Fulai Guo

THE FERMI BUBBLES. I. POSSIBLE EVIDENCE FOR RECENT AGN JET ACTIVITY IN THE GALAXY

The Fermi Gamma-ray Space Telescope reveals two large gamma-ray bubbles in the Galaxy, which extend about 50° (~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 η 0.1 relative to the ambient hot gas. The jets are energetically dominated by kinetic energy, and overpressured 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, providing 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.

THE FERMI BUBBLES. II. THE POTENTIAL ROLES OF VISCOSITY AND COSMIC-RAY DIFFUSION IN JET MODELS

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 active galactic nucleus (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 (μvisc 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 that 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.

COSMIC-RAY-DOMINATED AGN JETS AND THE FORMATION OF X-RAY CAVITIES IN GALAXY CLUSTERS

It is widely accepted that feedback from active galactic nuclei (AGNs) 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 non-thermal 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, and to 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 type I jets that appear to decelerate rapidly, produce strong synchrotron emission, and expand typically at distances of a few kpc from the central AGN.

SIMULATING X-RAY SUPERCAVITIES AND THEIR IMPACT ON GALAXY CLUSTERS

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 ~1062 erg, the most energetic active galactic nucleus (AGN) outburst currently known. We investigate the evolution of this outburst with two-dimensional axisymmetric gas dynamical calculations in which the cavities are inflated by relativistic cosmic rays (CRs). Many key observational features of the cavities and associated shocks are successfully reproduced. The radial elongation of the cavities indicates that CRs 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 109 M ☉ to recoil at ~6000 km s–1, exceeding kick velocities during black hole mergers, and be ejected from the cluster-center galaxy. Observed deviations from bipolar symmetry in the radio cavities can be caused by subsonic flows in the ambient cluster gas, but reflection shocks between symmetric cavities are likely to be visible in deep X-ray images. 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 CRs generates a long-lasting global cluster expansion that reduces the total gas thermal energy below that received from the cavity shock—even this most energetic AGN event has a net cooling effect on cluster gas. One gigayear after this single outburst, a gas mass of ~6 × 1011 M ☉ 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 Wilkinson Microwave Anisotropy Probe observations.

HOT VERSUS COLD: THE DICHOTOMY IN SPHERICAL ACCRETION OF COOLING FLOWS ONTO SUPERMASSIVE BLACK HOLES IN ELLIPTICAL GALAXIES, GALAXY GROUPS, AND CLUSTERS

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 have 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 cold-mode accretion onto SMBHs. However, in our four simulated systems on group and galaxy scales at a low metallicity Z = 0.3 Z ☉, the gas quickly settles into a long-term state that 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.

On the Importance of Very Light Internally Subsonic AGN Jets in Radio-mode AGN Feedback

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

THE SHAPE OF X-RAY CAVITIES IN GALAXY CLUSTERS: PROBING JET PROPERTIES AND VISCOSITY

0.01<\eta<1

Radiating Bondi and Cooling Site Flows

. Here we argue for the first time for the importance of very-light (

SELF-CONSISTENT EVOLUTION OF GAS AND COSMIC RAYS IN CYGNUS A AND SIMILAR FR II CLASSICAL DOUBLE RADIO SOURCES

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CONNECTING STAR FORMATION QUENCHING WITH GALAXY STRUCTURE AND SUPERMASSIVE BLACK HOLES THROUGH GRAVITATIONAL HEATING OF COOLING FLOWS

) 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 jets energy content, and internally-subsonic jets are energetically dominated by non-kinetic energy, such as thermal energy, cosmic rays, or magnetic fields.