Fields In Galaxy Clusters Research Articles

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

AbstractWe propose a model of magnetic‐field growth in galaxy clusters whereby the field is ampli.fied by a factor of about 108 over a cosmologically short time of ∼ 108 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 .uctuations, 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). (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Observations of magnetic fields in regular and irregular clusters

AbstractThe existence of magnetic fields associated with the intracluster medium in clusters of galaxies is now well established through different methods of analysis. The most detailed evidence for the presence of cluster magnetic fields comes from radio observations. Magnetic fields can be investigated through the synchrotron emission of cluster‐wide diffuse sources and from studies of the rotation measure of polarized radio galaxies. I will review efforts to measure magnetic field strengths and power spectra and the main issues that have led to our knowledge on magnetic fields in regular and irregular clusters of galaxies. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Cluster Magnetic Fields from Large-Scale Structure and Galaxy Cluster Shocks

The origin of the microgauss magnetic fields in galaxy clusters is one of the outstanding problems of modern cosmology. We have performed three-dimensional particle-in-cell simulations of the nonrelativistic Weibel instability in an electron-proton plasma, in conditions typical of cosmological shocks. These simulations indicate that cluster fields could have been produced by shocks propagating through the intergalactic medium during the formation of large-scale structure or by shocks within the cluster. The strengths of the shock-generated fields range from a few nanogauss in the intercluster medium to a fraction of a microgauss inside galaxy clusters. Further amplification of these fields by sheared turbulent motions and gas accretion in clusters may be expected. Thus, cluster fields may be explained without resorting to the amplification of a primordial field.

Magnetic field in galaxies, galaxy clusters, and intergalactic space

Magnetic fields of unknown origin appear to permeate the Universe on all large scales. We suggest that the winds and cosmic rays, which are produced by the spherical ejecta and jets from supernova explosions, magnetize the interstellar medium in galaxies, the intracluster medium in galaxy clusters, and the intergalactic space. We show that, if the cosmic rays generate magnetic fields in rough energy equipartition with the cosmic rays, the predicted magnetic-field strengths coincide with the ones observed.

Evolving turbulence and magnetic fields in galaxy clusters

We discuss, using simple analytical models and magnetohydrodynamic (MHD) simulations, the origin and parameters of turbulence and magnetic fields in galaxy clusters. Any pre-existing tangled magnetic field must decay in a few hundred million years by generating gas motions even if the electric conductivity of the intracluster gas is high. We argue that turbulent motions can be maintained in the intracluster gas and its dynamo action can prevent such a decay and amplify a random seed magnetic field by a net factor of typically 104 in 5 Gyr. Three physically distinct regimes can be identified in the evolution of turbulence and magnetic field in galaxy clusters. First, the fluctuation dynamo will produce microgauss (μG)-strong, random magnetic fields during the epoch of cluster formation and major mergers. At this stage pervasive turbulent flows with rms velocity of about 300 km s−1 can be maintained at scales of 100–200 kpc. The magnetic field is intermittent, has a smaller scale of 20–30 kpc and average strength of 2 μG. Secondly, turbulence will decay after the end of the major merger epoch; we discuss the dynamics of the decaying turbulence and the behaviour of magnetic field in it. Magnetic field and turbulent speed undergo a power-law decay, decreasing by a factor of 2 during this stage, whereas their scales increase by about the same factor. Thirdly, smaller-mass subclusters and cluster galaxies will produce turbulent wakes where magnetic fields will be generated as well. Although the wakes plausibly occupy only a small fraction of the cluster volume, we show that their area-covering factor can be close to unity, and thus they can produce some of the signatures of turbulence along virtually all lines of sight. The latter could potentially allow one to reconcile the possibility of turbulence with ordered filamentary gas structures, as in the Perseus cluster. The turbulent speeds and magnetic fields in the wakes are estimated to be of the order of 300 km s−1 and 2 μG, respectively, whereas the turbulent scales are of the order of 200 kpc for wakes behind subclusters of a mass 3 × 1013 M⊙ and about 10 kpc in the galactic wakes. Magnetic field in the wakes is intermittent and has the scale of about 30 and 1 kpc in the subcluster and galactic wakes, respectively. Random Faraday rotation measure is estimated to be typically 100–200 rad m−2, in agreement with observations. We predict detectable polarization of synchrotron emission from cluster radio haloes at wavelengths 3–6 cm, if observed at sufficiently high resolution.

Magnetic Fields in Galaxy Clusters

AbstractMagnetic fields in galaxy clusters can be investigated using a variety of techniques. Recent studies including radio halos, Inverse Compton hard X-ray emissions and Faraday rotation measure, are briefly outlined. A numerical approach for investigating cluster magnetic fields strength and structure is presented. It consists of producing simulated rotation measure, radio halo images, and radio halo polarization, obtained from 3-dimensional multi-scale cluster magnetic field models, and comparing with observations.

Evolution of cosmic magnetic fields: From the very early Universe, to recombination, to the present

A detailed numerical and analytical examination of the evolution of stochastic magnetic fields between a putative magnetogenesis era at high cosmic temperatures $T\ensuremath{\sim}100\text{ }\mathrm{M}\mathrm{e}\mathrm{V}--100\text{ }\mathrm{G}\mathrm{e}\mathrm{V}$ and the present epoch is presented. The analysis includes all relevant dissipation processes, such as neutrino- and photon-induced fluid viscosities as well as ambipolar and hydrogen diffusion. A simple and intuitive analytical model matching the results of the three-dimensional MHD simulations allows for the prediction of prerecombination and present day magnetic field correlation lengths and energy densities as a function of initial magnetic field energy density, helicity, and spectral index. Our conclusions are multifold. (a) Initial primordial fields with only a small amount of helicity are evolving into maximally helical fields at the present. Furthermore, the simulations show a self-similarity in the evolution of maximally helical fields implying a seemingly acausual amplification of magnetic fields on large scales is observed. (b) There exists a correlation between the strength of the magnetic field $B$ at the peak of its spectrum and the location of the peak, given at the present epoch by $B\ensuremath{\approx}5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}\text{ }\mathrm{G}(L/\mathrm{k}\mathrm{p}\mathrm{c})$, where $L$ is the magnetic field correlation length determined by the initial properties of the magnetic field. (c) Concerning studies of the generation of cosmic microwave background (CMBR) anisotropies due to primordial magnetic fields of $B\ensuremath{\sim}{10}^{\ensuremath{-}9}\text{ }\mathrm{G}$ on $\ensuremath{\gtrsim}10\text{ }\mathrm{M}\mathrm{p}\mathrm{c}$ scales, such fields are not only impossible to generate in early causal magnetogenesis scenarios but also seemingly ruled out by distortions of the CMBR spectrum due to magnetic field dissipation on smaller scales and the overproduction of cluster-magnetic fields. (d) The most promising detection possibility of CMBR distortions due to primordial magnetic fields may be on much smaller scales at higher multipoles $l\ensuremath{\sim}{10}^{6}$ where the signal is predicted to be the strongest. (e) It seems possible that magnetic fields in clusters of galaxies are entirely of primordial origin, without invoking dynamo amplification. Such fields would be of (precollapse) strength ${10}^{\ensuremath{-}12}--{10}^{\ensuremath{-}11}\text{ }\mathrm{G}$ with correlation lengths in the kpc range and would also exist in voids of galaxies.

Magnetic fields in clusters of galaxies

An important area of study of cosmic magnetic fields is on the largest scales, those of clusters of galaxies. In the last decade it has become clear that the intra-cluster medium in clusters of galaxies is magnetized and that magnetic fields play a critical role in the cluster formation and evolution. The observational evidence for the existence of cluster magnetic fields is obtained by the diffuse cluster-wide synchrotron radio emission and from rotation measure studies of extragalactic radio sources located within or behind the clusters. A significant breakthrough in the knowledge of the cluster magnetic fields will be reached through the SKA, owing to its capabilities, in particular the deep sensitivity and the polarization purity.

Large-scale magnetic fields from dilaton inflation in noncommutative spacetime

The generation of large-scale magnetic fields is studied in dilaton electromagnetism in noncommutative inflationary cosmology taking into account the effects of the spacetime uncertainty principle motivated by string theory. We show that it is possible to generate large-scale magnetic fields with sufficient strength to account for the observed fields in galaxies and clusters of galaxies through only adiabatic compression without dynamo amplification mechanism in models of power-law inflation based on spacetime noncommutativity without introducing a huge hierarchy between the dilaton's potential and its coupling to the electromagnetic fields.

MAGNETIC FIELDS IN CLUSTERS OF GALAXIES

The existence of magnetic fields associated with the intracluster medium in clusters of galaxies is now well established through different methods of analysis. Magnetic fields are investigated in the radio band from studies of the rotation measure of polarized radio galaxies and the synchrotron emission of cluster-wide diffuse sources. Other techniques include X-ray studies of the inverse Compton emission and of cold fronts and magneto hydrodynamic simulations. We review the main issues that have led to our knowledge on magnetic fields in clusters of galaxies. Observations show that cluster fields are at the μG level, with values up to tens of μG at the center of cooling core clusters. Estimates obtained from different observational approaches may differ by about an order of magnitude. However, the discrepancy may be alleviated by considering that the magnetic field is not constant throughout the cluster, and shows a complex structure. In particular, the magnetic field intensity declines with the cluster radius with a rough dependence on the thermal gas density. Moreover, cluster magnetic fields are likely to fluctuate over a wide range of spatial scales with values from a few kpc up to hundreds kpc. Important information on the cluster field are obtained by comparing the observational results with the prediction from numerical simulations. The origin of cluster magnetic fields is still debated. They might originate in the early Universe, either before or after the recombination, or they could have been deposited in the intracluster medium by normal galaxies, starburst galaxies, or AGN. In either case, magnetic fields undergo significant amplification during the cluster merger processes.

A submm survey of high-redshift galaxy clusters: a submm Butcher-Oemler effect?

We present the initial results of a deep submillimeter survey of ten galaxy clusters with the SCUBA bolometer array on the JCMT. Recently, an excess number of dust enshrouded starbursts in high-redshift galaxy cluster fields was reported (Best 2002) and has been interpreted as increased star-formation within young clusters. An alternative explanation is that these starbursts are background objects behind clusters with enhanced lensing cross-sections, over that predicted from simple lensing models, so-called “super-lenses” seen in recent optical cluster surveys. To investigate this issue, we are studying a set of clusters which show strong optical arcs (super-lenses) and a set of clusters of equal redshift and richness but which are not super-lenses. The preliminary results, based on 50% of the sample, tentatively suggest that, while the dusty starbursts are limited to the super-lensing cluster fields, they are not background objects but lie within the clusters themselves. Cluster-cluster merging offers a means of increasing both the lensing cross-section, thereby accounting for the multiple strong optical arcs, and increasing the star formation within the clusters through galaxy interactions.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Divergence of neighboring magnetic-field lines and fast-particle diffusion in strong magnetohydrodynamic turbulence, with application to thermal conduction in galaxy clusters.

Using direct numerical simulations, we calculate the rate of divergence of neighboring magnetic-field lines in different types of strong magnetohydrodynamic turbulence. In the static-magnetic-field approximation, our results imply that tangled magnetic fields in galaxy clusters reduce the electron diffusion coefficient and thermal conductivity by a factor of approximately 5-10, relative to their values in a nonmagnetized plasma.

Prospects for the Determination ofH0through Observation of Multiply Imaged Supernovae in Galaxy Cluster Fields

We assess the possibility of determining the Hubble constant H0 by measuring time delays between multiple images of supernovae gravitationally lensed by rich clusters of galaxies and combining these delay measurements with detailed cluster potential models based on other lensing constraints. Such a lensing determination of H0 would be complementary to those obtained from galaxy-quasar lensing studies and could potentially be better calibrated. We show that relatively low-redshift (z ~ 0.2), significantly elliptical clusters have appreciable lensing cross sections for observable image pairings with tractable time delays on the order of a few years despite large lensing mass scales. We find that a targeted search for such image pairs would be a significant undertaking for current observatories but that it would be appropriate for a facility such as the proposed Large-Aperture Synoptic Survey Telescope.

The Evolution of Magnetic Fields in Galaxy Clusters

Cosmological simulations of magnetic fields in galaxy clusters show that remarkable agreement between simulations and observations of Faraday rotation and radio haloes can be achieved assuming that seed fields of ∼ 10−9 G were present at redshifts ∼ 15−20. The structure of the seed field is irrelevant for the final intracluster field. On average, the field grows exponentially with decreasing cluster redshift, but merger events cause steep transient increases in the field strength. Typical field-reversal scales are of order 50h−1kpc. In most cases, the intracluster fields are dynamically unimportant. Assuming secondary electron models, the average structure of cluster radio haloes can naturally be reproduced.

Magnetic field in the intracluster medium: Rydberg matter with almost free electrons

Observations of the magnetic field in galaxy clusters by Faraday rotation give new important information. Since the magnetic field is coupled to matter, the Faraday rotation gives direct information on the intracluster matter. It was recently proposed that at least part of the dark matter in the Universe is so-called Rydberg matter. The magnetic field which arises as a result of Rydberg matter in space at low temperature is described. It is found to be of considerable strength, and strongly inhomogeneous, owing to the filamentary structure of Rydberg matter. The very low work function of Rydberg matter means that the electrons in the matter are almost free. The inhomogeneous distributions of magnetic field strength and electron density are calculated to give a Faraday rotation which easily covers the range of observed values.

Quantification of Uncertainty in the Measurement of Magnetic Fields in Clusters of Galaxies

We assess the principal statistical and physical uncertainties associated with the determination of magnetic field strengths in clusters of galaxies from measurements of Faraday rotation (FR) and Compton-synchrotron emissions. In the former case, a basic limitation is noted, that the relative uncertainty in the estimation of the mean-squared FR is generally at least . Even greater uncertainty stems from the crucial dependence of the Faraday-deduced field on the coherence length scale characterizing its random orientation; we further elaborate this dependence and argue that previous estimates of the field are likely to be too high by a factor of a few. Lack of detailed spatial information on the radio emission—and the recently deduced nonthermal X-ray emission in four clusters—has led to an underestimation of the mean value of the field in cluster cores. We conclude therefore that it is premature to draw definite quantitative conclusions from the previously claimed, seemingly discrepant values of the field determined by these two methods.

Evolution and structure of magnetic fields in simulated galaxy clusters

We use cosmological magneto-hydrodynamic simulations to study the evolution of magnetic fields in galaxy clusters in two different cosmological models, a standard-CDM and a Λ-CDM model. We show that the magnetic field strength profiles closely follow the cluster density profiles outside a core region of radius ~. The magnetic field has a correlation length of order and reverses on scales of ~ along typical lines-of-sight. The power spectrum of the magnetic field can well be approximated by a steep power law with an exponent of ~-2.7. The mean magnetic field in the cluster cores grows roughly exponentially with decreasing redshift, . Merger events have a pronounced effect on magnetic field evolution, which is strongly reflected in measurable quantities like the Faraday rotation. The field evolution in the two different cosmologies proceeds virtually identically. All our cluster models very well reproduce observed Faraday rotation measurements when starting with nG seed fields.

Supernovae in deep Hubble Space Telescope galaxy cluster fields: cluster rates and field counts

We have searched for high-redshift supernova (SN) candidates in multiple deep Hubble Space Telescope (HST) archival images of nine galaxy cluster fields. We detect six apparent SNe, with 21.6I81428.4 mag. There is roughly 1 SN per deep (I814>26 mag), doubly-imaged WFPC2 cluster field. Two SNe are associated with cluster galaxies (at redshifts z=0.18 and z=0.83), three are probably in galaxies not in the clusters (at z=0.49, z=0.60, and z=0.98), and one is at unknown z. After accounting for observational efficiencies and uncertainties (statistical and systematic) we derive the rate of type Ia SNe within the projected central of rich clusters: SNu in 0.18z0.37 clusters, and SNu in clusters at 0.83z1.27 (95 per cent confidence interval; 1 SNu≡1 SN century−1 per 1010LB⊙). Combining the two redshift bins, the mean rate is SNu. The upper bounds argue against SNe Ia being the dominant source of the large iron mass observed in the intracluster medium. We also compare our observed counts of field SNe (i.e. non-cluster SNe of all types) to recent model predictions. The observed field count is N1 SN with I81426 mag, and 1N3 SNe with I81427 mag. These counts are half of some of the predictions. Since the counts at these magnitudes are likely dominated by type II SNe, our observations may suggest obscuration of distant SNe II, or a SN II luminosity distribution devoid of a large high-luminosity tail.

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.

On the Thermal Conduction in Tangled Magnetic Fields in Clusters of Galaxies

Thermal conduction in tangled magnetic fields is reduced because heat-conducting electrons must travel longer distances along the field lines between hot and cold regions of space than if there were no fields. We consider the case in which the tangled magnetic field has a weak homogeneous component. We examine two simple models for temperature in clusters of galaxies: a time-independent model and a time-dependent one. We find that the actual value of the effective thermal conductivity in tangled magnetic fields depends on how it is defined for a particular astrophysical problem. Our final conclusion is that the heat conduction is never totally suppressed, but is usually important in the central regions of galaxy clusters, and therefore it should not be neglected.