We investigate the interplay of cosmic ray (CR) propagation and advection in galaxy clusters. Propagation in form of CR diffusion and streaming tends to drive the CR radial profiles towards being ...flat, with equal CR number density everywhere. Advection of CR by the turbulent gas motions tends to produce centrally enhanced profiles. We assume that the CR streaming velocity is of the order of the sound velocity. This is motivated by plasma physical arguments. The CR streaming is then usually larger than typical advection velocities and becomes comparable or lower than this only for periods with trans- and super-sonic cluster turbulence. As a consequence a bimodality of the CR spatial distribution results. Strongly turbulent, merging clusters should have a more centrally concentrated CR energy density profile with respect to relaxed ones with very subsonic turbulence. This translates into a bimodality of the expected diffuse radio and gamma-ray emission of clusters, since more centrally concentrated CR will find higher target densities for hadronic CR proton interactions, higher plasma wave energy densities for CR electron and proton re-acceleration, and stronger magnetic fields. Thus, the observed bimodality of cluster radio halos appears to be a natural consequence of the interplay of CR transport processes, independent of the model of radio halo formation, be it hadronic interactions of CR protons or re-acceleration of low-energy CR electrons. Energy dependence of the CR propagation should lead to spectral steepening of dying radio halos. Furthermore, we show that the interplay of CR diffusion with advection implies first order CR re-acceleration in the pressure-stratified atmospheres of galaxy clusters. Finally, we argue that CR streaming could be important in turbulent cool cores of galaxy clusters since it heats preferentially the central gas with highest cooling rate.
Magnetic fields are ubiquitous in the Universe. The energy density of these fields is typically comparable to the energy density of the fluid motions of the plasma in which they are embedded, making ...magnetic fields essential players in the dynamics of the luminous matter. The standard theoretical model for the origin of these strong magnetic fields is through the amplification of tiny seed fields via turbulent dynamo to the level consistent with current observations. However, experimental demonstration of the turbulent dynamo mechanism has remained elusive, since it requires plasma conditions that are extremely hard to re-create in terrestrial laboratories. Here we demonstrate, using laser-produced colliding plasma flows, that turbulence is indeed capable of rapidly amplifying seed fields to near equipartition with the turbulent fluid motions. These results support the notion that turbulent dynamo is a viable mechanism responsible for the observed present-day magnetization.
X-ray1, 2, 3 and radio4, 5, 6 observations of the supernova remnant Cassiopeia A reveal the presence of magnetic fields about 100 times stronger than those in the surrounding interstellar medium. ...Field coincident with the outer shock probably arises through a nonlinear feedback process involving cosmic rays2, 7, 8. The origin of the large magnetic field in the interior of the remnant is less clear but it is presumably stretched and amplified by turbulent motions. Turbulence may be generated by hydrodynamic instability at the contact discontinuity between the supernova ejecta and the circumstellar gas9. However, optical observations of Cassiopeia A indicate that the ejecta are interacting with a highly inhomogeneous, dense circumstellar cloud bank formed before the supernova explosion10, 11, 12. Here we investigate the possibility that turbulent amplification is induced when the outer shock overtakes dense clumps in the ambient medium13, 14, 15. We report laboratory experiments that indicate the magnetic field is amplified when the shock interacts with a plastic grid. We show that our experimental results can explain the observed synchrotron emission in the interior of the remnant. The experiment also provides a laboratory example of magnetic field amplification by turbulence in plasmas, a physical process thought to occur in many astrophysical phenomena.
We performed a set of cosmological simulations of major mergers in galaxy clusters, in order to study the evolution of merger shocks and the subsequent injection of turbulence in the post-shock ...region and in the intra-cluster medium (ICM). The computations have been performed with the grid-based, adaptive mesh refinement hydrodynamical code Enzo, using a refinement criterion especially designed for refining turbulent flows in the vicinity of shocks. When a major merger event occurs, a substantial amount of turbulence energy is injected in the ICM of the newly formed cluster. Our simulations show that the shock launched after a major merger develops an ellipsoidal shape and gets broken by the interaction with the filamentary cosmic web around the merging cluster. The size of the post-shock region along the direction of shock propagation is of the order of 300 kpc h --1, and the turbulent velocity dispersion in this region is larger than 100 km s-1. We performed a scaling analysis of the turbulence energy within our cluster sample. The best fit for the scaling of the turbulence energy with the cluster mass is consistent with M 5/3, which is also the scaling law for the thermal energy in the self-similar cluster model. This clearly indicates the close relation between virialization and injection of turbulence in the cluster evolution. As for the turbulence in the cluster core, we found that within 2 Gyr after the major merger (the timescale for the shock propagation in the ICM), the ratio of the turbulent to total pressure is larger than 10%, and after about 4 Gyr it is still larger than 5%, a typical value for nearly relaxed clusters. Turbulence at the cluster center is thus sustained for several gigayears, which is substantially longer than typically assumed in the turbulent re-acceleration models, invoked to explain the statistics of observed radio halos. Striking similarities in the morphology and other physical parameters between our simulations and the 'symmetrical radio relics' found at the periphery of the merging cluster A3376 are finally discussed. In particular, the interaction between the merger shock and the filaments surrounding the cluster could explain the presence of 'notch-like' features at the edges of the double relics.
We investigate the spatial and spectral properties of non-thermal emission from clusters of galaxies at γ-ray energies. We estimate the radiation flux between 10 keV and 10 TeV due to inverse-Compton ...(IC) emission, π0-decay and non-thermal bremsstrahlung (NTB) from cosmic ray (CR) ions and electrons accelerated at cosmic shocks as well as secondary e± generated in inelastic p–p collisions. We identify two main region of production of non-thermal radiation, namely the core (also bright in the thermal X-ray range) and the outskirts region where accretion shocks occur. We find that IC emission from shock-accelerated CR electrons dominate the emission at the outer regions of galaxy clusters, provided that at least a fraction of a per cent of the shock ram pressure is converted into CR electrons. A clear detection of this component and of its spatial distribution will allow us to probe the cosmic accretion shocks directly. In the cluster core, γ-ray emission above 100 MeV is dominated by the π0-decay mechanism. At lower energies, IC emission from secondary e± takes over. However, IC emission from shock-accelerated electrons projected on to the cluster core will not be negligible. We emphasize the importance of separating the aforementioned emission components for a correct interpretation of the experimental data and outline a strategy for that purpose. Failure to addresses this issue will produce unsound estimates of the intracluster magnetic field strength and CR ion content. According to our estimate future spaceborne and ground-based γ-ray facilities should be able to measure the whole non-thermal spectrum both in the cluster core and at its outskirts. The importance of such measurements in advancing our understanding of non-thermal processes in the intracluster medium is discussed.
Faraday rotation (rotation measure RM) probes of magnetic fields in the universe are sensitive to cosmological and evolutionary effects as z increases beyond image1 because of the scalings of ...electron density and magnetic fields, and the growth in the number of expected intersections with galaxy-scale intervenors, image. In this new global analysis of an unprecedented large sample of RMs of high-latitude quasars extending out to image, we find that the distribution of RM broadens with redshift in the 20-80 rad m super(-2 ) range, despite the super(-2) wavelength dilution expected in the observed Faraday rotation. Our results indicate that the universe becomes increasingly 'Faraday-opaque' to sources beyond image ; that is, as z increases, progressively fewer sources are found with a 'small' RM in the observer's frame. This is in contrast to sources at image. They suggest that the environments of galaxies were significantly magnetized at high redshifts, with magnetic field strengths that were at least as strong within a few Gyr of the big bang as at the current epoch. We separately investigate a simple unevolving toy model in which the RM is produced by Mg ii absorber systems, and find that it can approximately reproduce the observed trend with redshift. An additional possibility is that the intrinsic RM associated with the radio sources was much higher in the past, and we show that this is not a trivial consequence of the higher radio luminosities of the high-redshift sources.
Self-organization occurs in plasmas when energy progressively transfers from smaller to larger scales in an inverse cascade. Global structures that emerge from turbulent plasmas can be found in the ...laboratory and in astrophysical settings; for example, the cosmic magnetic field, collisionless shocks in supernova remnants and the internal structures of newly formed stars known as Herbig-Haro objects. Here we show that large, stable electromagnetic field structures can also arise within counter-streaming supersonic plasmas in the laboratory. These surprising structures, formed by a yet unexplained mechanism, are predominantly oriented transverse to the primary flow direction, extend for much larger distances than the intrinsic plasma spatial scales and persist for much longer than the plasma kinetic timescales. Our results challenge existing models of counter-streaming plasmas and can be used to better understand large-scale and long-time plasma self-organization. PUBLICATION ABSTRACT
Radio quasar sightlines with strong Mg II absorption lines display statistically enhanced Faraday rotation measures (RMs ), indicating the presence of additional magneto-active plasma with respect to ...sightlines free of such absorption. In this Letter, we use multi-color optical imaging to identify the galaxies likely hosting the magneto-active plasma, and to constrain the location of the latter with respect to the putative parent halo. We find that all of the sightlines with high RM pass within 50 kpc of a galaxy and that the RM distribution for low impact parameters, D < 50 kpc, is significantly different than for larger impact parameters. In addition, we find a decrease in the ratio of the polarization at 21 cm and 1.5 cm, p sub(21)/p sub(1.5), toward lower D. These two effects are most likely related, strengthen the association of excess RM with intervening galaxies, and suggest that intervening galaxies operate as inhomogeneous Faraday screens. These results are difficult to reconcile with only a disk model for the magnetic field, but are consistent with highly magnetized winds associated with Mg II systems. We infer strong magnetic fields of a few tens of mu G, consistent with the values required by the lack of evolution of the FIR-radio correlation at high redshifts. Finally, these findings lend support to the idea that the small-scale helicity bottleneck of alpha -Omega galactic dynamos can be significantly alleviated via galactic winds.
PDF
