Bubbles In Clusters Of Galaxies Research Articles

Effects of Anisotropic Viscosity on the Evolution of Active Galactic Nuclei Bubbles in Galaxy Clusters

Abstract The interaction between jets from active galactic nuclei (AGNs) and the intracluster medium (ICM) provides key constraints on the feeding and feedback of supermassive black holes. Much understanding about AGN feedback is gained from purely hydrodynamic models; however, whether such an approximation is adequate for the magnetized, weakly collisional ICM needs to be critically examined. For example, AGN-blown bubbles in hydrodynamic simulations are easily disrupted by fluid instabilities, making it difficult to explain the coherence of observed bubbles such as the northwest ghost bubble in Perseus. In order to investigate whether magnetic tension and viscosity in realistic conditions could preserve the bubble integrity, we performed the first Braginskii-magnetohydrodynamic simulations of jet-inflated bubbles in a medium with tangled magnetic field. We find that magnetic tension alone is insufficient to prevent bubble deformation due to large velocity shear at early stage of the evolution. Although unsuppressed anisotropic viscosity in tangled magnetic field can have similar effects as isotropic viscosity, when the pressure anisotropy is bounded by microscopic plasma instabilities, the level of viscosity is substantially limited, thereby failing to prevent bubble deformation as in the inviscid case. Our results suggest that Braginskii viscosity is unlikely to be the primary mechanism for suppressing the fluid instabilities for AGN bubbles, and it remains a challenging task to reproduce smooth and coherent bubbles as observed. Because the dynamical influence and heating of the ICM critically depend on the bubble morphology, our study highlights the fundamental role of “microphysics” on the macroscopic properties of AGN feedback processes.

Generation of internal waves by buoyant bubbles in galaxy clusters and heating of intracluster medium

Buoyant bubbles of relativistic plasma in cluster cores plausibly play a key role in conveying the energy from a supermassive black hole to the intracluster medium (ICM) - the process known as radio-mode AGN feedback. Energy conservation guarantees that a bubble loses most of its energy to the ICM after crossing several pressure scale heights. However, actual processes responsible for transferring the energy to the ICM are still being debated. One attractive possibility is the excitation of internal waves, which are trapped in the cluster's core and eventually dissipate. Here we show that a sufficient condition for efficient excitation of these waves in stratified cluster atmospheres is flattening of the bubbles in the radial direction. In our numerical simulations, we model the bubbles phenomenologically as rigid bodies buoyantly rising in the stratified cluster atmosphere. We find that the terminal velocities of the flattened bubbles are small enough so that the Froude number ${\rm Fr}\lesssim 1$. The effects of stratification make the dominant contribution to the total drag force balancing the buoyancy force. In particular, clear signs of internal waves are seen in the simulations. These waves propagate horizontally and downwards from the rising bubble, spreading their energy over large volumes of the ICM. If our findings are scaled to the conditions of the Perseus cluster, the expected terminal velocity is $\sim100-200{\,\rm km\,s^{-1}}$ near the cluster cores, which is in broad agreement with direct measurements by the Hitomi satellite.

Mass transport by buoyant bubbles in galaxy clusters

We investigate the effect of three important processes by which AGN-blown bubbles transport material: drift, wake transport and entrainment. The first of these, drift, occurs because a buoyant bubble pushes aside the adjacent material, giving rise to a net upward displacement of the fluid behind the bubble. For a spherical bubble, the mass of upwardly displaced material is roughly equal to half the mass displaced by the bubble, and should be ~ 10^{7-9} solar masses depending on the local ICM and bubble parameters. We show that in classical cool core clusters, the upward displacement by drift may be a key process in explaining the presence of filaments behind bubbles. A bubble also carries a parcel of material in a region at its rear, known as the wake. The mass of the wake is comparable to the drift mass and increases the average density of the bubble, trapping it closer to the cluster centre and reducing the amount of heating it can do during its ascent. Moreover, material dropping out of the wake will also contribute to the trailing filaments. Mass transport by the bubble wake can effectively prevent the build-up of cool material in the central galaxy, even if AGN heating does not balance ICM cooling. Finally, we consider entrainment, the process by which ambient material is incorporated into the bubble. Abridged

Rising jet-inflated bubbles in clusters of galaxies

Abstract We conduct two-dimensional axisymmetric (referred to as 2.5D) hydrodynamical numerical simulations of bubble evolution in clusters of galaxies. We inflate bubbles using slow, massive jets with a wide opening angle, and follow their evolution as they rise through the intracluster medium. We find that these jet-inflated bubbles are quite stable, and can reach large distances in the cluster while maintaining their basic structure. The stability of the jet-inflated bubble comes mainly from the dense shell that forms around it during its inflation stage, and from the outward momentum of the bubble and shell. On the contrary, bubbles that are inserted by hand on to the grid and not inflated by a jet, i.e. an artificial bubble, lack these stabilizing factors; therefore, they are rapidly destroyed. The stability of the jet-inflated bubble removes the demand for stabilizing magnetic fields in the bubble.

Sound waves excitation by jet-inflated bubbles in clusters of galaxies

We show that repeated sound waves in the intracluster medium (ICM) can be excited by a single inflation episode of an opposite bubble pair. To reproduce this behavior in numerical simulations the bubbles should be inflated by jets, rather than being injected artificially. The multiple sound waves are excited by the motion of the bubble-ICM boundary that is caused by vortices inside the inflated bubbles and the backflow (`cocoon') of the ICM around the bubble. These sound waves form a structure that can account for the ripples observed in the Perseus cooling flow cluster. We inflate the bubbles using slow massive jets, with either a wide opening angle or that are precessing. The jets are slow in the sense that they are highly sub-relativistic, $v_j \sim 0.01c-0.1c$, and they are massive in the sense that the pair of bubbles carry back to the ICM a large fraction of the cooling mass, i.e., $\sim 1-50 M_\odot \yr^{-1}$. We use a two-dimensional axisymmetric (referred to as 2.5D) hydrodynamical numerical code (VH-1).

Inflating fat bubbles in clusters of galaxies by precessing massive slow jets

We conduct hydrodynamical numerical simulations and find that precessing massive slow jets can inflate fat bubbles, i.e., more or less spherical bubbles, that are attached to the center of clusters of galaxies. To inflate a fat bubble the jet should precess fast. The precessing angle $\theta$ should be large, or change over a large range $ 0 \le \theta \le \theta_{\max} \sim 30-70 ^\circ$ (depending also on other parameters), where $\theta=0$ is the symmetry axis. The constraints on the velocity and mass outflow rate are similar to those on wide jets to inflate fat bubbles. The velocity should be $v_j \sim 10^4 \kms$, and the mass loss rate of the two jets should be $ 2 \dot M_j \simeq 1-50 \dot M_\odot \yr^{-1} $. These results, and our results from a previous paper dealing with slow wide jets, support the claim that a large fraction of the feedback heating in cooling flow clusters and in the processes of galaxy formation is done by slow massive jets.

Inflating Fat Bubbles in Clusters of Galaxies by Wide Jets

We conduct two-dimensional hydrodynamical simulations of jets expanding in the intra-cluster medium (ICM). We find that for a fat, i.e. more or less spherical, bubble attached to the center to be formed the jet should have high momentum flux and a large opening angle. Typically, the half opening angle should be >50 degrees, and the large momentum flux requires a jet speed of \~10,000 km/sec. The inflation process involves vortices and local instabilities which mix some ICM with the hot bubble. These results predict that most of the gas inside the bubble has a temperature of 3x10^8<T<3x10^9 K, and that large quantities of the cooling gas in cooling flow clusters are expelled back to the intra-cluster medium, and heated up. The magnetic fields and relativistic electrons that produce the synchrotron radio emission might be formed in the shock wave of the jet.

Metal mixing by buoyant bubbles in galaxy clusters

Using a series of three-dimensional, hydrodynamic simulations on an adaptive grid, we have performed a systematic study on the effect of bubble-induced motions on metallicity profiles in clusters of galaxies. In particular, we have studied the dependence on the bubble size and position, the recurrence times of the bubbles, the way these bubbles are inflated and the underlying cluster profile. We find that in hydrostatic cluster models, the resulting metal distribution is very elongated along the direction of the bubbles. Anisotropies in the cluster or ambient motions are needed if the metal distribution is to be spherical. In order to parametrise the metal transport by bubbles, we compute effective diffusion coefficients. The diffusion coefficients inferred from our simple experiments lie at values of around $\sim 10^{29}$ cm$^2$s$^{-1}$ at a radius of 10 kpc. The runs modelled on the Perseus cluster yield diffusion coefficients that agree very well with those inferred from observations.

Magnetic draping of merging cores and radio bubbles in clusters of galaxies

Sharp fronts observed by the Chandra satellite between dense cool cluster cores moving with near-sonic velocity through the hotter intergalactic gas, require strong suppression of thermal conductivity across the boundary. This may be due to magnetic fields tangential to the contact surface separating the two plasma components. We point out that a super-Alfvenic motion of a plasma cloud (a core of a merging galaxy) through a weakly magnetized intercluster medium leads to 'magnetic draping', formation of a thin, strongly magnetized boundary layer with a tangential magnetic field. For supersonic cloud motion, M S ≥ 1, magnetic field inside the layer reaches near-equipartition values with thermal pressure. Typical thickness of the layer is ∼L/M 2 A « L, where L is the size of the obstacle (plasma cloud) moving with Alfven Mach number M A >> 1. To a various degree, magnetic draping occurs for both subsonic and supersonic flows, random and ordered magnetic fields and it does not require plasma compressibility. The strongly magnetized layer will thermally isolate the two media and may contribute to the Kelvin-Helmholtz stability of the interface. Similar effects occur for radio bubbles, quasi-spherical expanding cavities blown up by active galactic nucleus jets; in this case, the thickness of the external magnetized layer is smaller, ∼L/M 3 A << L.

On the Rayleigh-Taylor instability of radio bubbles in galaxy clusters

We consider the Rayleigh‐Taylor instability in the early evolution of the rarefied radio bubbles (cavities) observed in many cooling-flow clusters of galaxies. The top of a bubble becomes prone to the Rayleigh‐Taylor instability as the bubble rises through the intracluster medium (ICM). We show that while the jet is powering the inflation, the deceleration of the bubble‐ICM interface is able to reverse the Rayleigh‐Taylor instability criterion. In addition, the inflation introduces a drag effect which increases substantially the instability growth time. The combined action of these two effects considerably delays the onset of the instability. Later on, when the magnitude of the deceleration drops or the jet fades, the Rayleigh‐Taylor and the Kelvin‐ Helmholtz instabilities set in and eventually disrupt the bubble. We conclude that the initial deceleration and drag, albeit unable to prevent the disruption of a bubble, may significantly lengthen its lifetime, removing the need to invoke stabilizing magnetic fields.

Magnetohydrodynamic Simulations of Relic Radio Bubbles in Clusters

In order to better understand the origin and evolution of relic radio bubbles in clusters of galaxies, we report on an extensive set of two-dimensional MHD simulations of hot buoyant bubbles evolving in a realistic intracluster medium (ICM). Our bubbles are inflated near the base of the ICM over a finite time interval from a region whose magnetic field is isolated from the ICM. We confirm both the early conjecture from linear analysis and the later results based on preformed MHD bubbles, namely, that very modest ICM magnetic fields can stabilize the rising bubbles against disruption by Rayleigh-Taylor and Kelvin-Helmholtz instabilities. We find in addition that amplification of the ambient fields as they stretch around the bubbles can be sufficient to protect the bubbles or their initial fragments even if the fields are initially much too weak to play a significant role early in the evolution of the bubbles. Indeed, even with initial fields less than 1 μG and values of β = Pg/Pb approaching 105, magnetic stresses in our simulations eventually became large enough to influence the bubble evolution. Magnetic field influence also depends significantly on the geometry of the ICM field and on the topology of the field at the bubble/ICM interface. For example, reconnection of antiparallel fields across the bubble top greatly reduced the ability of the magnetic field to inhibit disruptive instabilities. Our results confirm earlier estimates of 108 yr for relic radio bubble lifetimes and show that magnetic fields can account for the long-term stability of these objects against disruption by surface instabilities. In addition, these calculations show that lifting and mixing of the ambient ICM may be a critical function of field geometries in both the ICM and the bubble interior.

Unveiling the composition of radio plasma bubbles in galaxy clusters with the Sunyaev-Zel'dovich effect

The Chandra X-ray Observatory is finding a large number of cavities in the X-ray emitting intra-cluster medium which often coincide with the lobes of the central radio galaxy. We propose high-resolution Sunyaev-Zel'dovich (SZ) observations in order to infer the still unknown dynamically dominating component of the radio plasma bubbles. This work calculates the thermal and relativistic SZ emission of different compositions of these plasma bubbles while simultaneously allowing for the cluster's kinetic SZ effect. As examples, we present simulations of an Atacama Large Millimeter Array (ALMA) observation and of a Green Bank Telescope (GBT) observation of the cores of the Perseus cluster and Abell 2052. We predict a 5 sigma detection of the southern radio bubble of Perseus in a few hours with the GBT and ALMA while assuming a relativistic electron population within the bubble. In Abell 2052, a similar detection would require a few ten hours with either telescope, the longer exposures mainly being the result of the higher redshift and the lower central temperature of this cluster. Future high-sensitivity multi-frequency SZ observations will be able to infer the energy spectrum of the dynamically dominating electron population in order to measure its temperature or spectral characteristics. This knowledge can yield indirect indications for an underlying radio jet model.

Bubbles in planetary nebulae and clusters of galaxies: Jet properties

I derive constraints on jet properties for inflating pairs of bubbles in planetary nebulae and clusters of galaxies. This work is motivated by the similarity in morphology and some non-dimensional quantities between X-ray-deficient bubbles in clusters of galaxies and the optical-deficient bubbles in planetary nebulae, which were pointed out in an earlier work. In the present Paper I find that for inflating fat bubbles, the opening angle of the jets must be large, i.e., the half opening angle measured from the symmetry axis of the jets should typically be . For such wide-opening angle jets, a collimated fast wind (CFW) is a more appropriate term. Narrow jets will form elongated lobes rather than fat bubbles. I emphasize the need to include jets with large opening angle, i.e., , in simulating bubble inflation in both planetary nebulae and (cooling flow) clusters of galaxies.

Bubbles in planetary nebulae and clusters of galaxies: instabilities at bubble’ fronts

I study the stability of off-center low-density more or less spherical (fat) bubbles in clusters of galaxies and in planetary nebulae (PNs) to Rayleigh–Taylor (RT) instability. As the bubble expands and decelerates, the interface between the low-density bubble’s interior and the dense shell formed from the accreted ambient medium is RT-stable. If, however, in a specific direction the density decreases such that this segment is accelerated by the pressure inside the bubble, then this accelerated region is RT-unstable. The outermost region, relative to the center of the system, is the most likely to become unstable because there the density gradient is the steepest. Using simple analytical analysis, I find that off-center fat bubbles in PNs are much less stable than in clusters. In PNs bubbles become unstable when they are very small relative to their distance from the center; they can be stabilized somewhat if the mass loss rate from the stellar progenitor decreases for a time, such that the negative density gradient is much shallower. In clusters fat bubbles become unstable when their size is comparable to their distance from the center. I discuss some implications of this instability in clusters and in PNs.

Pairs of Bubbles in Planetary Nebulae and Clusters of Galaxies

ABSTRACTI point to an interesting similarity in the morphology and some nondimensional quantities between pairs of X‐ray–deficient bubbles in clusters of galaxies and pairs of optical‐deficient bubbles in planetary nebulae (PNs). This similarity leads me to postulate a similar formation mechanism. This postulate is used to strengthen models for PN shaping by jets (or collimated fast winds [CFWs]). The presence of dense material in the equatorial plane observed in the two classes of bubbles constrains the jets and CFW activity in PNs to occur while the asymptotic giant branch star still blows its dense wind, or very shortly after. I argue that only a stellar companion can account for such jets and CFWs.