F. Vazza

Turbulent gas motions in galaxy cluster simulations: the role of smoothed particle hydrodynamics viscosity

Smoothed particle hydrodynamics (SPH) employs an artificial viscosity to properly capture hydrodynamic shock waves. In its original formulation, the resulting numerical viscosity is large enough to suppress structure in the velocity field on scales well above the nominal resolution limit, and to damp the generation of turbulence by fluid instabilities. This could artificially suppress random gas motions in the intracluster medium (ICM), which are driven by infalling structures during the hierarchical structure formation process. We show that this is indeed the case by analysing results obtained with an SPH formulation where an individual, time-variable viscosity is used for each particle, following a suggestion by Morris & Monaghan. Using test calculations involving strong shocks, we demonstrate that this scheme captures shocks as well as the original formulation of SPH, but, in regions away from shocks, the numerical viscosity is much smaller. In a set of nine high-resolution simulations of cosmological galaxy cluster formation, we find that this low-viscosity formulation of SPH produces substantially higher levels of turbulent gas motions in the ICM, reaching a kinetic energy content in random gas motions (measured within a 1-Mpc cube) of up to 5‐30 per cent of the thermal energy content, depending on cluster mass. This also has significant effects on radial gas profiles and bulk cluster properties. We find a central flattening of the entropy profile and a reduction of the central gas density in the low-viscosity scheme. As a consequence, the bolometric X-ray luminosity is decreased by about a factor of 2. However, the cluster temperature profile remains essentially unchanged. Interestingly, this tends to reduce the differences seen in SPH and adaptive mesh refinement simulations of cluster formation. Finally, invoking a model for particle acceleration by magnetohydrodynamics waves driven by turbulence, we find that efficient electron acceleration and thus diffuse radio emission can be powered in the clusters simulated with the low-viscosity scheme provided that more than 5‐10 per cent of the turbulent energy density is associated with fast magneto-sonic modes.

Turbulent motions and shocks waves in galaxy clusters simulated with adaptive mesh refinement

We have implemented an Adaptive Mesh Refinement criterion explicitly designed to increase spatial resolution around discontinuities in the velocity field in ENZO cosmological simulations. With this technique, shocks and turbulent eddies developed during the hierarchical assembly of galaxy clusters are followed with unprecedented spatial resolution, even at large distances from the clusters center. By measuring the spectral properties of the gas velocity field, its time evolution and the properties of shocks for a reference galaxy cluster, we investigate the connection between accretion processes and the onset of chaotic motions in the simulated Inter Galactic Medium over a wide range of scales

Massive and refined - II. The statistical properties of turbulent motions in massive galaxy clusters with high spatial resolution

We study the properties of chaotic motions in the intra clust er medium using a set of 20 galaxy clusters simulated with large dynamical range, using the Adaptive Mesh Refinement co de ENZO (e.g. Norman et al.2007). The adopted setup allows us to study the spectral and spatial properties of turbulent motions in galaxy clusters with unprecedented detail, achi eving an maximum available Reynolds number of the order of Re ∼ 500− 1000 for the largest eddies. The correlations between the energy of these motions in the Intra Cluster Medium and the dy namical state of the host systems are studied, and the statis tical properties of turbulent motions and their evolution with ti me support that major merger events are responsible for the i njection of the bulk of turbulent kinetic energy inside cluster. Turb lence is found to account for a ∼ 20− 30 per cent of the thermal energy in merging clusters, while it accounts for a ∼ 5 per cent in relaxed clusters. A comparison of the energies o f turbulence and motions in our simulated clusters with present upper-li mits in real nearby clusters, recently derived with XMM-New ton (Sanders et al.2010), is provided. When the same spatial sca les of turbulent motions are compared, the data from simulat ions result well within the range presently allowed by observati ons. Finally, we comment on the possibility that turbulence may accelerate relativistic particles leading to the formatio n of giant radio halos in turbulent (merging) clusters. Base d on our simulations we confirm previous semi-analytical studies th at suggest that the fraction of turbulent clusters is consis tent with that of clusters hosting radio halos.We study the properties of chaotic motions in the intra cluster medium using a set of 20 galaxy clusters simulated with large dynamical range, using the adaptive mesh refinement code ENZO. The adopted setup allows us to study the spectral and spatial properties of turbulent motions in galaxy clusters with unprecedented detail, achieving an maximum available Reynolds number of the order of Re ∼ 500−1000 for the largest eddies. We investigated the correlations between the energy of these motions in the intra cluster medium and the dynamical state of the host systems. We find that the statistical properties of turbulent motions and their evolution with time imply that major merger events are responsible for the injection of the bulk of turbulent kinetic energy into the cluster. Turbulence is found to account for ∼20−30 per cent of the thermal energy in merging clusters, and ∼5 per cent in relaxed clusters. We compare the energies of turbulence and motions in our simulated clusters with upper-limits for real nearby clusters derived from XMM-Newton data. When turbulent motions are compared on the same spatial scales, the data from simulations are well within the range presently allowed by observations. Finally, we comment on the possibility that turbulence may accelerate relativistic particles leading to the formation of giant radio halos in turbulent (merging) clusters. On the basis of our simulations, we confirm the conclusions of previous semi-analytical studies that the fraction of turbulent clusters appears to be consistent with that of clusters hosting radio halos.

The gas distribution in the outer regions of galaxy clusters

Aims. We present our analysis of a local (z = 0.04-0.2) sample of 31 galaxy clusters with the aim of measuring the density of the X-ray emitting gas in cluster outskirts. We compare our results with numerical simulations to set constraints on the azimuthal symmetry and gas clumping in the outer regions of galaxy clusters.

Discovery of radio haloes and double relics in distant MACS galaxy clusters: clues to the efficiency of particle acceleration

We have performed 323 MHz observations with the Giant Metrewave Radio Telescope of the most promising candidates selected from the MACS catalogue. The aim of the work is to extend our knowledge of the radio halo and relic populations to z > 0.3, the epoch in which massive clusters are formed. In MACSJ1149.5+2223 and MACSJ1752.1+4440, we discovered two double-relic systems with a radio halo, and in MACSJ0553.4−3342 we found a radio halo. Archival Very Large Array observations and Westerbork Synthesis Radio Telescope observations have been used to study the polarization and spectral-index properties. The radio halo in MACSJ1149.5+2223 has the steepest spectrum ever found so far in these objects (α ≥ 2). The double relics in MACSJ1149.5+2223 are peculiar in their position that is misaligned with the main merger axis. The relics are polarized up to 30 and 40 per cent in MACSJ1149.5+2223 and MACSJ1752.040+44, respectively. In both cases, the magnetic field is roughly aligned with the relics’ main axes. The spectra in the relics in MACSJ1752.040+44 steepen towards the cluster centre, in agreement with model expectations. X-ray data on MACSJ0553.4−3342 suggest that this cluster is undergoing a major merger, with the merger axis close to the plane of the sky. The cores of the disrupted clusters have just passed each other, but no radio relic is detected in this system. If turbulence is responsible for the radio emission, we argue that it must develop before the core passage. A comparison of double-relic plus halo system with cosmological simulations allows a simultaneous estimate of the acceleration efficiencies at shocks (to produce relics) and of turbulence (to produce the halo).

A comparison of cosmological codes: properties of thermal gas and shock waves in large-scale structures

Cosmological hydrodynamical simulations are a valuable to ol f r understanding the growth of large scale structure and the observables connect ed wi h this. Yet, comparably little attention has been given to validation studies of the proper ties of shocks and of the resulting thermal gas between different numerical methods – somet hing of immediate importance as gravitational shocks are responsible for generating mos t of the entropy of the large scale structure in the Universe. Here, we present results for the s tatistics of thermal gas and the shock wave properties for a large volume simulated with thre e different cosmological numerical codes: the Eulerian total variations diminishing c ode TVD, the Eulerian piecewise parabolic method-based code ENZO, and the Lagrangian smoot hed-particle hydrodynamics code GADGET. Starting from a shared set of initial condition s, we present convergence tests for a cosmological volume of side-length 100Mpc/h, studying in detail the morphological and statistical properties of the thermal gas as a function o f mass and spatial resolution in all codes. By applying shock finding methods to each code, we meas ur the statistics of shock waves and the related cosmic ray acceleration efficiencies, within the sample of simulations and for the results of the different approaches. We discuss t he regimes of uncertainties and disagreement among codes, with a particular focus on the res ults at the scale of galaxy clusters. Even if the bulk of thermal and shock properties are rea son bly in agreement among the three codes, yet some significant differences exist (esp ecially between Eulerian methods and smoothed particle hydrodynamics). In particular, we re po t: a) differences of huge factors (∼ 10 − 100) in the values of average gas density, temperature, entropy , Mach number and shock thermal energy flux in the most rarefied regions of the si mulations (ρ/ρcr < 1) between grid and SPH methods; b) the hint of an entropy core inside clu sters simulated in grid codes; c) significantly different phase diagrams of shocked cells i n gr d codes compared to SPH; d) sizable differences in the morphologies of accretion shock s between grid and SPH methods.

The X-ray/SZ view of the virial region - I. Thermodynamic properties

We measure the thermodynamic properties of cluster outer regions to provide constraints on the processes that rule the formation of large scale structures. We derived the thermodynamic properties of the intracluster gas (temperature, entropy) by combining the SZ thermal pressure from Planck and the X-ray gas density from ROSAT. This method allowed us to reconstruct for the first time temperature and entropy profiles out to the virial radius and beyond in a large sample of objects. At variance with several recent Suzaku studies, we find that the entropy rises steadily with radius, albeit at at a somewhat lower rate than predicted by self-similar expectations. We note significant differences between relaxed, cool-core systems and unrelaxed clusters in the outer regions. Relaxed systems appear to follow the self-similar expectations more closely than perturbed objects. Our results indicate that the well-known entropy excess observed in cluster cores extends well beyond the central regions. When correcting for the gas depletion, the observed entropy profiles agree with the prediction from gravitational collapse only, especially for cool-core clusters.

Turbulent velocity fields in smoothed particle hydrodymanics simulated galaxy clusters: Scaling laws for the turbulent energy

We present a study of the turbulent velocity fields in the Intra Cluster Medium of a sample of 21 galaxy clusters simulated by the SPH–code Gadget2, using a new numerical scheme where the artificial viscosity is suppressed outside shocks. The turbulent motions in the ICM of our simulated clusters are detected with a novel method devised to better disentangle laminar bulk motions from chaotic ones. We focus on the scaling law between the turbulent energy content of the gas particles and the total mass, and find that the energy in the form of turbulence scales approximatively with the thermal energy of clusters. We follow the evolution with time of the scaling laws and discuss the physical origin of the observed trends. The simulated data are in agreement with independent semi–analytical calculations, and the combination between the two methods allows to constrain the scaling law over more than two decades in cluster mass.

Properties of gas clumps and gas clumping factor in the intra-cluster medium

ABSTRACT The spatial distribution of gas matter inside galaxy clusters is not completely smooth,but may host gas clumps associated with substructures. These overdensegas substructures aregenerallya source of unresolved bias of X-ray observationstowards high density gas, but theirbright luminosity peaks may be resolved sources within the ICM, that deep X-ray exposuresmay be (already) capable to detect. In this paper we aim at investigating both features, usinga set of high-resolution cosmological simulations with ENZO . First, we monitor how the biasby unresolved gas clumping may yield incorrect estimates of global cluster parameters andaffects the measurements of baryon fractions by X-ray observations. We find that based onX-ray observations of narrow radial strips, it is difficult to recover the real baryon fraction tobetter than 10 - 20 percent uncertainty. Second, we investigated the possibility of observingbright X-ray clumps in the nearby Universe (z60.3). We produced simple mock X-ray ob-servations for several instruments (XMM, Suzaku and ROSAT) and extracted the statistics ofpotentially detectable bright clumps. Some of the brightest clumps predicted by simulationsmay already have been already detected in X- ray images with alarge field of view. However,their small projected size makes it difficult to prove their e xistence based on X-ray morphol-ogy only. Preheating, AGN feedback and cosmic rays are found to have little impact on thestatistical properties of gas clumps.Key words: galaxy clusters, ICM

On the amplification of magnetic fields in cosmic filaments and galaxy clusters

The amplification of primordial magnetic fields via a small-scale turbulent dynamo during structure formation might be able to explain the observed magnetic fields in galaxy clusters. The magnetisation of more tenuous large-scale structures such as cosmic filaments is more uncertain, as it is challenging for numerical simulations to achieve the required dynamical range. In this work, we present magneto-hydrodynamical cosmological simulations on large uniform grids to study the amplification of primordial seed fields in the intracluster medium (ICM) and in the warm-hot-intergalactic medium (WHIM). In the ICM, we confirm that turbulence caused by structure formation can produce a significant dynamo amplification, even if the amplification is smaller than what is reported in other papers. In the WHIM inside filaments, we do not observe significant dynamo amplification, even though we achieve Reynolds numbers of