Exploring the magnetized cosmic web through low-frequency radio emission

Recent improvements in the capabilities of low-frequency radio telescopes provide a unique opportunity to study thermal and non-thermal properties of the cosmic web. We argue that the diffuse, polarized emission from giant radio relics traces structure formation shock waves and illuminates the large-scale magnetic field. To show this, we model the population of shock-accelerated relativistic electrons in high-resolution cosmological simulations of galaxy clusters and calculate the resulting radio synchrotron emission. We find that individual shock waves correspond to localized peaks in the radio surface brightness map which enables us to measure Mach numbers for these shocks. We show that the luminosities and number counts of the relics strongly depend on the magnetic field properties, the cluster mass and dynamical state. By suitably combining different cluster data, including Faraday rotation measures, we are able to constrain some macroscopic parameters of the plasma at the structure formation shocks, such as models of turbulence. We also predict upper limits for the properties of the warm-hot intergalactic medium, such as its temperature and density. We predict that the current generation of radio telescopes [Low-Frequency Array (LOFAR), Giant Meter Radio Telescope (GMRT), the Murchison Wide-field Array (MWA) and Long Wavelength Array (LWA)] have the potential to discover a substantially larger sample of radio relics, with multiple relics expected for each violently merging cluster. Future experiments [(Square Kilometre Array (SKA)] should enable us to further probe the macroscopic parameters of plasma physics in clusters.     

Double Relics in the Outskirts of A3376: Accretion Flows Meet Merger Shocks?

The case of spectacular ring-like double radio relics in the merging, rich galaxy cluster A3376 is of great interest to study non-thermal phenomena at cluster outskirts. We present the first low frequency (330 and 150 MHz) images of the double relics using the GMRT. With our GMRT 330 MHz map and the VLA 1400 MHz map (Bagchi et al. 2006), we have constructed and analyzed the distribution of spectral indices over the radio relics. We find flat spectral indices at the outer edges of both the relics and a gradual steepening of spectral indices toward the inner regions. This supports the model of outgoing merger shock waves. The eastern relic has a complex morphology and spectral index distribution toward the inner region. This will be discussed in the context of the effect of large-scale accretion flows on the outgoing merger shocks as reported in the recent simulations.     

On the formation of cluster radio relics

In several merging clusters of galaxies so-called cluster radio relics have been observed. These are extended radio sources which do not seem to be associated with any radio galaxies. Two competing physical mechanisms to accelerate the radio-emitting electrons have been proposed: (i) diffusive shock acceleration and (ii) adiabatic compression of fossil radio plasma by merger shock waves. Here the second scenario is investigated. We present detailed three-dimensional magneto-hydrodynamical simulations of the passage of a radio plasma cocoon filled with turbulent magnetic fields through a shock wave. Taking into account synchrotron, inverse Compton and adiabatic energy losses and gains, we evolved the relativistic electron population to produce synthetic polarization radio maps. On contact with the shock wave the radio cocoons are first compressed and finally torn into filamentary structures, as is observed in several cluster radio relics. In the synthetic radio maps the electric polarization vectors are mostly perpendicular to the filamentary radio structures. If the magnetic field inside the cocoon is not too strong, the initially spherical radio cocoon is transformed into a torus after the passage of the shock wave. Very recent, high-resolution radio maps of cluster radio relics seem to exhibit such toroidal geometries in some cases. This supports the hypothesis that cluster radio relics are fossil radio cocoons that have been revived by a shock wave. For a late-stage relic the ratio of its global diameter to the filament diameter should correlate with the shock strength. Finally, we argue that the total radio polarization of a radio relic should be well correlated with the three-dimensional orientation of the shock wave that produced the relic.     

Radio signature of cosmological structure formation shocks

In the course of the formation of cosmological structures, large shock waves are generated in the intracluster medium (ICM). In analogy to processes in supernova remnants, these shock waves may generate a significant population of relativistic electrons which, in turn, produce observable synchrotron emission. The extended radio relics found at the periphery of several clusters and possibly also a fraction of radio halo emission may have this origin. Here, we derive an analytic expression for (i) the total radio power in the downstream region of a cosmological shock wave, and (ii) the width of the radio-emitting region. These expressions predict a spectral slope close to −1 for strong shocks. Moderate shocks, such as those produced in mergers between clusters of galaxies, lead to a somewhat steeper spectrum. Moreover, we predict an upper limit for the radio power of cosmological shocks. Comparing our results to the radio relics in Abell 115, 2256 and 3667, we conclude that the magnetic field in these relics is typically at a level of 0.1 μG. Magnetic fields in the ICM are presumably generated by the shocks themselves; this allows us to calculate the radio emission as a function of the cluster temperature. The resulting emissions agree very well with the radio power–temperature relation found for cluster haloes. Finally, we show that cosmic accretion shocks generate less radio emission than merger shock waves. The latter may, however, be detected with upcoming radio telescopes.     

The redshift dependence of spectral index in powerful radio galaxies

We present and discuss in this paper the rest frame radio spectra (1–25 GHz) of a sample of fourteen radio galaxies atz >2 from the newly defined MRC/1Jy complete sample of 558 radio sources. These galaxies are among the most powerful radio sources known and range in luminosity from 1028-1028·8 watt Hz-1 at 1 GHz. We find that the median rest frame spectral index of this sample of galaxies atz >2 is significantly steeper than that of a matched luminosity sample of 3CRR galaxies which are at a much lower redshift (0.85 <z < 1.7). This indicates that spectral index correlates primarily with redshift, at least in the luminosity range considered here. The difference between the distributions of rest frame spectral curvatures for the two samples does not appear to be statistically significant. We suggest a new explanation for the steeper spectra of radio galaxies at high redshift involving steeper electron energy spectra at injection. Electron energy spectra are expected to steepen in a first-order Fermi acceleration process, at both non-relativistic and relativistic shock fronts, as the upstream fluid velocity decreases. This may well be the case at high redshifts: the hotter and denser circum-galactic medium at high redshifts could result in slower speeds for the hotspot and the jet material behind it. The smaller sizes of radio sources at higher redshifts provide support to this scenario. Since deceased.     

Radio jet interactions with massive clouds

Three-dimensional simulations of light hydrodynamic jets are computed using the Zeus-3D code. We employ parameters corresponding to moderate to high power radio jets emerging through a galactic atmosphere or halo, and eventually crossing a tilted pressure matched interface with a hotter intracluster medium. These simulations aim the jets so that they hit massive dense clouds within the galactic halo. Such clouds are set up with radii several times that of the jet, and nominally correspond to giant molecular cloud complexes or small cannibalized galaxies. We find that powerful jets eventually disperse the clouds, but that, for the off-center collisions considered, non-axisymmetric instabilities are induced in those jets. Those instabilities grow faster for lower Mach number jets, and can produce disruptions substantially sooner than occurred in our earlier work on jets in the absence of collisions with massive clouds. Such interactions could be related to some Compact Steep Spectrum source morphologies. Very weak jets can be effectively halted by reasonably massive clouds, and this may have relevance for the paucity of radio jets in spiral galaxies. Slow, dense jets may be bent, yet remain stable for fairly extended times, thereby explaining some Wide-Angle-Tail and most “dog-leg” morphologies.     

Centaurus A as a source of extragalactic cosmic rays with arrival energies well beyond the GZK cutoff

The ultra-high energy cosmic rays recently detected by several air shower experiments could have an extragalactic origin. In this case, the nearest active galaxy Centaurus A might be the source of the most energetic particles ever detected on Earth. We have used recent radio observations in order to estimate the arrival energy of the protons accelerated by strong shock fronts in the outer parts of this southern radio source. We expect detections coresponding to particles with energies up to 2.2 × 10²¹ eV and an arrival direction of (l ≈ 310°, b ≈ 20°) in galactic coordinates. The future Southern Hemisphere Pierre Auger Observatory might provide a decisive test for extragalactic models of the origin of the ultra-high energy cosmic rays.     

The influence of the Mach number on the stability of radiative shocks

We study the stability properties of hydrodynamic shocks with finite Mach numbers. The linear analysis supplements previous analyses which took the strong shock limit. We derive the linearized equations for a general specific heat ratio as well as temperature and density power-law cooling functions, corresponding to a range of conditions relevant to interstellar atomic and molecular cooling processes. Boundary conditions corresponding to a return to the upstream temperature  ( R = 1)  and to a cold wall  ( R = 0)  are investigated. We find that for Mach number   M > 5  , the strong shock overstability limits are not significantly modified. For   M < 3  , however, shocks are considerably more stable for most cases. In general, as the shock weakens, the critical values of the temperature power-law index (below which shocks are overstable) are reduced for the overtones more than for the fundamental, which signifies a change in basic behaviour. In the   R = 0  scenario, however, we find that the overstability regime and growth rate of the fundamental mode are increased when cooling is under local thermodynamic equilibrium. We provide a possible explanation for the results in terms of a stabilizing influence provided downstream but a destabilizing effect associated with the shock front. We conclude that the regime of overstability for interstellar atomic shocks is well represented by the strong shock limit unless the upstream gas is hot. Although molecular shocks can be overstable to overtones, the magnetic field provides a significant stabilizing influence.     

Interaction of Weak Shock Waves with Current Sheets in an Active Region to Produce Nanoflares and Chains of Type I Radio Bursts

Interaction of weak shock waves with a current sheet is investigated by a two-dimensional numerical magnetohydrodynamic model. In accordance with solar coronal conditions, a ratio of thermal to magnetic pressures of 0.1 and a shock Alfvén Mach number slightly above 1 are considered. It is found that even weak shock waves trigger magnetic field reconnection in current sheets. Based on this result, it is suggested that drifting chains of type I radio bursts are radio manifestations of the interactions of weakly super-Alfvénic shock waves with pre-existing current sheets distributed in an active region. This model of type I noise storms is then discussed in connection with the concept of nanoflares (localized reconnections) and the heating of the solar corona.     

On the formation of cold fronts in massive mergers

Using adiabatic hydrodynamical simulations, we follow the evolution of two symmetric cold fronts forming in the remnant of a violent   z = 0.3  massive cluster merger. Because the fronts develop after the first passage of the two gas cores of the merging subclusters, and because they soon move ahead of their associated dark matter cores, both the structure and the location of our simulated cold fronts may correspond to a stage that is later than that of most cold fronts observed so far. The cold fronts are preceded by a roughly spherical shock that originates in the centre of the cluster and disappears in the outer regions after 1.6 Gyr. The cold fronts last longer, until   z ∼ 0  . We follow the spatial evolution of the gas of the subcluster cores, and find that a fraction of this gas is liberated in the intracluster medium after core passage, but mainly at apocentre, and that it does not fall back onto the cluster centre. Conversely, we trace back the low-temperature gas constituting the fronts and find that it is initially associated with the two dense cores of the merging clusters. In addition, we find some evidence for discontinuity of the gas velocity field across the edge of the forming cold fronts, suggesting the presence of a contact discontinuity there. In the light of other recent work, we then speculate on the physical mechanism resulting in the cold fronts. We suggest that sloshing induced by strongly varying ram pressure along the subcluster's orbit and/or spatial segregation between the dark matter and gas components of the cores of the subclusters results in strong tidal forces on the gas, and that these forces could be responsible for the deposition of part of the cold dense gas in the surrounding hot intracluster medium. This deposited gas then expands, cools down further, and constitutes the cold fronts.     

Observations of a 2→3 Type Interplanetary Intermediate Shock

An intermediate shock-like event was observed by Voyager 2 on 9 January 1979. The discontinuity structure is well identified to be a 2→3 type intermediate shock by fitting the Rankine – Hugoniot relations. The shock satisfies the following conditions: i) the plasma density increases from the upstream region to the downstream region, ii) The normal Alfvén Mach number (M _AN) is greater than unity in the preshock state and less than unity in the postshock state, iii) The fast-mode Mach numbers in the upstream and downstream regions are less than unity and both the slow-mode Mach numbers are greater than unity, iv) The tangential component of the magnetic field changes sign across the shock front.     

Relics as Probes of Galaxy Cluster Mergers

Galaxy clusters grow by mergers with other clusters and galaxy groups. These mergers create shocks within the intracluster medium (ICM). It is proposed that particles can be accelerated to extreme energies within the shocks. In the presence of a magnetic field these particles should then form large regions emitting synchrotron radiation, creating the so-called radio relics. An example of a cluster with relics is CIZA J2242.8+5301. Here we present hydrodynamical simulations of idealized binary cluster collisions with the aim of constraining the merger scenario for this cluster. We conclude that by using the location, size and width of double radio relics we can set constraints on the mass ratios, impact parameters, time scales, and viewing geometries of binary cluster merger events.     

Reverse shock emission as a probe of gamma-ray burst ejecta

We calculate the reverse shock (RS) synchrotron emission in the optical and the radio wavelength bands from electron–positron pair-enriched gamma-ray burst ejecta with the goal of determining the pair content of gamma-ray bursts (GRBs) using early-time observations. We take into account an extensive number of physical effects that influence radiation from the RS-heated GRB ejecta. We find that optical/infrared flux depends very weakly on the number of pairs in the ejecta, and there is no unique signature of ejecta pair enrichment if observations are confined to a single wavelength band. It may be possible to determine if the number of pairs per proton in the ejecta is ≳100 by using observations in optical and radio bands; the ratio of flux in the optical and radio at the peak of each respective RS light curve is dependent on the number of pairs per proton. We also find that over a large parameter space, RS emission is expected to be very weak; GRB 990123 seems to have been an exceptional burst in that only a very small fraction of the parameter space produces optical flashes this bright. Also, it is often the case that the optical flux from the forward shock is brighter than the RS flux at deceleration. This could be another possible reason for the paucity of prompt optical flashes with a rapidly declining light curve at early times as was seen in GRBs 990123 and 021211. Some of these results are a generalization of similar results reported in Nakar & Piran.     

A Zoo of Radio Relics: Cluster Cores to Filaments

Radio relics in galaxy clusters can be electrons accelerated at cluster merger shocks or adiabatically compressed fossil radio cocoons or dying radio galaxies. The spectral evolution of radio relics is affected by the surrounding thermal plasma. We present a low frequency study of three radio relics representing environments of dense cluster core (A4038), cluster outskirts (A1664) and filaments (A786). The properties of the relics are found to be consistent with the effect of confinement by external medium if the effects of projection are ignored.     

A numerical and experimental study of the time-evolution of a low Mach number jet

We present a comparison between a plasma-generated 'starting jet' experiment and an axisymmetric numerical simulation of the flow. The experimental flow and the numerical simulation give results that agree both qualitatively and quantitatively, showing that the complex vortical structures arising in the flow are surprisingly well reproduced by the numerical model. This result inspires confidence in the accuracy of astrophysical jet numerical simulations. Also, even though the Mach number of our laboratory jet is somewhat low ( M ∼0.5), the dimensionless parameters of this jet are not very far from those expected for Faranoff–Riley class I radio jets.     

The structure of the turbulent shock wave propagating in the solar atmosphere across the magnetic field

A consistent account of plasma turbulence in magnetohydrodynamics equations describing transport processes across the magnetic field is presented. The structure of the perpendicular shock wave generated in the solar atmosphere, as a result of either local disturbance of the magnetic field or dense plasma cloud motion with a frozen-in magnetic field, has been investigated. The region of parameters in the solar atmosphere at which the electron-ion relative drift velocity u exceeds the electron thermal velocity V _eand generation of radio emission becomes possible, has been determined. The plasma turbulence inside the front has been shown, under conditions of solar corona, not to cause the oscillation structure of shock front to break down. Under chromospheric conditions, the shock profile is aperiodical. Then, the condition u > V_ecan be satisfied and shock waves having an Alfvén Mach number M which exceeds the critical value M _c 3.3 for aperiodical shock waves can exist (Eselevich et al., 1971a). Arguments are given in favour of the fact that perpendicular shock waves are generated in the Sun's atmosphere when dense plasma clouds, with a frozen-in magnetic field, are expanded.     

Supersonic motions of galaxies in clusters

We study motions of galaxies in galaxy clusters formed in the concordance Λ cold dark matter cosmology. We use high-resolution cosmological simulations that follow the dynamics of dark matter and gas and include various physical processes critical for galaxy formation: gas cooling, heating and star formation. Analysing the motions of galaxies and the properties of intracluster gas in a sample of eight simulated clusters at z = 0, we study the velocity dispersion profiles of the dark matter, gas and galaxies. We measure the mean velocity of galaxy motions and gas sound speed as a function of radius and calculate the average Mach number of galaxy motions. The simulations show that galaxies, on average, move supersonically with the average Mach number of ≈1.4, approximately independent of the cluster-centric radius. The supersonic motions of galaxies may potentially provide an important source of heating for the intracluster gas by driving weak shocks and via dynamical friction, although these heating processes appear to be inefficient in our simulations. We also find that galaxies move slightly faster than the dark matter particles. The magnitude of the velocity bias,   b _v ≈ 1.1  , is, however, smaller than the bias estimated for subhaloes in dissipationless simulations. Interestingly, we find velocity bias in the tangential component of the velocity dispersion, but not in the radial component. Finally, we find significant random bulk motions of gas. The typical gas velocities are of order ≈20–30 per cent of the gas sound speed. These random motions provide about 10 per cent of the total pressure support in our simulated clusters. The non-thermal pressure support, if neglected, will bias measurements of the total mass in the hydrostatic analyses of the X-ray cluster observations.     

The properties of powerful radio sources at 90 GHz

We have observed a small sample of powerful double radio sources (radio galaxies and quasars) at frequencies around 90 GHz with the Berkeley Illinois Maryland Association (BIMA) millimetre array, with the intention of constraining the resolved high-frequency spectra of radio galaxies. When combined with other sources we have previously observed and with data from the BIMA archive, these observations allow us for the first time to make general statements about the high-frequency behaviour of compact components of radio galaxies – cores, jets and hotspots. We find that cores in our sample remain flat-spectrum up to 90 GHz; jets in some of our targets are detected at 90 GHz for the first time in our new observations and hotspots are found to be almost universal, but show a wide range of spectral properties. Emission from the extended lobes of radio galaxies is detected in a few cases and shows rough consistency with the expectations from standard spectral ageing models, though our ability to probe this in detail is limited by the sensitivity of BIMA. We briefly discuss the prospects for radio galaxy astrophysics with Atacama Large Millimeter Array.