On the detectability of turbulence and bulk flows in X-ray clusters

Sunyaev, R.; Norman, Michael L.; Bryan, Greg L.

doi:10.1134/1.1631411

Cited by 117 publications

(138 citation statements)

References 29 publications

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“…Various indirect observational estimates (e.g., Schuecker et al, 2004;Rebusco et al, 2005;Fujita, 2005) appear to converge in expecting random flows with velocity fluctuations U ∼ 10 2 − 10 3 km/s at the outer scale L ∼ 10 2 − 10 3 kpc (a direct detection may be achieved in the near future; see Inogamov & Sunyaev, 2003). Similar numbers are obtained in numerical simulations of cluster formation (Norman & Bryan, 1999;Sunyaev, Norman & Bryan, 2003;Ricker & Sarazin, 2001;Takizawa, 2005). There is, however, no consensus on whether turbulence, at least in the usual hydrodynamic sense of a Kolmogorov energy cascade across a broad inertial range of scales, is a generic feature of clusters (Fabian et al, 2003).…”

Section: Introductionsupporting

confidence: 65%

Fast growth of magnetic fields in galaxy clusters: a self‐accelerating dynamo

Schekochihin¹,

Cowley²

2006

Astron Nachr

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Abstract. We propose a model of magnetic-field growth in galaxy clusters whereby the field is amplified by a factor of about 10 8 over a cosmologically short time of ∼ 10 8 yr. Our model is based on the idea that the viscosity of the intracluster medium during the field-amplification epoch is determined not by particle collisions but by plasma microinstabilities: these give rise to small-scale fluctuations, which scatter particles, increasing their effective collision rate and, therefore, the effective Reynolds number. This gives rise to a bootstrap effect as the growth of the field triggers the instabilities which increase the Reynolds number which, in turn, accelerates the growth of the field. The growth is explosive and the result is that the observed field strength is reached over a fraction of the cluster lifetime independent of the exact strength of the seed field (which only needs to be above ∼ 10 −15 G to trigger the explosive growth).

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Section: Introductionsupporting

confidence: 65%

Fast growth of magnetic fields in galaxy clusters: a self‐accelerating dynamo

Schekochihin¹,

Cowley²

2006

Astron Nachr

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“…Dolag et al (2005) argue that the bulk of turbulence is injected in the core of galaxy clusters, implying a more developed turbulence in the innermost regions, compared to the outer parts. On the other hand, cosmological numerical simulations suggest that turbulence is expected to be greater at increasing radial distances from the cluster center (Bryan & Norman 1998;Sunyaev et al 2003). Moreover, Govoni et al (2006) showed that to reproduce the RM distribution for three of the radio galaxies of A2255 one needs a power spectrum of the magnetic field of the cluster steepening from the center towards the periphery and the presence of filamentary structures on large scale.…”

Section: Halo Originmentioning

confidence: 99%

Radio spectral study of the cluster of galaxies Abell 2255

Pizzo¹,

Bruyn

2009

A&A

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Context. The study of the nonthermal components associated with the intra cluster medium (ICM) in galaxy clusters is important in understanding the history and evolution of clusters. Aims. Spectral index studies of halos, relics, and radio galaxies provide useful information on their origin and connection with merger processes. Moreover, they reveal the energy spectrum of the relativistic particles and the magnetic field distribution in galaxy clusters. Methods. We present WSRT multi-wavelength observations of the galaxy cluster Abell 2255 at 25 cm, 85 cm, and 2 m. The spectral index images allowed us to study the integrated spectrum of halo and relic and to investigate the physical properties of the Beaver head-tail radio galaxy belonging to the cluster. Results. In the radio halo, the spectral index is steeper at the center and flatter at the locations of the radio filaments, clearly detected at 25 cm. In the relics, the spectral index flattens, moving away from the cluster center. For the Beaver radio galaxy, the spectrum severely steepens from the head towards the end of the tail, because of the energy losses suffered by the relativistic particles. In the 2 m map, which is the first high-sensitivity image presented in the literature at such a long wavelength, a new Mpc-size emission region is detected between the known radio halo and the NW relic. Not detecting this feature in the more sensitive 85 cm observations implies that it must have a very steep spectrum (α ≤ −2.6). Conclusions. The observational properties of the radio halo suggest that either we are looking at a superposition of different structures (filaments in the foreground plus real halo in the background) seen in projection across the cluster center or that the halo is intrinsically peculiar. The newly detected extended region to the NW of the halo could be considered as an asymmetric extension of the halo itself. However, since radio halos are known in the literature as structures showing a regular morphology, the new feature could represent the first example of steep Mpc-size diffuse structures (MDS), detected around clusters at very low frequencies. The spectrum of the initial part of the tail of the Beaver, detected at the three wavelengths, is well-fitted by a single injection model.

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“…In the absence of A&A 526, A158 (2011) magnetic fields, classical values are given by Re = UL/ν ∼ 100 (Sunyaev et al 2003), where U is the characteristic injection scale and V is the characteristic velocity. In a magnetized plasma, Reynolds numbers are expected to be much higher because of the reduction in the transport coefficients and the subsequent suppression of viscosity due to the presence of magnetic fields (Iapichino & Niemeyer 2008).…”

Section: Introductionmentioning

confidence: 99%

The impact of numerical viscosity in SPH simulations of galaxy clusters

Valdarnini

2011

A&A

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The goal of this paper is to investigate in N-body/SPH hydrodynamical cluster simulations the impact of artificial viscosity on the ICM thermal and velocity field statistical properties. To properly reduce the effects of artificial viscosity, a time-dependent artificial viscosity scheme is implemented in an SPH code in which each particle has its own viscosity parameter, whose time evolution is governed by the local shock conditions. The new SPH code is verified in a number of test problems with known analytical or numerical reference solutions and is then used to construct a large set of N-body/SPH hydrodynamical cluster simulations. These simulations are designed to study in SPH simulations the impact of artificial viscosity on the thermodynamics of the ICM and its velocity field statistical properties by comparing results extracted at the present epoch from runs with different artificial viscosity parameters, cluster dynamical states, numerical resolution, and physical modeling of the gas. Spectral properties of the gas velocity field are investigated by measuring the velocity power spectrum E(k) for the simulated clusters. Over a limited range, the longitudinal component E c (k) exhibits a Kolgomorov-like scaling ∝k −5/3 , whilst the solenoidal power spectrum component E s (k) is strongly influenced by numerical resolution effects. The dependence of the spectra E(k) on dissipative effects is found to be significant at length scales < ∼ 100−300 kpc, with viscous damping of the velocities being less pronounced in the runs with the lowest artificial viscosity. The turbulent energy density radial profile E turb (r) is strongly affected by the numerical viscosity scheme adopted in the simulations, with the turbulent-to-total energy density ratios being higher in the runs with the lowest artificial viscosity settings and lying in the range between a few percent and ∼10%. These values are in accord with the corresponding ratios extracted from previous cluster simulations based on mesh-based codes. The radial entropy profiles show a weak dependence on the artificial viscosity parameters of the simulations, with a small amount of entropy mixing being present in cluster cores. At large cluster radii, the mass correction terms to the hydrostatic equilibrium equation are affected little by the numerical viscosity of the simulations, indicating that the X-ray mass bias is already accurately estimated in standard SPH simulations. The results presented here indicate that in individual SPH cluster simulations at least N > ∼ 256 3 gas particles are necessary to provide a correct description of turbulent spectral properties over a decade in wavenumbers, whilst radial profiles of thermodynamic variables can be reliably obtained using N > ∼ 64 3 particles. Finally, simulations in which the gas can cool radiatively are characterized by the presence in the cluster inner regions of high levels of turbulence, generated by the interaction of the compact cool gas core with the ambient medium. These findings strongly support t...

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On the detectability of turbulence and bulk flows in X-ray clusters

Cited by 117 publications

References 29 publications

Fast growth of magnetic fields in galaxy clusters: a self‐accelerating dynamo

Fast growth of magnetic fields in galaxy clusters: a self‐accelerating dynamo

Radio spectral study of the cluster of galaxies Abell 2255

The impact of numerical viscosity in SPH simulations of galaxy clusters

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