Torsten A. Ensslin

Radio observational constraints on Galactic 3D-emission models

Context. Our position inside the Galaxy requires 3D-modelling to obtain the distribution of the Galactic magnetic field, cosmic- ray (CR) electrons and thermal electrons. Aims. Our intention is to find a Galactic 3D-model which agrees best with available radio observations. Methods. We constrain simulated all-sky maps in total intensity, lin ear polarization, and rotation measure (RM) by observations. For the simulated maps as a function of frequency we integrate in 15 ′ wide cones the emission along the line of sight calculated from Galactic 3D-models. We test a number of large-scale magnetic field configurations and take the properties of the warm inte rstellar medium into account. Results. From a comparison of simulated and observed maps we are able to constrain the regular large-scale Galactic magnetic field in the disk and the halo of the Galaxy. The local regular fi eld is 2� G and the average random field is about 3 � G. The known local excess of synchrotron emission originating either fr om enhanced CR electrons or random magnetic fields is able to e xplain the observed high-latitude synchrotron emission. The thermal electron model (NE2001) in conjunction with a proper fill ing factor accounts for the observed optically thin thermal emission and low frequency absorption by optically thick emission. A coupling factor between thermal electrons and the random magnetic field comp onent is proposed, which in addition to the small filling fact or of thermal electrons increases small-scale RM fluctuations an d thus accounts for the observed depolarization at 1.4 GHz. Conclusions. We conclude that an axisymmetric magnetic disk field configur ation with reversals inside the solar circle fits available observations best. Out of the plane a strong toroidal magnetic field with di fferent signs above and below the plane is needed to account for the observed high-latitude RMs. The large field strength is a consequence of the small thermal electron scale height of 1 kpc, which also limits the CR electron extent up to a height of 1 kpc not to contradict with the observed synchrotron emission out of the plane. Our preferred 3D-model fits the observed Galactic total intensi ty and polarized emission better than other models over a wide frequency range and also agrees with the observed RM from extragalactic sources.

Cosmic-ray protons and magnetic fields in clusters of galaxies and their cosmological consequences

The masses of clusters of galaxies estimated by gravitational lensing exceed in many cases the mass estimates based on hydrostatic equilibrium. This may suggest the existence of nonthermal pressure. We ask if radio galaxies can heat and support the cluster gas with injected cosmic-ray protons and magnetic field densities, which are permitted by Faraday rotation and gamma-ray observations of clusters of galaxies. We conclude that they are powerful enough to do this within a cluster radius of roughly 1 Mpc. If present, nonthermal pressures could lead to a revised estimate of the ratio of baryonic mass to total mass, and the apparent baryonic overdensity in clusters would disappear. In consequence, Ωcold, the clumping part of the cosmological density Ω0, would be larger than 0.4 h -->−1/250.

On the formation of cluster radio relics

ABSTRA C T 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.

Deep 1.4 GHz very large array observations of the radio halo and relic in Abell 2256

We present deep Very Large Array observations of the merging galaxy cluster A2256. This cluster is known to possess diffuse steep-spectrum radio relic emission in the peripheral regions. Our new observations provide the first detailed image of the central diffuse radio halo emission in this cluster. The radio halo extends over more than 800 kpc in the cluster core, while the relic emission covers a region of ~1125 kpc × 520 kpc. A spectral index map of the radio relic shows a spectral steepening from the northwest toward the southeast edge of the emission, with an average spectral index between 1369 and 1703 MHz of α = -1.2 across the relic. Polarization maps reveal high fractional polarization of up to 45% in the relic region with an average polarization of 20% across the relic region. The observed Faraday rotation measure is consistent with the Galactic estimate, and the dispersion in the rotation measure is small, suggesting that there is very little contribution to the rotation measure of the relic from the intracluster medium. We use these Faraday properties of the relic to argue that it is located on the front side of the cluster.

Radio observations of ZwCl 2341.1+0000: a double radio relic cluster

Context. Hierarchal models of large-scale structure (LSS) formation predict that galaxy clusters grow via gravitational infall and mergers of smaller subclusters and galaxy groups. Diffuse radio emission, in the form of radio halos and relics, is found in clusters undergoing a merger, indicating that shocks or turbulence associated with the merger are capable of accelerating electrons to highly relativistic energies. Double relics are a rare class of radio sources found in the periphery of clusters, with the two components located symmetrically on the opposite sides of the cluster center. These relics are important probes of the cluster periphery as (i) they provide an estimate of the magnetic field strength, and (ii) together with detailed modeling can be used to derive information about the merger geometry, mass, and timescale. Observations of these double relics can thus be used to test the framework of LSS formation. Here we report on radio observations of ZwCl 2341.1+0000, a complex merging structure of galaxies located at z = 0.27, using Giant Metrewave Radio Telescope (GMRT) observations. Aims. The main aim of the observations is to study the nature of the diffuse radio emission in the galaxy cluster ZwCl 2341.1+0000. Methods. We carried out GMRT 610, 241, and 157 MHz continuum observations of ZwCl 2341.1+0000. The radio observations are combined with X-ray and optical data of the cluster. Results. The GMRT observations show a double peripheral radio relic in the cluster ZwCl 2341.1+0000. The spectral index is −0.49 ± 0.18 for the northern relic and −0.76 ± 0.17 for the southern relic. We have derived values of 0.48−0.93 μGauss for the equipartition magnetic field strength. The relics are probably associated with outward traveling merger shock waves.

The radio luminosity function of cluster radio halos

A significant fraction of galaxy clusters exhibits cluster-wide radio halos. We give a simple prediction of the local and higher redshift radio halo luminosity function (RHLF) on the basis of (i) an observed and a theoretical X-ray cluster luminosity function (XCLF) (ii) the observed radio-X-ray luminosity correlation (RXLC) of galaxy clusters with radio halos (iii) an assumed fraction of frh ≈ 1 galaxy clusters to have radio halos as supported by observations. We then find 300-700 radio halos with S 1.4 GHz > 1mJy, and 10 5 -10 6 radio halos with S 1.4 GHz > 1 µJy should be visible on the sky. 14% of the S 1.4 GHz > 1mJy and 56% of the S 1.4 GHz > 1 µJy halos are located at z > 0.3. Subsequently, we give more realisticpredictions taking into account (iv) a refined estimate of the radio halo fraction as a function of redshift and cluster mass, and (v) a decrease in intrinsic radio halo luminosity with redshift due to increased inverse Compton electron energy losses on the Cosmic Microwave Background (CMB). We find that this reduces the radio halo counts from the simple prediction by only 30 % totally, but the high redshift (z > 0.3) counts are more strongly reduced by 50-70%. These calculations show that the new generation of sensitive radio telescopes, including LOFAR, ATA, EVLA, SKA and the already-operating GMRT should be able to detect large numbers of radio halos and will provide unique information for studies of galaxy cluster merger rates and associated non-thermal processes.

Cosmic cartography of the large-scale structure with sloan digital sky survey data release 6

We present the largest Wiener reconstruction of the cosmic density field made to date. The reconstruction is based on the Sloan Digital Sky Survey (SDSS) data release 6 covering the northern Galactic cap. We use a novel supersampling algorithm to suppress aliasing effects and a Krylov-space inversion method to enable high performance with high resolution. These techniques are implemented in the argo computer code. We reconstruct the field over a 500 Mpc cube with Mpc grid resolution while accounting for both the angular and the radial selection functions of the SDSS, and the shot noise giving an effective resolution of the order of similar to 10 Mpc. In addition, we correct for the redshift distortions in the linear and non-linear regimes in an approximate way. We show that the commonly used method of inverse weighting the galaxies by the corresponding selection function heads to excess noise in regions where the density of the observed galaxies is small. It is more accurate and conservative to adopt a Bayesian framework in which we model the galaxy selection/detection process to be Poisson binomial. This results in heavier smoothing in regions of reduced sampling density. Our results show a complex cosmic web structure with huge void regions indicating that the recovered matter distribution is highly non-Gaussian. Filamentary structures are clearly visible on scales of up to similar to 20 Mpc. We also calculate the statistical distribution of density after smoothing the reconstruction with Gaussian kernels of different radii r(S) and find good agreement with a lognormal distribution for 10 Mpc less than or similar to r(S) less than or similar to 30 Mpc.

Ultrahigh energy cosmic rays in a structured and magnetized universe

We simulate propagation of cosmic ray nucleons above 10^{19} eV in scenarios where both the source distribution and magnetic fields within about 50 Mpc from us are obtained from an unconstrained large scale structure simulation. We find that consistency of predicted sky distributions with current data above 4 x 10^{19} eV requires magnetic fields of ~0.1 microGauss in our immediate environment, and a nearby source density of ~10^{-4}-10^{-3} Mpc^{-3}. Radio galaxies could provide the required sources, but only if both high and low-luminosity radio galaxies are very efficient cosmic ray accelerators. Moreover, at ~10^{19} eV an additional isotropic flux component, presumably of cosmological origin, should dominate over the local flux component by about a factor three in order to explain the observed isotropy. This argues against the scenario in which local astrophysical sources of cosmic rays above ~10^{19} eV reside in strongly magnetized (B~0.1 microGauss) and structured intergalactic medium. Finally we discuss how future large scale full-sky detectors such as the Pierre Auger project will allow to put much more stringent constraints on source and magnetic field distributions.

Constraining models of the large scale Galactic magnetic field with WMAP5 polarization data and extragalactic rotation measure sources

We introduce a method to quantify the quality-of-fit between data and observables depending on the large scale Galactic magnetic field. We combine WMAP5 polarized synchrotron data and rotation measures of extragalactic sources in a joint analysis to obtain best fit parameters and confidence levels for GMF models common in the literature. None of the existing models provide a good fit in both the disk and halo regions, and in many instances best-fit parameters are quite different than the original values. We note that probing a very large parameter space is necessary to avoid false likelihood maxima. The thermal and relativistic electron densities are critical for determining the GMF from the observables but they are not well constrained. We show that some characteristics of the electron densities can already be constrained using our method and with future data it may be possible to carry out a self-consistent analysis in which models of the GMF and electron densities are simultaneously optimized.

Probing the cosmic ray population of the giant elliptical galaxy M 87 with observed TeV

We examine the cosmic ray proton (CRp) population within the giant elliptical galaxy M 87 using the TeV γ-ray detection of the HEGRA collaboration. In our scenario the γ-rays are produced by decaying pious which result from hadronic CRp interactions with thermal gas of the interstellar medium of M 87. By comparing the γ-ray emission to upper limits from EGRET, we constrain the spectral index of the CRp population to α T e V GeV < 2.275 within our scenario. Both the expected radial γ-ray profile and the required amount of CRp support this hadronic scenario. The accompanying radio mini-halo of hadronically originating cosmic ray electrons is outshone by the synchrotron emission of the relativistic jet of M 87 by one order of magnitude. According to our predictions, the future GLAST mission should allow us to test this hadronic scenario.