Plasma In Clusters Of Galaxies Research Articles

The Magnetized Intracluster Plasma

AbstractWe now know that the plasma in clusters of galaxies is magnetized, at strong levels in some regions. It follows that the dynamics of the cluster plasma is more complex than has been thought up to now.

Observational Properties of Diffuse Halos in Clusters

The number of known diffuse radio sources in clusters of galaxies has grown in recent years, making it possible to derive statistical properties of these sources and of the hosting clusters. We show that diffuse sources are associated with X-ray luminous clusters which have undergone recent merger processes. The radio and X-ray structures are often similar, and correlations are found between radio and X-ray parameters. This is indication of a link between the diffuse relativistic and thermal plasma in clusters of galaxies.

Simulation of radio plasma in clusters of galaxies

We present three-dimensional hydrodynamical simulations of buoyant gas in a typical cluster environment. In these simulations, hot matter was injected continuously into a small region offset from the cluster centre. In agreement with previous analytic estimates, we found that the bubbles evolve very differently depending on the rate of energy injection. Using tracer particles we computed the efficiency of the bubbles to stir the intracluster medium (ICM) and find that recurrent low-power sources are more effective in mixing the inner cluster region than rarer large outbursts. Moreover, we computed radio maps of the bubbles based on different assumptions about the magnetic field. In the radio band the bubbles closely resemble FR I sources. For the bubbles to be detectable for long enough to account for FR I sources, we found that reacceleration has to take place. The bubbles are generally difficult to detect, both in the radio and in the X-ray band. Thus it is possible to hide a significant amount of energy in the form of bubbles in clusters.

Isotropization of ultra-high energy cosmic ray arrival directions by radio ghosts

The isotropy in the ultra-high energy cosmic ray (UHECR) flux observed by Yakutsk and AGASA experiments, is a very strong constraint to production and propagation models alike. Most of the scenarios proposed in the literature should produce a sizable anisotropy as either extragalactic luminous or dark matter is normally associated with the invoked particle sources. We explore the possibility that the magnetic fields in fossil cocoons of former radio galaxies – so called radio ghosts [Enßlin, Ringberg Workshop on `Diffuse Thermal and Relativistic Plasma in Galaxy Clusters', vol. 271, MPE Report, 1999, p. 275] – are able to scatter UHECR in the intergalactic medium giving rise to the observed isotropy. We show, through numerical simulations, under which conditions this process can be operative and the magnitude of the effect. We further demonstrate, that if radio ghosts mix with the ambient medium, they might be able to produce the observed magnetic fields in clusters of galaxies. In the case of mixing, the UHECR isotropization would be even stronger than in our conservative estimates.

Reviving fossil radio plasma in clusters of galaxies by adiabatic compression in environmental shock waves

We give for a plasma with a history of several expansion and contraction phases an analytical model of the evolution of a contained relativistic electron population under synchrotron, inverse Compton and adiabatic energy losses or gains. This is applied to different scenarios for evolution of radio plasma inside the cocoons of radio galaxies, after the activity of the central engine has ceased. It is demonstrated that fossil radio plasma with an age of even up to 2 Gyr can be revived by compression in a shock wave of large-scale structure formation, caused during the merging events of galaxy clusters, or by the accretion onto galaxy clusters. We argue, that this is a highly plausible explanation for the observed cluster radio relics, which are the regions of diffuse radio emission found in clusters of galaxies, without any likely parent radio galaxy seen nearby. An implication of this model is the existence of a population of diffuse, ultra-steep spectrum, very low frequency radio sources located inside and possibly outside of clusters of galaxies, tracing the revival of aged fossil radio plasma by the shock waves associated with large-scale structure formation.

Buoyant radio plasma in clusters of galaxies

Radio galaxies are known to inflate lobes of hot relativistic plasmas into the intergalactic medium. Here we present hydrodynamical and magnetohydrodynamical simulations of these hot plasma bubbles in FR II objects. We focus on the later stages of their evolution after the jet has died down and after the bow shock that surrounded the lobes at earlier stages has vanished. We investigate the evolution of the plasma bubbles as they become subject to Rayleigh–Taylor instabilities. From our simulations we calculate the radio and X-ray emissivities of the bubbles and discuss their appearance in observations. Finally, we investigate the influence of large-scale magnetic fields on the evolution of the bubbles. The issues of re-acceleration and diffusion of relativistic particles are briefly discussed.

Dynamical Collapse of the Intracluster Gas and the Formation of a Hot Plasma in Clusters of Galaxies

Dynamical collapse of the intracluster gas in clusters of galaxies is studied numerically by taking account of both the gravity of background galaxies and the self-gravity. The adiabatic case, the case with the thermal bremsstrahlung cooling and the case with both the heat conduction and thermal bremsstrahlung cooling have been calculated. In the first two cases, a hot plasma which is formed after the bounce reaches an equilib­ rium state if the initial density po is lower than a certain critical value. In the last case, the gas secularly contracts. The results are compared with X-ray observation of the Coma cluster.