erg/s, and the light curve characteristics depend on the parameters of winds and tori. We apply the model to two TDE candidates, and provide lower limits on the masses of the disrupted stars, as well as rigorous constraints on the gas densities of tori. Our results suggest that the observations of the time delay, spectral shape, luminosity and the light curve of the X-ray afterglow can be used to constrain the physical parameters of both TDE winds and tori, including the wind velocity, wind density and torus density., Comment: 13 pages, submitted to ApJ

Mechanical active galactic nucleus (AGN) feedback plays a key role in massive galaxies, galaxy groups and clusters. However, the energy content of AGN jets that mediate this feedback process is still far from clear. Here we present a preliminary study of radial elongations $\tau$ of a large sample of X-ray cavities, which are apparently produced by mechanical AGN feedback. All the cavities in our sample are elongated along the angular (type-I) or jet directions (type-II), or nearly circular (type-III). The observed value of $\tau$ roughly decreases as the cavities rise buoyantly, confirming the same trend found in hydrodynamic simulations. For young cavities, both type-I and II cavities exist, and the latter dominates. Assuming a spheroidal cavity shape, we derive an analytical relation between the intrinsic radial elongation $\bar{\tau}$ and the inclination-angle-dependent value of $\tau$, showing that projection effect makes cavities appear more circular, but does not change type-I cavities into type-II ones, or vice versa. We summarize radial elongations of young cavities in simulations, finding that $\bar{\tau}$ increases with the kinetic fraction of AGN jets. While mild jets always produce type-II cavities, thermal-energy-dominated strong jets produce type-I cavities, and kinetic-energy-dominated strong jets produce type-II cavities. Our results suggest that some AGN jets are strong and dominated by thermal energy (or cosmic rays). However, these jets do not dominate in AGN feedback. If most jets are dominated by non-kinetic energies, they should be mainly mild jets. If most jets are strong, they must be mainly dominated by the kinetic energy., Comment: Slightly revised version accepted for publication in ApJ. 11 pages, 6 figures

We propose a novel method to constrain the Milky Way (MW) mass $M_{\rm vir}$ with its corona temperature observations. For a given corona density profile, one can derive its temperature distribution assuming a generalized equilibrium model with non-thermal pressure support. While the derived temperature profile decreases substantially with radius, the X-ray-emission-weighted average temperature, which depends most sensitively on $M_{\rm vir}$, is quite uniform toward different sight lines, consistent with X-ray observations. For an Navarro-Frenk-White (NFW) total matter distribution, the corona density profile should be cored, and we constrain $M_{\rm vir}=(1.19$ - $2.95) \times 10^{12} M_{\rm sun}$. For a total matter distribution contributed by an NFW dark matter profile and central baryons, the corona density profile should be cuspy and $M_{\rm vir,dm}=(1.34$ - $5.44) \times 10^{12} M_{\rm sun}$. Non-thermal pressure support leads to even higher values of $M_{\rm vir}$, while a lower MW mass may be possible if the corona is accelerating outward. This method is independent of the total corona mass, its metallicity, and temperature at very large radii., Comment: Significantly revised version, accepted for publication in ApJ Letters. Discussions largely expanded. 7 pages, 4 figures

Active galactic nucleus (AGN) jets are believed to be important in solving the cooling flow problem in the intracluster medium (ICM), while the detailed mechanism is still in debate. Here we present a systematic study on the energy coupling efficiency $\eta_{\rm cp}$, the fraction of AGN jet energy transferred to the ICM. We first estimate the values of $\eta_{\rm cp}$ analytically in two extreme cases, which are further confirmed and extended with a parameter study of spherical outbursts in a uniform medium using hydrodynamic simulations. We find that $\eta_{\rm cp}$ increases from $\sim 0.4$ for a weak isobaric injection to $\gtrsim 0.8$ for a powerful point injection. For any given outburst energy, we find two characteristic outburst powers that separate these two extreme cases. We then investigate the energy coupling efficiency of AGN jet outbursts in a realistic ICM with hydrodynamic simulations, finding that jet outbursts are intrinsically different from spherical outbursts. For both powerful and weak jet outbursts, $\eta_{\rm cp}$ is typically around $0.7-0.9$, partly due to the non-spherical nature of jet outbursts, which produce backflows emanating from the hotspots, significantly enhancing the ejecta-ICM interaction. While for powerful outbursts a dominant fraction of the energy transferred from the jet to the ICM is dissipated by shocks, shock dissipation only accounts for $\lesssim 30\%$ of the injected jet energy for weak outbursts. While both powerful and weak outbursts could efficiently heat cooling flows, powerful thermal-energy-dominated jets are most effective in delaying the onset of the central cooling catastrophe., Comment: 20 pages, 12 figures, slightly revised version, Accepted for publication in ApJ

The Fermi bubbles are two giant bubbles in gamma rays lying above and below the Galactic center (GC). Despite numerous studies on the bubbles, their origin and emission mechanism remain elusive. Here we use a suite of hydrodynamic simulations to study the scenario where the cosmic rays (CRs) in the bubbles are mainly accelerated at the forward shocks driven by a pair of opposing jets from Sgr A*. We find that an active galactic nucleus (AGN) jet event happened $5-6$ Myr ago can naturally reproduce the bilobular morphology of the bubbles, and the postshock gas temperature in the bubbles is heated to $\sim0.4$ keV, consistent with recent X-ray observations. The forward shocks compress the hot halo gas, and at low latitudes, the compressed gas shows an X-shaped structure, naturally explaining the biconical X-ray structure in the ROSAT 1.5 keV map in both morphology and X-ray surface brightness. CR acceleration is most efficient in the head regions of the bubbles during the first 2 Myrs. The opposing jets release a total energy of $\sim 10^{55}$ erg with an Eddington ratio of $\sim 10^{-3}$, which falls well in the range of the hot accretion flow mode for black holes. Our simulations further show that the forward shocks driven by spherical winds at the GC typically produce bubbles with much wider bases than observed, and could not reproduce the biconical X-ray structure at low latitudes. This suggests that starburst or AGN winds are unlikely the origin of the bubbles in the shock scenario., Comment: Slightly revised version, accepted for publication in ApJ. 15 pages, 15 figures

Theoretical and observational arguments suggest that there is a large amount of hot ($\sim 10^6$ K), diffuse gas residing in the Milky Way's halo, while its total mass and spatial distribution are still unclear. In this work, we present a general model for the gas density distribution in the Galactic halo, and investigate the gas evolution under radiative cooling with a series of 2D hydrodynamic simulations. We find that the mass inflow rate in the developed cooling flow increases with gas metallicity and the total gas mass in the halo. For a fixed halo gas mass, the spatial gas distribution affects the onset time of the cooling catastrophe, which starts earlier when the gas distribution is more centrally-peaked, but does not substantially affect the final mass inflow rate. The gravity from the Galactic bulge and disk affects gas properties in inner regions, but has little effect on the final inflow rate either. We confirm our results by investigating cooling flows in several density models adopted from the literature, including the Navarro-Frenk-White (NFW) model, the cored-NFW model, the Maller & Bullock model, and the $\beta$ model. Typical mass inflow rates in our simulations range from $\sim 5 M_{\odot}$ yr$^{-1}$ to $\sim 60 M_{\odot}$ yr$^{-1}$, and are much higher than the observed star formation rate in our Galaxy, suggesting that stellar and active galactic nucleus feedback processes may play important roles in the evolution of the Milky Way (MW) and MW-type galaxies., Comment: Slightly revised version, accepted for publication in ApJ. 15 pages, 13 figures

This paper summarizes our recent works of studying AGN feedback in an isolated elliptical galaxy by performing high-resolution hydrodynamical numerical simulations. Bondi radius is resolved and the mass accretion rate of the black hole is calculated. The most updated AGN physics, namely the discrimination of cold and hot accretion modes and the exact descriptions of the AGN radiation and wind for a given accretion rate are adopted and their interaction with the gas in the host galaxy is calculated. Physical processes such as star formation and SNe feedback are taken into account. Consistent with observation, we find the AGN spends most of the time in the low-luminosity regime. AGN feedback overall suppresses the star formation; but depending on location in the galaxy and time, it can also enhance it. The light curve of specific star formation rate is not synchronous with the AGN light curve. We find that wind usually plays a dominant role in controlling the AGN luminosity and star formation, but radiation also cannot be neglected., Comment: 7 pages, 3 figures; proceedings of the IAUS342 "Perseus in Sicily: from black hole to cluster outskirts"

Recent Chandra X-ray observations of many galaxy clusters find evidence for hot metal-rich outflows preferentially aligned with the large-scale axes of X-ray cavities with typical outflow masses of around $10^{9} $ - $10^{10} M_{\odot}$. Here we perform a suite of three hydrodynamic simulations to investigate whether AGN jets could drive these metal-rich outflows in a representative cluster. By using both the tracer variable and virtual particle methods, and additionally following the gas metallicity evolution, we show that metal-rich gas initially located in central regions can indeed be uplifted by the AGN bubble to large distances, a phenomenon called Darwin drift in fluid mechanics, and forming a filamentary trailing outflow extending beyond $100$ kpc behind the bubble. The gas entrained in the trailing outflow is entirely outflowing with an average outflow rate of nearly $100M_{\odot}$/yr during the first $100$ Myr, and at later times, a growing lower part flows back towards the cluster center due to gravity. The outflow mass rises up to about $10^{10} M_{\odot}$ with entrained iron mass of about $10^{6} - 10^{7}M_{\odot}$, consistent with observations and predictions from the drift model. By the end of our simulation ($\sim 800$ Myr after the AGN event), several $10^{9}M_{\odot}$ of the uplifted high-metallicity gas still remains at large altitudes, potentially contributing to the enrichment of the bulk ICM and the broadening of central metallicity peaks observed in cool core clusters., Comment: 9 pages, 5 figures, slightly revised version, Accepted for publication in ApJ

Galaxy clusters host a large reservoir of diffuse plasma with radially-varying temperature profiles. The efficiency of thermal conduction in the intracluster medium (ICM) is complicated by the existence of turbulence and magnetic fields, and has received a lot of attention in the literature. Previous studies suggest that the magnetothermal instability developed in outer regions of galaxy clusters would drive magnetic field lines preferentially radial, resulting in efficient conduction along the radial direction. Using a series of spherically-symmetric simulations, here we investigate the impact of thermal conduction on the observed temperature distributions in outer regions of three massive clusters, and find that thermal conduction substantially modifies the ICM temperature profile. Within 3 Gyr, the gas temperature at a representative radius of $0.3r_{500}$ typically decreases by ~10 - 20% and the average temperature slope between $0.3r_{500}$ and $r_{500}$ drops by ~ 30 - 40%, indicating that the observed ICM would not stay in a long-term equilibrium state in the presence of thermal conduction. However, X-ray observations show that the outer regions of massive clusters have remarkably similar radially-declining temperature profiles, suggesting that they should be quite stable. Our study thus suggests that the effective conductivity along the radial direction must be suppressed below the Spitzer value by a factor of 10 or more, unless additional heating sources offset conductive cooling and maintain the observed temperature distributions. Our study provides a smoking-gun evidence for the suppression of parallel conduction along magnetic field lines in low-collisionality plasmas by kinetic mirror or whistler instabilities., Comment: Slightly revised version, accepted for publication in ApJ. 11 pages, 7 figures

We investigate the effects of AGN feedback on the cosmological evolution of an isolated elliptical galaxy by performing two-dimensional high-resolution hydrodynamical numerical simulations. The inner boundary of the simulation is chosen so that the Bondi radius is resolved. Compared to previous works, the two accretion modes, namely hot and cold, which correspond to different accretion rates and have different radiation and wind outputs, are carefully discriminated and the feedback effects by radiation and wind in each mode are taken into account. The most updated AGN physics, including the descriptions of radiation and wind from the hot accretion flows and wind from cold accretion disks, are adopted. Physical processes like star formation, Type Ia and Type II supernovae are taken into account. We study the AGN light curve, typical AGN lifetime, growth of the black hole mass, AGN duty-cycle, star formation, and the X-ray surface brightness of the galaxy. We compare our simulation results with observations and find general consistency. Comparisons with previous simulation works find significant differences, indicating the importance of AGN physics. The respective roles of radiation and wind feedbacks are examined and it is found that they are different for different problems of interest such as AGN luminosity and star formation. We find that it is hard to neglect any of them, so we suggest to use the names of "cold feedback mode" and "hot feedback mode" to replace the currently used ones., Comment: 25 pages, 14 figures, accepted for publication to ApJ (Revised to match version published in ApJ)

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 about 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-of- sight motion of 80 to 180 km/s 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 (less than 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., Comment: 12 pages, 6 figures; Accepted for publication in AJ

The cooling flow problem is one of the central problems in galaxy clusters, and active galactic nucleus (AGN) feedback is considered to play a key role in offsetting cooling. However, how AGN jets heat and suppress cooling flows remains highly debated. Using an idealized simulation of a cool-core cluster, we study the development of central cooling catastrophe and how a subsequent powerful AGN jet event averts cooling flows, with a focus on complex gasdynamical processes involved. We find that the jet drives a bow shock, which reverses cooling inflows and overheats inner cool core regions. The shocked gas moves outward in a rarefaction wave, which rarefies the dense core and adiabatically transports a significant fraction of heated energy to outer regions. As the rarefaction wave propagates away, inflows resume in the cluster core, but a trailing outflow is uplifted by the AGN bubble, preventing gas accumulation and catastrophic cooling in central regions. Inflows and trailing outflows constitute meridional circulations in the cluster core. At later times, trailing outflows fall back to the cluster centre, triggering central cooling catastrophe and potentially a new generation of AGN feedback. We thus envisage a picture of cool cluster cores going through cycles of cooling-induced contraction and AGN-induced expansion. This picture naturally predicts an anti-correlation between the gas fraction (or X-ray luminosity) of cool cores and the central gas entropy, which may be tested by X-ray observations., Comment: Slightly revised version, accepted for publication in MNRAS. 14 pages, 10 figures

The nature and origin of the Fermi bubbles detected in the inner Galaxy remain elusive. In this paper, we briefly discuss some recent theoretical and observational developments, with a focus on the AGN jet model. Analogous to radio lobes observed in massive galaxies, the Fermi bubbles could be naturally produced by a pair of opposing jets emanating nearly along the Galaxy's rotation axis from the Galactic center. Our two-fluid hydrodynamic simulations reproduce quite well the bubble location and shape, and interface instabilities at the bubble surface could be effectively suppressed by shear viscosity. We briefly comment on some potential issues related to our model, which may lead to future progress., Comment: 4 pages, 1 figure. Proceedings of IAU Symposium 322 (The Multi-Messenger Astrophysics of the Galactic Centre)

Radio-mode AGN feedback plays a key role in the evolution of galaxy groups and clusters. Its physical origin lies in the kpc-scale interaction of AGN jets with the intracluster medium. Large-scale jet simulations often initiate light internally-supersonic jets with density contrast $0.01<\eta<1$. Here we argue for the first time for the importance of very-light ($\eta<0.01$) internally-subsonic jets. We investigated the shapes of young X-ray cavities produced in a suite of hydrodynamic simulations, and found that bottom-wide cavities are always produced by internally-subsonic jets, while internally-supersonic jets inflate cylindrical, center-wide, or top-wide cavities. We found examples of real cavities with shapes analogous to those inflated in our simulations by internally-subsonic and internally-supersonic jets, suggesting a dichotomy of AGN jets according to their internal Mach numbers. We further studied the long-term cavity evolution, and found that old cavities resulted from light jets spread along the jet direction, while those produced by very light jets are significantly elongated along the perpendicular direction. The northwestern ghost cavity in Perseus is pancake-shaped, providing tentative evidence for the existence of very light jets. Our simulations show that very-light internally-subsonic jets decelerate faster and rise much slower in the ICM than light internally-supersonic jets, possibly depositing a larger fraction of jet energy to cluster cores and alleviating the problem of low coupling efficiencies found previously. The internal Mach number points to the jet's energy content, and internally-subsonic jets are energetically dominated by non-kinetic energy, such as thermal energy, cosmic rays, or magnetic fields., Comment: Slightly revised version accepted for publication in ApJ. 8 pages, 5 figures

X-ray observations of galaxy clusters have detected numerous X-ray cavities, evolved from the interaction of AGN jets with the intracluster medium (ICM) and providing compelling evidence for the importance of jet-mode AGN feedback. Here we argue for the physical importance of the cavity shape, which we characterize with two geometric parameters: radial elongation $\tau$ and top wideness $b$. We study the cavity shape with 16 hydrodynamic jet simulations in two representative clusters, and find that the shapes of young cavities are mainly determined by various jet properties. Our simulations successfully reproduce two observed types of young cavities elongated along either the jet ($\tau>1$; type-II) or perpendicular ($\tau\leq1$; type-I) direction. Bottom-wide type-I cavities are produced by very light internally-subsonic jets, while top-wide type-II cavities are produced by heavier, internally-supersonic jets, which may also produce center-wide cavities with $\tau\sim 1$ if the jets are only slightly supersonic. Bottom-wide type-II cavities can be produced by very light jets with very long durations and cylindrical cavities are produced by very light internally-supersonic jets. While not appreciably affecting the shapes of young cavities, viscosity significantly affects the long-term cavity evolution, suppressing both interface instabilities and the formation of torus-like morphology. We encourage observers to study the shapes of young and old X-ray cavities separately, the former probing the properties of AGN jets and the latter potentially probing the ICM viscosity level., Comment: Significantly revised version accepted for publication in ApJ. 12 pages, 9 figures

Recent observations suggested that star formation quenching in galaxies is related to galaxy structure. Here we propose a new mechanism to explain the physical origin of this correlation. We assume that while quiescent galaxies are maintained quenched by a feedback mechanism, cooling flows in the hot halo gas can still develop intermittently. We study cooling flows in a large suite of around 90 hydrodynamic simulations of an isolated galaxy group, and find that the flow development depends significantly on the gravitational potential well in the central galaxy. If the galaxy's gravity is not strong enough, cooling flows result in a central cooling catastrophe, supplying cold gas and feeding star formation to galactic bulges. When the bulge grows prominent enough, compressional heating starts to offset radiative cooling and maintains cooling flows in a long-term hot mode without producing cooling catastrophe. Our model thus describes a self-limited growth channel for galaxy bulges, and naturally explains the connection between quenching and bulge prominence. In particular, we explicitly demonstrate that $M_{*}/R_{\rm eff}^{1.5}$ is a good structural predictor of quenching. We further find that the gravity from the central supermassive black hole also affects the bimodal fate of cooling flows, and predict a more general quenching predictor to be $M_{\rm bh}^{1.6}M_{*}/R_{\rm eff}^{1.5}$, which may be tested in future observational studies., Comment: Slightly revised version accepted for publication in ApJL. 5 pages, 4 figures

Feedback heating from active galactic nuclei (AGNs) has been commonly invoked to suppress cooling flows predicted in hot gas in elliptical galaxies, galaxy groups and clusters. Previous studies have focused on if and how AGN feedback heats the gas, but little paid attention to its triggering mechanism. Using spherically symmetric simulations, we investigate how large-scale cooling flows are accreted by central supermassive black holes (SMBHs) in eight well-observed systems and find an interesting dichotomy. In massive clusters, the gas develops a central cooling catastrophe within about the cooling time (typically ~ 100 - 300 Myr), resulting in a cold-mode accretion onto SMBHs. However, in our four simulated systems on group and galaxy scales at a low metallicity Z=0.3Z_{sun}, the gas quickly settles into a long-term state which has a cuspy central temperature profile extending to several tens to about 100 pc. At the more realistic solar metallicity, two groups (with R_{e} ~ 4 kpc) still host the long-term hot-mode accretion. Both accretion modes naturally appear in our idealized calculations where only cooling, gas inflow, and compressional heating are considered. The long-term hot-mode accretion is maintained by the quickly-established closeness between the timescales of these processes, preferably in systems with low gas densities, low gas metallicities, and importantly, compact central galaxies, which result in strong gravitational acceleration and compressional heating at the intermediate radii. Our calculations predict that central cuspy temperature profiles appear more often in smaller systems than galaxy clusters, which instead often host significant cold gas and star formation., Comment: Significantly revised version accepted for publication in ApJ. 18 pages, 13 figures

The observed bimodality in radio luminosity in galaxy clusters is puzzling. We investigate the possibility that cosmic-ray (CR) streaming in the intra-cluster medium can 'switch off' hadronically induced radio and gamma-ray emission. For self-confined CRs, this depends on the source of MHD wave damping: if only non-linear Landau damping operates, then CRs stream on the slow Alfvenic timescale, but if turbulent wave damping operates, super-Alfvenic streaming is possible. As turbulence increases, it promotes outward streaming more than it enables inward turbulent advection. Curiously, the CR flux is independent of $\nabla f$ (as long as it is non-zero) and depends only on plasma parameters; this enables radio halos with flat inferred CR profiles to turn off. We perform 1D time-dependent calculations of a radio mini-halo (Perseus) and giant radio halo (Coma) and find that both diminish in radio luminosity by an order of magnitude in several hundred Myr, given plausible estimates for the magnetic field in the outskirts of the cluster. Due to the energy dependence of CR streaming, spectral curvature develops, and radio halos turn off more slowly at low frequencies -- properties consistent with observations. Similarly, CR streaming rapidly turns off gamma-ray emission at the high-energies probed by Cherenkov telescopes, but not at the low energies probed by Fermi. CR mediated wave-heating of the ICM is unaffected, as it is dominated by ~GeV CRs which stream Alfvenically.

We describe approximate axisymmetric computations of the dynamical evolution of material inside radio lobes and X-ray cluster gas cavities in Fanaroff-Riley II sources such as Cygnus A. All energy is delivered by a jet to the lobe/cavity via a moving hotspot where jet energy dissipates in a reverse shock. Our calculations describe the evolution of hot plasma, cosmic rays (CRs) and toroidal magnetic fields flowing from the hotspot into the cavity. Many observed features are explained. Gas, CRs and field flow back along the cavity surface in a "boundary backflow" consistent with detailed FRII observations. Computed ages of backflowing CRs are consistent with observed radio-synchrotron age variations only if shear instabilities in the boundary backflow are damped and we assume this is done with viscosity of unknown origin. Magnetic fields estimated from synchrotron self-Compton (SSC) X-radiation observed near the hotspot evolve into radio lobe fields. Computed profiles of radio synchrotron lobe emission perpendicular to the jet are dramatically limb-brightened in excellent agreement with FRII observations although computed lobe fields exceed those observed. Strong winds flowing from hotspots naturally create kpc-sized spatial offsets between hotspot inverse Compton (IC-CMB) X-ray emission and radio synchrotron emission that peaks 1-2 kpc ahead where the field increases due to wind compression. In our computed version of Cygnus A, nonthermal X-ray emission increases from the hotspot (some IC-CMB, mostly SSC) toward the offset radio synchrotron peak (mostly SSC). A faint thermal jet along the symmetry axis may be responsible for redirecting the Cygnus A non-thermal jet., Comment: 24 pages, 10 figures, accepted by ApJ

Steady accretion of a radiating gas onto a central mass point is described and compared to classic Bondi accretion. Radiation losses are essential for accretion flows to be observed. Unlike Bondi flows, radiating Bondi flows pass through a sonic point at a finite radius and become supersonic near the center. The morphology of all radiating Bondi flows is described by a single dimensionless parameter. In radiating Bondi flows the mass accretion rate varies approximately as the first power of the central mass -- this differs significantly from the quadratic dependence on the central mass in classical Bondi flows. Mass accretion rates onto galaxy or cluster-centered black holes estimated from traditional and radiating Bondi flows are significantly different. In radiating Bondi flows the gas temperature increases at large radii, as in the cores of many galaxy groups and clusters, allowing radiating Bondi flows to merge naturally with gas arriving from their cluster environments. Some radiating flows cool completely before reaching the center of the flow, and this also occurs in cooling site flows in which there is no central gravitating mass., Comment: 9 pages with 3 figures; accepted by ApJ

The origin of the Fermi bubbles recently detected by the Fermi Gamma-ray Space Telescope in the inner Galaxy is mysterious. In the companion paper Guo & Mathews (Paper I), we use hydrodynamic simulations to show that they could be produced by a recent powerful AGN jet event. Here we further explore this scenario to study the potential roles of shear viscosity and cosmic ray (CR) diffusion on the morphology and CR distribution of the bubbles. We show that even a relatively low level of viscosity (\mu_{visc} >~ 3 g cm^{-1} s^{-1}, or ~0.1% - 1% of Braginskii viscosity in this context) could effectively suppress the development of Kelvin-Helmholtz instabilities at the bubble surface, resulting in smooth bubble edges as observed. Furthermore, viscosity reduces circulating motions within the bubbles, which would otherwise mix the CR-carrying jet backflow near bubble edges with the bubble interior. Thus viscosity naturally produces an edge-favored CR distribution, an important ingredient to produce the observed flat gamma-ray surface brightness distribution. Generically, such a CR distribution often produces a limb-brightened gamma-ray intensity distribution. However, we show that by incorporating CR diffusion which is strongly suppressed across the bubble surface (as inferred from sharp bubble edges) but is close to canonical values in the bubble interior, we obtain a reasonably flat gamma-ray intensity profile. The similarity of the resulting CR bubble with the observed Fermi bubbles strengthens our previous result in Paper I that the Fermi bubbles were produced by a recent AGN jet event. Studies of the nearby Fermi bubbles may provide a unique opportunity to study the potential roles of plasma viscosity and CR diffusion on the evolution of AGN jets and bubbles., Comment: Revised version, accepted for publication in ApJ. 14 pages, 9 figures

The total feedback energy injected into hot gas in galaxy clusters by central black holes can be estimated by comparing the potential energy of observed cluster gas profiles with the potential energy of non-radiating, feedback-free hot gas atmospheres resulting from gravitational collapse in clusters of the same total mass. Feedback energy from cluster-centered black holes expands the cluster gas, lowering the gas-to-dark matter mass ratio below the cosmic value. Feedback energy is unnecessarily delivered by radio-emitting jets to distant gas far beyond the cooling radius where the cooling time equals the cluster lifetime. For clusters of mass 4-11 X 10^14 Msun estimates of the total feedback energy, 1-3 X 10^63 ergs, far exceed feedback energies estimated from observations of X-ray cavities and shocks in the cluster gas, energies gained from supernovae, and energies lost from cluster gas by radiation. The time-averaged mean feedback luminosity is comparable to those of powerful quasars, implying that some significant fraction of this energy may arise from the spin of the black hole. The universal entropy profile in feedback-free gaseous atmospheres in NFW cluster halos can be recovered by multiplying the observed gas entropy profile of any relaxed cluster by a factor involving the gas fraction profile. While the feedback energy and associated mass outflow in the clusters we consider far exceed that necessary to stop cooling inflow, the time-averaged mass outflow at the cooling radius almost exactly balances the mass that cools within this radius, an essential condition to shut down cluster cooling flows., Comment: 17 pages, 5 figures, Accepted by ApJ; typos and references corrected, small revisions for clarity

The pair of large radio lobes in the Virgo cluster, each about 23 kpc in radius, have curiously sharp outer edges where the radio-synchrotron continuum flux declines abruptly. However, just adjacent to this sharp transition, the radio flux increases. This radio limb-brightening is observed over at least half of the perimeter of both lobes. We describe slowly propagating steady state diffusion fronts that explain these counterintuitive features. Because of the natural buoyancy of radio lobes, the magnetic field is largely tangent to the lobe boundary, an alignment that polarizes the radio emission and dramatically reduces the diffusion coefficient of relativistic electrons. As cosmic ray electrons diffuse slowly into the cluster gas, the local magnetic field and gas density are reduced as gas flows back toward the radio lobe. Radio emission peaks can occur because the synchrotron emissivity increases with magnetic field and then decreases with the density of non-thermal electrons. A detailed comparison of steady diffusion fronts with quantitative radio observations may reveal information about the spatial variation of magnetic fields and the diffusion coefficient of relativistic electrons. On larger scales, some reduction of the gas density inside the Virgo lobes due to cosmic ray pressure must occur and may be measurable. Such X-ray observations could reveal important information about the presence of otherwise unobservable non-thermal components such as relativistic electrons of low energy or proton cosmic rays., Comment: 11 pages, 5 figures, Accepted by ApJ

The Fermi Gamma-ray Space Telescope reveals two large gamma-ray bubbles in the Galaxy, which extend about 50 degrees (~ 10 kpc) above and below the Galactic center (GC) and are symmetric about the Galactic plane. Using axisymmetric hydrodynamic simulations with a self-consistent treatment of the dynamical cosmic ray (CR) - gas interaction, we show that the bubbles can be created with a recent active galactic nucleus (AGN) jet activity about 1 - 3 Myr ago, which was active for a duration of ~ 0.1 - 0.5 Myr. The bipolar jets were ejected into the Galactic halo along the rotation axis of the Galaxy. Near the GC, the jets must be moderately light with a typical density contrast 0.001 <~ \eta <~ 0.1 relative to the ambient hot gas. The jets are energetically dominated by kinetic energy, and over-pressured with either CR or thermal pressure which induces lateral jet expansion, creating fat CR bubbles as observed. The sharp edges of the bubbles imply that CR diffusion across the bubble surface is strongly suppressed. The jet activity induces a strong shock, which heats and compresses the ambient gas in the Galactic halo, potentially explaining the ROSAT X-ray shell features surrounding the bubbles. The Fermi bubbles provide plausible evidence for a recent powerful AGN jet activity in our Galaxy, shedding new insights into the origin of the halo CR population and the channel through which massive black holes in disk galaxies release feedback energy during their growth., Comment: Revised version, accepted for publication in ApJ. 17 pages, 11 figures

In Cygnus A and other classical FR II double radio sources, powerful opposing jets from the cores of halo-centered galaxies drive out into the surrounding cluster gas, forming hotspots of shocked and compressed cluster gas at the jet extremities. The moving hotspots are sandwiched between two shocks. An inner-facing shock receives momentum and cosmic rays from the jet and creates additional cosmic rays that form a radio lobe elongated along the jet axis. An outer-facing bow shock moves directly into the undisturbed group or cluster gas, creating a cocoon of shocked gas enclosing the radio lobe. We describe computations that follow the self-consistent dynamical evolution of the shocked cluster gas and the relativistic synchrotron-emitting gas inside the lobes. Relativistic and non-relativistic components exchange momentum by interacting with small magnetic fields having dynamically negligible energy densities. The evolution of Cygnus A is governed almost entirely by cosmic ray energy flowing from the hotspots. Mass flowing into hotspots from the jets is assumed to be small, greatly reducing the mass of gas flowing back along the jet, common in previous calculations, that would disrupt the spatial segregation of synchrotron-loss ages observed inside FR II radio lobes. We compute the evolution of the cocoon when the velocity and cosmic ray luminosity of the hotspots are constant and when they vary with time. If cosmic rays mix with cluster gas in hotspots before flowing into the radio lobe, the thermal gas is heated to mildly relativistic temperatures, producing an unobserved pressure inside the lobe., Comment: ApJ accepted, 23 pages, 12 figures

It is widely accepted that feedback from active galactic nuclei (AGN) plays a key role in the evolution of gas in groups and clusters of galaxies. Unequivocal evidence comes from quasi-spherical X-ray cavities observed near cluster centers having sizes ranging from a few to tens of kpc, some containing radio emission. Cavities apparently evolve from the interaction of AGN jets with the intracluster medium (ICM). However, in numerical simulations it has been difficult to create such fat cavities from narrow jets. Ultra-hot thermal jets dominated by kinetic energy typically penetrate deep into the ICM, forming radially elongated cavities at large radii unlike those observed. Here, we study very light jets dominated energetically by relativistic cosmic rays (CRs) with axisymmetric hydrodynamic simulations, investigating the jet evolution both when they are active and when they are later turned off. We find that, when the thermal gas density in a CR-dominated jet is sufficiently low, the jet has a correspondingly low inertia, and thus decelerates quickly in the ICM. Furthermore, CR pressure causes the jet to expand laterally, encounter and displace more decelerating ICM gas, naturally producing fat cavities near cluster centers similar to those observed. Our calculations of cavity formation imply that AGN jets responsible for creating fat X-ray cavities (radio bubbles) are very light, and dominated by CRs. This scenario is consistent with radio observations of Fanaroff-Riley I jets that appear to decelerate rapidly, produce strong synchrotron emission and expand typically at distances of a few kpc from the central AGN., Comment: Slightly revised version, accepted for publication in ApJ. 9 pages, 8 figures

Recent X-ray observations of galaxy clusters suggest that cluster populations are bimodally distributed according to central gas entropy and are separated into two distinct classes: cool core (CC) and non-cool core (NCC) clusters. While it is widely accepted that AGN feedback plays a key role in offsetting radiative losses and maintaining many clusters in the CC state, the origin of NCC clusters is much less clear. At the same time, a handful of extremely powerful AGN outbursts have recently been detected in clusters, with a total energy ~10^{61}-10^{62} erg. Using two dimensional hydrodynamic simulations, we show that if a large fraction of this energy is deposited near the centers of CC clusters, which is likely common due to dense cores, these AGN outbursts can completely remove CCs, transforming them to NCC clusters. Our model also has interesting implications for cluster abundance profiles, which usually show a central peak in CC systems. Our calculations indicate that during the CC to NCC transformation, AGN outbursts efficiently mix metals in cluster central regions, and may even remove central abundance peaks if they are not broad enough. For CC clusters with broad central abundance peaks, AGN outbursts decrease peak abundances, but can not effectively destroy the peaks. Our model may simultaneously explain the contradictory (possibly bimodal) results of abundance profiles in NCC clusters, some of which are nearly flat, while others have strong central peaks similar to those in CC clusters. A statistical analysis of the sizes of central abundance peaks and their redshift evolution may shed interesting insights on the origin of both types of NCC clusters and the evolution history of thermodynamics and AGN activity in clusters., Comment: Slightly revised version, accepted for publication in ApJ. 12 pages, 11 figures

We investigate a series of steady-state models of galaxy clusters, in which the hot intracluster gas is efficiently heated by active galactic nucleus (AGN) feedback and thermal conduction, and in which the mass accretion rates are highly reduced compared to those predicted by the standard cooling flow models. We perform a global Lagrangian stability analysis. We show for the first time that the global radial instability in cool core clusters can be suppressed by the AGN feedback mechanism, provided that the feedback efficiency exceeds a critical lower limit. Furthermore, our analysis naturally shows that the clusters can exist in two distinct forms. Globally stable clusters are expected to have either: 1) cool cores stabilized by both AGN feedback and conduction, or 2) non-cool cores stabilized primarily by conduction. Intermediate central temperatures typically lead to globally unstable solutions. This bimodality is consistent with the recently observed anticorrelation between the flatness of the temperature profiles and the AGN activity (Dunn & Fabian 2008) and the observation by Rafferty et al. (2008) that the shorter central cooling times tend to correspond to significantly younger AGN X-ray cavities., Comment: 4 pages, to appear in the proceedings of "The Monster's Fiery Breath: Feedback in Galaxies, Groups, and Clusters", Eds. Sebastian Heinz, Eric Wilcots (AIP conference series)

Recent X-ray observations of hot gas in the galaxy cluster MS 0735.6+7421 reveal huge radio-bright, quasi-bipolar X-ray cavities having a total energy ~10^{62} ergs, the most energetic AGN outburst currently known. We investigate the evolution of this outburst with two-dimensional axisymmetric gasdynamical calculations in which the cavities are inflated by relativistic cosmic rays. Many key observational features of the cavities and associated shocks are successfully reproduced. The radial elongation of the cavities indicates that cosmic rays were injected into the cluster gas by a (jet) source moving out from the central AGN. AGN jets of this magnitude must be almost perfectly identically bipolar. The relativistic momentum of a single jet would cause a central AGN black hole of mass 10^9 M_{sun} to recoil at ~6000 km s^{-1}, exceeding kick velocities during black hole mergers, and be ejected from the cluster-center galaxy. When the cavity inflation is complete, 4PV underestimates the total energy received by the cluster gas. Deviations of the cluster gas from hydrostatic equilibrium are most pronounced during the early cavity evolution when the integrated cluster mass found from the observed gas pressure gradient can have systematic errors near the cavities of ~10-30%. The creation of the cavity with cosmic rays generates a long-lasting global cluster expansion that reduces the total gas thermal energy below that received from the cavity shock. One Gyr after this single outburst, a gas mass of ~ 6 \times 10^{11} M_{sun} is transported out beyond a cluster radius of 500 kpc. Such post-cavity outflows can naturally produce the discrepancy observed between the cluster gas mass fraction and the universal baryon fraction inferred from WMAP observations. (Abridged), Comment: Slightly revised version, accepted for publication in ApJ. 11 pages, 6 figures

The origin of the bimodality in cluster core entropy is still unknown. At the same time, recent work has shown that thermal conduction in clusters is likely a time-variable phenomenon. We consider if time-variable conduction and AGN outbursts could be responsible for the cool-core (CC), non cool-core (NCC) dichotomy. We show that strong AGN heating can bring a CC cluster to a NCC state, which can be stably maintained by conductive heating from the cluster outskirts. On the other hand, if conduction is shut off by the heat-flux driven buoyancy instability, then the cluster will cool to the CC state again, where it is stabilized by low-level AGN heating. Thus, the cluster cycles between CC and NCC states. In contrast with massive clusters, we predict the CC/NCC bimodality should vanish in groups, due to the lesser role of conductive heating there. We find tentative support from the distribution of central entropy in groups, though firm conclusions require a larger sample carefully controlled for selection effects., Comment: Slightly revised version, accepted for publication in MNRAS. 9 pages, 3 figures

We investigate a series of steady-state models of galaxy clusters, in which the hot intracluster gas is efficiently heated by active galactic nucleus (AGN) feedback and thermal conduction, and in which the mass accretion rates are highly reduced compared to those predicted by the standard cooling flow models. We perform a global Lagrangian stability analysis. We show for the first time that the global radial instability in cool core clusters can be suppressed by the AGN feedback mechanism, provided that the feedback efficiency exceeds a critical lower limit. Furthermore, our analysis naturally shows that the clusters can exist in two distinct forms. Globally stable clusters are expected to have either: 1) cool cores stabilized by both AGN feedback and conduction, or 2) non-cool cores stabilized primarily by conduction. Intermediate central temperatures typically lead to globally unstable solutions. This bimodality is consistent with the recently observed anticorrelation between the flatness of the temperature profiles and the AGN activity (Dunn & Fabian 2008) and the observation by Rafferty et al. (2008) that the shorter central cooling times tend to correspond to significantly younger AGN X-ray cavities., Comment: 17 pages, 12 figures. Revised version accepted for publication in ApJ. Expanded discussions with three new figures; conclusions unchanged

Recent observations show that the cooling flows in the central regions of galaxy clusters are highly suppressed. Observed AGN-induced cavities/bubbles are a leading candidate for suppressing cooling, usually via some form of mechanical heating. At the same time, observed X-ray cavities and synchrotron emission point toward a significant non-thermal particle population. Previous studies have focused on the dynamical effects of cosmic-ray pressure support, but none have built successful models in which cosmic-ray heating is significant. Here we investigate a new model of AGN heating, in which the intracluster medium is efficiently heated by cosmic-rays, which are injected into the ICM through diffusion or the shredding of the bubbles by Rayleigh-Taylor or Kelvin-Helmholtz instabilities. We include thermal conduction as well. Using numerical simulations, we show that the cooling catastrophe is efficiently suppressed. The cluster quickly relaxes to a quasi-equilibrium state with a highly reduced accretion rate and temperature and density profiles which match observations. Unlike the conduction-only case, no fine-tuning of the Spitzer conduction suppression factor f is needed. The cosmic ray pressure, P_c/P_g <~ 0.1 and dP_c/dr <~ 0.1 \rho g, is well within observational bounds. Cosmic ray heating is a very attractive alternative to mechanical heating, and may become particularly compelling if GLAST detects the gamma-ray signature of cosmic-rays in clusters., Comment: Revised version accepted for publication in MNRAS. Significantly expanded discussion and new simulations exploring parameter space/model robustness; conclusions unchanged