Evolution of shocks and turbulence in the formation of galaxy clusters embedded in Megaparsec-scale filaments (original) (raw)
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Evolution of Shocks and Turbulence in Major Cluster Mergers
The Astrophysical Journal, 2011
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 (AMR) 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 for explaining 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 Abell 3376 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.
Numerical Simulations of Merging Clusters of Galaxies
The Astrophysical Journal Supplement Series, 1997
We present results from three-dimensional numerical simulations of head-on mergers between two clusters of galaxies using a hybrid hydro/N-body code. In these simulations, the gaseous intracluster medium (ICM) is evolved as a massless Ñuid within a changing gravitational potential deÐned by the collisionless dark matter component. The ICM is represented by the equations of hydrodynamics which are solved by an Eulerian, Ðnite-di †erence method. The cluster dark matter component is represented by the N-body particle distribution. A series of simulations have been conducted in which we have systematically varied the cluster-subcluster mass ratio between 8 : 1 and 1 : 1. We Ðnd that cluster-subcluster mergers result in an elongation of both the cluster dark matter and gas distributions. The dark matter distribution is elongated parallel to the merger axis and accompanied by anisotropy in the dark matter velocity dispersion. Both the elongation and corresponding velocity anisotropy are sustained for more than 5 Gyr after the merger. The elongation of the gas distribution is also generally along the merger axis, although shocks and adiabatic compressions produce elongations perpendicular to the merger axis at various times during the merger. We also Ðnd a signiÐcant o †set between dark matter and gas centroids in the period following core passage. The gasdynamics is also severely a †ected by the cluster-subcluster merger. In these simulations, the subcluster enters the primary at supersonic speeds initiating bulk Ñows that can exceed 2000 km s~1. The width of the bulk Ñows are seen to range between several hundred kiloparsecs to nearly 1 Mpc. We believe the bulk Ñows can produce the bending of wide-angle tailed (WAT) radio sources. The most signiÐcant gasdynamics is seen to subside on timescales of 2 Gyr, although still signiÐcant dynamics is seen even after 5 Gyr. The merger-induced gasdynamics may also play a role in the formation of radio halo sources, and, consequently, the sustained nature of the gasdynamics may extend the lifetime of halos beyond the canonical synchrotron lifetime of the source. Substructure, shocks, and adiabatic cooling during the merger can result in a very complex temperature structure within the intracluster medium. As a result of these mergers, we Ðnd temperature inhomogeneities of several keV on linear scales of ¹0.5 Mpc. Finally, these simulations indicate that even relatively high mass-ratio mergers (e.g., 8 : 1) result in nonequilibrium conditions for an extended period of time. The period of time with the most signiÐcant dynamical evolution is within 2 Gyr after core passage. The nonequilibrium conditions have implications for cluster mass estimates. The observable consequences of cluster mergers and their inÑuence on cluster mass estimates are addressed in Roettiger, Burns, & Loken (1996).
The Cluster-Merger Shock in 1E 0657-56: Faster than a Speeding Bullet?
The Astrophysical Journal, 2007
Shock waves driven in the intergalactic medium during the merging of galaxy clusters have been observed in X-ray imaging and spectroscopy. Fluid motions inferred from the shock strength and morphology can be compared to the cold dark matter (CDM) distribution inferred from gravitational lensing. A detailed reconstruction of the CDM kinematics, however, must take into account the nontrivial response of the fluid intracluster medium to the collisionless CDM motions. We have carried out two-dimensional simulations of gas dynamics in cluster collisions. We analyze the relative motion of the clusters, the bow shock wave, and the contact discontinuity and relate these to X-ray data. We focus on the "bullet cluster," 1E 0657−56, a near head-on collision of unequal-mass clusters, for which the gas density and temperature jumps across the prominent bow shock imply a high shock velocity 4, 700 km s −1 . The velocity of the fluid shock has been widely interpreted as the relative velocity of the CDM components. This need not be the case, however. An illustrative simulation finds that the present relative velocity of the CDM halos is ∼ 16% lower than that of the shock. While this conclusion is sensitive to the detailed initial mass and gas density profile of the colliding clusters, such a decrease of the inferred halo relative velocity would increase the likelihood of finding 1E 0657−56 in a ΛCDM universe.
Properties of Cosmic Shock Waves in Large‐Scale Structure Formation
The Astrophysical Journal, 2000
We have examined the properties of shock waves in simulations of large scale structure formation. Two cosmological scenarios have been considered: a standard cold dark matter model with Ω M = 1 (SCDM) and a cold dark matter model with cosmological constant and Ω M + Ω Λ = 1 (ΛCDM) having Ω Λ = 0.55. Large-scale shocks result from accretion onto sheets, filaments and knots of mass distribution on a scale of order of ∼ 5h −1 Mpc in both scenarios. Energetic motions, partly residual of past accretion processes and partly caused by current asymmetric inflow along filaments, end up generating additional shocks. These extend on a scale of order of ∼ 1h −1 Mpc and envelop and penetrate deep inside the clusters. Also collisions between substructures inside clusters form merger shocks. Consequently, the topology of the shocks is very complex and highly connected. During cosmic evolution the comoving shock surface density decreases, reflecting the ongoing structure merger process in both scenarios.
Galaxies in clusters: the observational characteristics of bow shocks, wakes and tails
Monthly Notices of the Royal Astronomical Society, 1999
The dynamical signatures of the interaction between galaxies in clusters and the intracluster medium (ICM) can potentially yield significant information about the structure and dynamical history of clusters. To develop our understanding of this phenomenon we present results from numerical modelling of the galaxy/ICM interaction, as the galaxy moves through the cluster. The simulations have been performed for a broad range, of ICM temperatures (kT cl = 1, 4 and 8 keV), representative of poor clusters or groups through to rich clusters.
Shocks and Non-thermal Particles in Clusters of Galaxies
Space Science Reviews, 2019
Galaxy clusters grow by gas accretion, mostly from mergers of substructures, which release powerful shock waves into cosmic plasmas and convert a fraction of kinetic energy into thermal energy, amplification of magnetic fields and into the acceleration of energetic particles. The modeling of the radio signature of cosmic shocks, combined with the lack of detected γ-rays from cosmic ray (CR) protons, poses challenges to our understanding of how cosmic rays get accelerated and stored in the intracluster medium. Here we review the injection of CRs by cosmic shocks of different strengths, combining the detailed "microscopic" view of collisionless processes governing the creation of non-thermal distributions of electrons and protons in cluster shocks (based on analytic theory and particle-in-cell simulations), with the "macroscopic" view of the large-scale distribution of cosmic rays, suggested by modern cosmological simulations. Time dependent non-linear kinetic models of particle acceleration by multiple internal shocks with large scale compressible motions of plasma with soft CR spectra containing a noticable energy density in the superthermal protons of energies below a few GeV which is difficult to constrain by Fermi
Turbulence and vorticity in Galaxy clusters generated by structure formation
Monthly Notices of the Royal Astronomical Society, 2016
Turbulence is a key ingredient for the evolution of the intracluster medium, whose properties can be predicted with high-resolution numerical simulations. We present initial results on the generation of solenoidal and compressive turbulence in the intracluster medium during the formation of a small-size cluster using highly resolved, non-radiative cosmological simulations, with a refined monitoring in time. In this first of a series of papers, we closely look at one simulated cluster whose formation was distinguished by a merger around z ∼ 0.3. We separate laminar gas motions, turbulence and shocks with dedicated filtering strategies and distinguish the solenoidal and compressive components of the gas flows using Hodge-Helmholtz decomposition. Solenoidal turbulence dominates the dissipation of turbulent motions (∼95 per cent) in the central cluster volume at all epochs. The dissipation via compressive modes is found to be more important (∼30 per cent of the total) only at large radii (≥0.5r vir) and close to merger events. We show that enstrophy (vorticity squared) is good proxy of solenoidal turbulence. All terms ruling the evolution of enstrophy (i.e. baroclinic, compressive, stretching and advective terms) are found to be significant, but in amounts that vary with time and location. Two important trends for the growth of enstrophy in our simulation are identified: first, enstrophy is continuously accreted into the cluster from the outside, and most of that accreted enstrophy is generated near the outer accretion shocks by baroclinic and compressive processes. Secondly, in the cluster interior vortex, stretching is dominant, although the other terms also contribute substantially.
Merging clusters and the formation of radio haloes
Computer Physics Communications, 2005
In this paper we present the preliminary result obtained from a set of cosmological numerical simulations run with the goal of describing self-consistently the evolution of the gas diffused in clusters of galaxies (Intra-Cluster Medium-ICM), including hydrodynamical and gravitational processes, in a cosmological framework. We have followed the details of the physical evolution of the ICM with particular attention to shock processes and merging events and their possible feedbacks the relativistic plasma. Furthermore, we have analyzed the details of the ICM dynamics in the most massive clusters, using tracers particles which follows the fluid in its motion.
On the evolution of giant radio halos and their connection with cluster mergers
Astronomy & Astrophysics, 2009
Giant radio halos are diffuse, Mpc-scale, synchrotron sources located in the central regions of galaxy clusters and provide the most relevant example of cluster non-thermal activity. Radio and X-ray surveys allow to investigate the statistics of halos and may contribute to constrain their origin and evolution. We investigate the distribution of clusters in the plane X-ray (thermal, L_X) vs synchrotron (P_{1.4})luminosity, where clusters hosting giant radio halos trace the P_{1.4}--L_X correlation and clusters without radio halos populate a region that is well separated from that spanned by the above correlation. The connection between radio halos and cluster mergers suggests that the cluster Mpc-scale synchrotron emission is amplified during these mergers and then suppressed when clusters become more dynamically relaxed. In this context, by analysing the distribution in the P_{1.4}--L_X plane of clusters from X-ray selected samples with adequate radio follow up, we constrain the typical time-scale of evolution of diffuse radio emission in clusters and discuss the implications for the origin of radio halos. We conclude that cluster synchrotron emission is suppressed (and amplified) in a time-scale significantly smaller than 1 Gyr. We show that this constraint appears difficult to reconcile with the hypothesis that the halo's radio power is suppressed due to dissipation of magnetic field in galaxy clusters. On the other hand, in agreement with models where turbulent acceleration plays a role, present constraints suggest that relativistic electrons are accelerated in Mpc-scale regions, in connection with cluster mergers and for a time-interval of about 1 Gyr, and then they cool in a relatively small time-scale, when the hosting cluster becomes more dynamically relaxed.
arXiv: Astrophysics, 2002
New multi-wavelength (radio, optical & X-rays) observational evidencesare presented which show that the nearby (z = 0.046), rich cluster of galaxiesAbell 3376 is experiencing a major event of binary subcluster merger. Thekey evidence is the discovery of a pair of large, optically unidentified diffuseradio sources (‘arcs’), symmetrically located about 2.6h −150 Mpc apart at theopposite ends of the hot intra-cluster gas mapped by ROSAT in X-rays. Itis argued that the gas-dynamical shock-waves, which occur naturally duringcluster formation, are accelerating charged particles (cosmic rays) to relativis-tic energies, leading to synchrotron emission from the megaparsec scale radioarcs. If this is so, cluster Abell 3376 would also be a potential source capa-ble of accelerating cosmic ray particles upto ultra-high energies (UHECR) ofE max ∼ 10 18−19 eV. Thus this cluster is an excellent test-bed for understand-ing the physics of merger shocks and origin of enigmatic UHECR particlesin structur...