Showing papers by "Klaus Dolag published in 2005"

Constrained simulations of the magnetic field in the local Universe and the propagation of ultrahigh energy cosmic rays

Abstract: We use simulations of large-scale structure formation to study the build-up of magnetic fields (MFs) in the intergalactic medium. Our basic assumption is that cosmological MFs grow in a magnetohydrodynamical (MHD) amplification process driven by structure formation out of a magnetic seed field present at high redshift. This approach is motivated by previous simulations of the MFs in galaxy clusters which, under the same hypothesis that we adopt here, succeeded in reproducing Faraday rotation measurements (RMs) in clusters of galaxies. OurCDM initial conditions for the dark matter density fluctuations have been statistically constrained by the observed large-scale density field within a sphere of 110 Mpc around the Milky Way, based on the IRAS 1.2-Jy all-sky redshift survey. As a result, the positions and masses of prominent galaxy clusters in our simulation coincide closely with their real counterparts in the Local Universe. We find excellent agreement between RMs of our simulated galaxy clusters and observational data. The improved numerical resolution of our simulations compared to previous work also allows us to study the MF in large-scale filaments, sheets and voids. By tracing the propagation of ultra high energy (UHE) protons in the simulated MF we construct full-sky maps of expected deflection angles of protons with arrival energies E = 10 20 eV and 4 × 10 19 eV, respectively. Accounting only for the structures within 110 Mpc, we find that strong deflections are only produced if UHE protons cross galaxy clusters. The total area on the sky covered by these structures is however very small. Over still larger distances, multiple crossings of sheets and filaments may give rise to noticeable deflections over a significant fraction of the sky; the exact amount and angular distribution depends on the model adopted for the magnetic seed field. Based on our results we argue that over a large fraction of the sky the deflections are likely to remain smaller than the present experimental angular sensitivity. Therefore, we conclude that forthcoming air shower experiments should be able to locate sources of UHE protons and shed more light on the nature of cosmological MFs.

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

Abstract: 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 gas motions in galaxy cluster simulations: The role of SPH viscosity

Abstract: Smoothed particle hydrodynamics (SPH) employs an artificial viscosity to properly capture hydrodynamical 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 (Monaghan 1997). 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 1Mpc cube) of up to 5%-30% of the thermal energy content, depending on cluster mass. This has also significant effects on radial gas profile. We find a central flattening of the entropy profile and a reduction of the central gas density in the low--viscosity scheme. 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 MHD waves driven by turbulence, we find efficient electron acceleration to power diffuse radio emission.

Mismatch between X-Ray and Emission-weighted Temperatures in Galaxy Clusters: Cosmological Implications

Abstract: The thermal properties of hydrodynamical simulations of galaxy clusters are usually compared to observations by relying on the emission-weighted temperature Tew instead of on the spectroscopic X-ray temperature Tspec, which is obtained by actual observational data. In a recent paper, Mazzotta et al. show that if the intracluster medium is thermally complex, Tew fails at reproducing Tspec. They propose a new formula, the spectroscopic-like temperature, Tsl, which approximates Tspec better than a few percent. By analyzing a set of hydrodynamical simulations of galaxy clusters, we find that Tsl is lower than Tew by 20%-30%. As a consequence, the normalization of the M-Tsl relation from the simulations is larger than the observed one by about 50%. If masses in simulated clusters are estimated by following the same assumptions of hydrostatic equilibrium and β-model gas density profile, as is often done for observed clusters, then the M-T relation decreases by about 40% and significantly reduces its scatter. On the basis of this result, we conclude that using the observed M-T relation to infer the amplitude of the power spectrum from the X-ray temperature function could bias low σ8 by 10%-20%. This may alleviate the tension between the value of σ8 inferred from the cluster number density and those from the cosmic microwave background and large-scale structure.

The imprints of local superclusters on the Sunyaev-Zel'dovich signals and their detectability with Planck

Abstract: We use high-resolution hydrodynamical simulations of large-scale structure formation to study the imprints of the local superclusters on to the full-sky Sunyaev–Zel'dovich (SZ) signals. Following Mathis et al., the initial conditions have been statistically constrained to reproduce the density field within a sphere of 110 Mpc around the Milky Way, as observed in the IRAS 1.2-Jy all-sky redshift survey. As a result, the positions and masses of prominent galaxy clusters and superclusters in our simulations coincide closely with their real counterparts in the local Universe. We present the results of two different runs: one with adiabatic gas physics only, and one also including cooling, star formation and feedback. By analysing the full-sky maps for the thermal and kinetic SZ signals extracted from these simulations, we find that for multipoles with l < 100 the power spectrum is dominated by the prominent local superclusters, and its amplitude at these scales is a factor of 2 higher than that obtained from unconstrained simulations; at lower multipoles (l < 20) this factor can even reach one order of magnitude. We check the influence of the SZ effect from local superclusters on the cosmic microwave background (CMB) power spectrum at small multipoles and find it negligible and with no signs of quadrupole–octopole alignment. However, performing simulations of the CMB radiation including the experimental noise at the frequencies which will be observed by the Planck satellite, we find results suggesting that an estimate of the SZ power spectrum at large scales can be extracted.

A simulation pipeline for the Planck mission

Abstract: We describe an assembly of numerical tools to model the output data of the Planck satellite. These start with the generation of a CMB sky in a chosen cosmology, add in various foreground sources, convolve the sky signal with arbitrary, even non-symmetric and polarised beam patterns, derive the time ordered data streams measured by the detectors depending on the chosen satellite-scanning strategy, and include noise signals for the individual detectors and electronic systems. The simulation products are needed to develop, verify, optimise, and characterise the accuracy and performance of all data processing and scientific analysis steps of the Planck mission, including data handling, data integrity checking, calibration, map making, physical component separation, and power spectrum estimation. In addition, the simulations allow detailed studies of the impact of many stochastic and systematic effects on the scientific results. The efficient implementation of the simulation allows the build-up of extended statistics of signal variances and co-variances. Although being developed specifically for the Planck mission, it is expected that the employed framework as well as most of the simulation tools will be of use for other experiments and CMB-related science in general.

Hot and Cooled baryons in SPH simulations of galaxy clusters: physics and numerics

Abstract: We discuss an extended set of Tree+SPH simulations of galaxy clusters, with the goal of investigating the interplay between numerical resolution effects and star-formation/feedback processes. The simulated clusters span the mass range (0.1-2.3) 10^{15}Msun/h, with mass resolution varying by several decades. At the highest achieved resolution, we resolve the virial region of a Virgo-like cluster with more than 2 million gas particles and with at least as many dark-matter (DM) particles. Our resolution study confirms that, in the absence of an efficient feedback mechanism, runaway cooling leads to about 35 per cent of baryons in clusters to be locked up in long lived stars at our highest resolution, with no evidence of convergence. However, including feedback causes the fraction of cooled baryons to converge at about 15 per cent already at modest resolution. Feedback also stabilizes other gas-related quantities, such as radial profiles of entropy, gas density and temperature, against variations due to changes in resolution. We also investigate the influence of the gravitational force softening length, and that of numerical heating of the gas induced by two-body encounters between DM and lighter gas particles. We show that simulations where more DM than gas particles are used, show a significantly enhanced efficiency of star formation at z>3. Our results are important for establishing and delineating the regime of numerical reliability of the present generation of hydrodynamical simulations of galaxy clusters.

Measuring cluster peculiar velocities with the Sunyaev-Zel'dovich effect: scaling relations and systematics

Abstract: The fluctuations in the cosmic microwave background (CMB) intensity due to the Sunyaev-Zel'dovich (SZ) effect are the sum of a thermal and a kinetic contribution. Separating the two components to measure the peculiar velocity of galaxy clusters requires radio and microwave observations at three or more frequencies, and knowledge of the temperature T e of the intracluster medium (ICM) weighted by the electron number density. To quantify the systematics of this procedure, we extract a sample of 117 massive clusters at redshift z = 0 from an N-body hydrodynamical simulation, with 2 x 480 3 particles, of a cosmological volume 192 h -1 Mpc on a side of a flat cold dark matter model with Ω 0 = 0.3 and Ω Λ = 0.7. Our simulation includes radiative cooling, star formation and the effect of feedback and galactic winds from supernovae. We find that: (i) our simulated clusters reproduce the observed scaling relations between X-ray and SZ properties; (ii) bulk flows internal to the ICM affect the velocity estimate by less than 200 km s -1 in 93 per cent of the cases; (iii) using the X-ray emission weighted temperature, as an estimate of T e , can overestimate the peculiar velocity by 20-50 per cent, if the microwave observations do not spatially resolve the cluster. For spatially resolved clusters, the assumptions on the spatial distribution of the ICM, required to separate the two SZ components, still produce a velocity overestimate of 10-20 per cent, even with an unbiased measure of T e . Thanks to the large size of our cluster samples, these results set a robust lower limit of ∼200 km s -1 to the systematic errors that will affect upcoming measures of cluster peculiar velocities with the SZ effect.

Strong lensing efficiency of galaxy clusters in dark energy cosmologies

Abstract: We study the efficiency for producing strong gravitational lensing events of galaxy clusters numerically simulated in different dark-energy cosmologies with constant and time-variable equation of state, and we compare the results with those obtained in standard ACDM and OCDM models. Our main results are (1) that the expected abundance of gravitational arcs with large length-to-width ratio depends on the equation of state of dark energy at the epoch of formation of the halo; and (2) that the high sensitivity of strong-lensing cross sections of galaxy clusters to dynamical processes like mergers, which was found in earlier studies, varies substantially between different cosmologies, being stronger for models in which halos are less concentrated. As expected, the largest differences in the lensing optical depth occur at intermediate and high redshift.

pacerman — I. A new algorithm to calculate Faraday rotation maps

Abstract: We propose a new method to calculate Faraday rotation measure maps from multifrequency polarization angle data. In order to solve the so-called nπ-ambiguity problem which arises from the observational ambiguity of the polarization angle which is only determined up to additions of ±nπ, where n is an integer, we suggest using a global scheme. Instead of solving the nπ-ambiguity for each data point independently, our algorithm, which we chose to call PACERMAN (Polarization Angle CorrEcting Rotation Measure ANalysis), solves the nπ-ambiguity for a high signal-to-noise region ‘democratically’ and uses this information to assist computations in adjacent low signal-to-noise areas. Ke yw ords: galaxies: clusters: general ‐ intergalactic medium.

A full-sky prediction of the Sunyaev-Zeldovich effect from diffuse hot gas in the local universe and the upper limit from the WMAP data

Abstract: We use the Point Source Catalogue Redshift Survey galaxy redshift catalogue combined with constrained simulations based on the IRAS 1.2-Jy galaxy density field to estimate the contribution of hot gas in the local universe to the Sunyaev‐Zeldovich (SZ) effect on a large scale. We produce a full-sky HEALPIX map predicting the SZ effect from clusters as well as diffuse hot gas within 80 h −1 Mpc. Performing cross-correlation tests between this map and the WMAP data in pixel, harmonic and wavelet space we can put an upper limit on the effect. We conclude that the SZ effect from diffuse gas in the local universe cannot be detected in current cosmic microwave background (CMB) data and is not a large-scale contaminating factor ( �< 60) in studies of CMB angular anisotropies. We derive an upper limit for the mean temperature

A full sky prediction of the SZ effect from diffuse hot gas in the local universe and the upper limit from the WMAP data

Abstract: We use the PSCz galaxy redshift catalogue combined with constrained simulations based on the IRAS 1.2 Jy galaxy density field to estimate the contribution of hot gas in the local universe to the SZ-effect on large scales. We produce a full sky Healpix map predicting the SZ-effect from clusters as well as diffuse hot gas within 80Mpc/h. Performing cross-correlation tests between this map and the WMAP data in pixel, harmonic and wavelet space we can put an upper limit on the effect. We conclude that the SZ effect from diffuse gas in the local universe cannot be detected in current CMB data and is not a contaminating factor on large scales (l<60) in studies of the CMB angular anisotropies. However, for future high sensitivity experiments observing at a wider range of frequencies, the predicted large scale SZ effect could be of importance.

pacerman– II. Application and statistical characterization of improved RM maps

Abstract: We have proposed a new method - PACERMAN - to calculate Faraday rotation measure (RM) maps from multifrequency polarization angle data in order to avoid the so-called nπ-ambiguity. Here, we apply our PACERMAN algorithm to two polarization data sets of extended radio sources in the Abell 2255 and the Hydra A cluster, and compare the RM maps obtained using PACERMAN to RM maps obtained employing already existing methods. Thereby, we provide a new high-quality RM map of the Hydra north lobe which is in good agreement with the existing one but find significant differences in the case of the south lobe of Hydra A. We demonstrate the reliability and the robustness of PACERMAN. In order to study the influence of map-making artefacts, which are imprinted by wrong solutions to the nπ-ambiguities, and of the error treatment of the data, we calculated and compared magnetic field power spectra from various RM maps. The power spectra were derived using the method recently proposed by EnBlin & Vogt. We demonstrate the sensitivity of statistical analysis to artefacts and noise in the RM maps and thus we demonstrate the importance of an unambiguous determination of RM maps and an understanding of the nature of the noise in the data. We introduce and perform statistical tests to estimate the quality of the derived RM maps, which demonstrate the quality improvements due to PACERMAN.

Angular diameter distance estimates from the Sunyaev-Zel'dovich effect in hydrodynamical cluster simulations

Abstract: The angular-diameter distance D_A of a galaxy cluster can be measuread by combining its X-ray emission with the cosmic microwave background fluctution due to the Sunyaev-Zeldovich effect. The application of this distance indicator usually assumes that the cluster is spherically symmetric, the gas is distributed according to the isothermal beta-model, and the X-ray temperature is an unbiased measure of the electron temperature. We test these assumptions with galaxy clusters extracted from an extended set of cosmological N-body/hydrodynamical simulations of a LCDM concordance cosmology, which include the effect of radiative cooling, star formation and energy feedback from supernovae. We find that, due to the steep temperature gradients which are present in the central regions of simulated clusters, the assumption of isothermal gas leads to a significant underestimate of D_A. This bias is efficiently corrected by using the polytropic version of the beta-model to account for the presence of temperature gradients. In this case, once irregular clusters are removed, the correct value of D_A is recovered with a ~ 5 per cent accuracy on average, with a ~ 20 per cent intrinsic scatter due to cluster asphericity. This result is valid when using either the electron temperature or a spectroscopic-like temperature. When using instead the emission-weighted definition for the temperature of the simulated clusters, D_A is biased low by \~ 20 per cent. We discuss the implications of our results for an accurate determination of the Hubble constant H_0 and of the density parameter Omega_m. We find that H_0 can be potentially recovered with exquisite precision, while the resulting estimate of Omega_m, which is unbiased, has typical errors Delta(Omega_m) ~ 0.05.

Simulating the magnetic field in the local supercluster

Abstract: We construct the rst realistic map of deections of ultra-high energy cosmic rays by extragalactic magnetic elds, using a magneto-hydrodynamical simulation of cosmic structure formation that reproduces the positions of known galaxy clusters in the local universe. Large deection angles occur in cluster regions, which however cover only an insignicant fraction of the sky. More typical deections of order 1 are caused by crossings of laments. For protons with energies 4 10 eV, deections do not exceed a few degrees over most of the sky up to a propagation distance of 500 Mpc. Given that the eld strength of our simulated intergalactic magnetic eld forms a plausible upper limit, we conclude that charged particle astronomy is in principle possible.

The imprints of local superclusters on the Sunyaev-Zel'dovich signals and their detectability with Planck

Abstract: We use high-resolution hydrodynamical simulations of large-scale structure formation to study the imprints of the local superclusters onto the full-sky Sunyaev-Zel'dovich (SZ) signals. Following (Mathis et al. 2002), the initial conditions have been statistically constrained to reproduce the density field within a sphere of 110 Mpc around the Milky Way, as observed in the IRAS 1.2-Jy all-sky redshift survey. As a result, the positions and masses of prominent galaxy clusters and superclusters in our simulations coincide closely with their real counterparts in the local universe. We present the results of two different runs, one with adiabatic gas physics only, and one also including cooling, star formation and feedback. By analysing the full-sky maps for the thermal and kinetic SZ signals extracted from these simulations, we find that for multipoles with l<100 the power spectrum is dominated by the prominent local superclusters, and its amplitude at these scales is a factor of two higher than that obtained from unconstrained simulations; at lower multipoles (l<20) this factor can even reach one order of magnitude. We check the influence of the SZ effect from local superclusters on the cosmic microwave background (CMB) power spectrum at small multipoles and find it negligible and with no signs of quadrupole-octopole alignment. However, performing simulations of the CMB radiation including the experimental noise at the frequencies which will be observed by the Planck satellite, we find results suggesting that an estimate of the SZ power spectrum at large scales can be extracted.