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Register a new userBuoyant radio-lobes in a viscous intracluster medium
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Added by
Christopher S. Reynolds
Publication date
2004
fields
Physics
and research's language is
English
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(Abridged) Ideal hydrodynamic models of the intracluster medium (ICM) in the core regions of galaxy clusters fail to explain both the observed temperature structure of this gas, and the observed morphology of radio-galaxy/ICM interactions. It has recently been suggested that, even in the presence of reasonable magnetic fields, thermal conduction in the ICM may be crucial for reproducing the temperature floor seen in many systems. If this is indeed correct, it raises the possibility that other transport processes may be important. With this motivation, we present a numerical investigation of the buoyant evolution of AGN-blown cavities in ICM that has a non-negligible shear viscosity. We use the ZEUS-MP code to follow the 3-d evolution of an initially static, hot bubble in a beta-model ICM atmosphere with varying degrees of shear viscosity. With no explicit viscosity, it is found that the combined action of Rayleigh-Taylor and Kelvin-Helmholtz instabilities shred the ICM cavity and one does not reproduce the intact and detached ``ghost cavities observed in systems such as Perseus-A. On the other hand, even a modest level of shear viscosity can be important in quenching the fluid instabilities and maintaining the integrity of the bubble. In particular, we show that the morphology of the NW ghost cavity found in Perseus-A can be reproduced, as can the flow pattern inferred from the morphology of H-alpha filaments. Finally, we discuss the possible relevance of ICM viscosity to the fact that many of the active ICM cavities are not bounded by strong shocks.
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In the recent literature there is circumstantial evidence that the viscosity of the intracluster medium may not be too far from the Spitzer value. In this letter, we present two-dimensional hydrodynamical simulations of ram pressure stripping of disc galaxies in a viscous intracluster medium. The values of viscosity explored range between 0.1 and 1.0 times the Spitzer value. We find that viscosity affects the appearance and the dimensions of the galactic wakes but has very little effect on the evolution of the gas mass of the galaxy.
The radio lobes of Hydra A lie within cavities surrounded by a rim of enhanced X-ray emission in the intracluster gas. Although the bright rim appears cooler than the surrounding gas, existing Chandra data do not exclude the possibility that the rim is produced by a weak shock. A temperature map shows that cool gas extends out along the radio axis of Hydra A. The age of the radio source and equipartition pressure of the radio lobe argue against a shock, and comparison with similar structure in the Perseus Cluster also suggests that the rim is cool. We show that the cool bright rim cannot be the result of shock induced cooling, or due to the effect of magnetic fields in shocks. The most likely source of low entropy (cool) gas is entrainment by the rising cavity. This requires some means of communicating the bouyant force on the cavity to the surrounding gas. The magnetic field required to produce the Faraday rotation in Hydra A has the appropriate properties for this, if the Faraday screen is mainly in this bright rim. In Hydra A, the mass outflow due to the rising cavities could be sufficient to balance cooling driven inflow, so preventing the build up of low entropy gas in the cluster core.
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 clusters 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.
An important aspect of the radio emission from galaxy clusters is represented by the diffuse radio sources associated with the intracluster medium: radio halos, relics and mini-halos. The radio halos and relics are indicators of cluster mergers, whereas mini-halos are detected at the center of cooling core clusters. SKA will dramatically improve the knowledge of these sources, thanks to the detection of new objects, and to detailed studies of their spectra and polarized emission. SKA will also provide the opportunity to investigate the presence of halos produced by radiation scattered by a powerful radio galaxy at the cluster centers.
The intracluster medium of galaxy clusters is a weakly collisional, high-beta plasma in which the transport of heat and momentum occurs primarily along magnetic-field lines. Anisotropic heat conduction allows convective instabilities to be driven by temperature gradients of either sign, the magnetothermal instability (MTI) in the outskirts of non-isothermal clusters and the heat-flux buoyancy-driven instability (HBI) in their cooling cores. We employ the Athena MHD code to investigate the nonlinear evolution of these instabilities, self-consistently including the effects of anisotropic viscosity (i.e. Braginskii pressure anisotropy), anisotropic conduction, and radiative cooling. We highlight the importance of the microscale instabilities that inevitably accompany and regulate the pressure anisotropies generated by the HBI and MTI. We find that, in all but the innermost regions of cool-core clusters, anisotropic viscosity significantly impairs the ability of the HBI to reorient magnetic-field lines orthogonal to the temperature gradient. Thus, while radio-mode feedback appears necessary in the central few tens of kpc, conduction may be capable of offsetting radiative losses throughout most of a cool core over a significant fraction of the Hubble time. Magnetically-aligned cold filaments are then able to form by local thermal instability. Viscous dissipation during the formation of a cold filament produces accompanying hot filaments, which can be searched for in deep Chandra observations of nearby cool-core clusters. In the case of the MTI, anisotropic viscosity maintains the coherence of magnetic-field lines over larger distances than in the inviscid case, providing a natural lower limit for the scale on which the field can fluctuate freely. In the nonlinear state, the magnetic field exhibits a folded structure in which the field-line curvature and field strength are anti-correlated.
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