Showing papers by "Klaus Dolag published in 2007"

Cosmic rays and radio halos in galaxy clusters: new constraints from radio observations

Abstract: Clusters of galaxies are sites of acceleration of charged particles and sources of nonthermal radiation. We report on new constraints on the population of cosmic rays in the intracluster medium (ICM) obtained via radio observations of a fairly large sample of massive, X-ray-luminous galaxy clusters in the redshift interval 0.2-0.4. The bulk of the observed galaxy clusters does not show any hint of megaparsec-scale synchrotron radio emission at the cluster center (radio halo). We obtained solid upper limits to the diffuse radio emission and discuss their implications for the models for the origin of radio halos. Our measurements allow us to also derive a limit to the content of cosmic-ray protons in the ICM. Assuming spectral indices of these protons ? = 2.1-2.4 and ?G level magnetic fields, as from rotation measures, these limits are 1 order of magnitude deeper than present EGRET upper limits, while they are less stringent for steeper spectra and lower magnetic fields.

Simulating cosmic rays in clusters of galaxies – I. Effects on the Sunyaev–Zel'dovich effect and the X-ray emission

Abstract: We performed high-resolution simulations of a sample of 14 galaxy clusters that span a mass range from 5 × 10 13 to 2 × 10 15 h −1 Mto study the effects of cosmic rays (CRs) on thermal cluster observables such as X-ray emission and the Sunyaev-Zel'dovich effect. We analyse the CR effects on the intra-cluster medium while simultaneously taking into account the cluster's dynamical state as well as the mass of the cluster. The modelling of the CR physics includes adiabatic CR transport processes, injection by supernovae and cosmological structure formation shocks, as well as CR thermalization by the Coulomb interaction and catastrophic losses by hadronic interactions. While the relative pressure contained in CRs within the virial radius is of the order of 2 per cent in our non-radiative simulations, their contribution rises to 32 per cent in our simulations with dissipative gas physics including radiative cooling, star formation and supernova feedback. The relative CR pressure rises towards the outer regions due to a combination of the following effects: CR acceleration is more efficient at the peripheral strong accretion shocks compared to weak central flow shocks, adiabatic compression of a composite of CRs and thermal gas disfavours the CR pressure relative to the thermal pressure due to the softer equation of state of CRs and CR loss processes are more important at the dense centres. Interestingly, in the radiative simulations the relative CR pressure reaches high values of the order of equipartition with the thermal gas in each cluster galaxy due to the fast thermal cooling of gas which diminishes the thermal pressure support relative to that in CRs. This also leads to a lower effective adiabatic index of the composite gas that increases the compressibility of the intra-cluster medium. This effect slightly increases the central density, thermal pressure and the gas fraction. While the X-ray luminosity in low-mass cool core clusters is boosted by up to 40 per cent, the integrated Sunyaev-Zel'dovich effect appears to be remarkably robust and the total flux decrement only slightly reduced by typically 2 per cent. The resolved Sunyaev-Zel'dovich maps, however, show a larger variation with an increased central flux decrement.

New scaling relations in cluster radio halos and the re-acceleration model

Abstract: In this paper we derive new expected scaling relations for clusters with giant radio halos in the framework of the re-acceleration scenario in a simplified, but physically motivated, form, namely: radio power (P_R) vs size of the radio emitting region (R_H), and P_R vs total cluster mass (M_H) contained in the emitting region and cluster velocity dispersion (sigma_H) in this region. We search for these correlations by analyzing the most recent radio and X-ray data available in the literature for a well known sample of clusters with giant radio halos. In particular we find a good correlation between P_R and R_H and a very tight ``geometrical'' scaling between M_H and R_H. From these correlations P_R is also expected to scale with M_H and sigma_H and this is confirmed by our analysis. We show that all the observed trends can be well reconciled with expectations in the case of a slight variation of the mean magnetic field strength in the radio halo volume with M_H. A byproduct correlation between R_H and sigma_H is also found, and can be further tested by optical studies. In addition, we find that observationally R_H scales non-linearly with the virial radius of the host cluster, and this immediately means that the fraction of the cluster volume which is radio emitting increases with cluster mass and thus that the non-thermal component in clusters is not self-similar.

New scaling relations in cluster radio haloes and the re‐acceleration model

Abstract: In this paper we derive new expected scaling relations for clusters with giant radio haloes in the framework of the re-acceleration scenario in a simplified, but physically motivated, form, namely: radio power (P R ) versus size of the radio emitting region (R H ), and P R versus total cluster mass (M H ) contained in the emitting region and cluster velocity dispersion (σ H ) in this region. We search for these correlations by analysing the most recent radio and X-ray data available in the literature for a well-known sample of clusters with giant radio haloes. In particular we find a good correlation between P R and R H and a very tight 'geometrical' scaling between M H and R H . From these correlations P R is also expected to scale with M H and σ H and this is confirmed by our analysis. We show that all the observed trends can be well reconciled with expectations in the case of a slight variation of the mean magnetic field strength in the radio halo volume with M H . A byproduct correlation between R H and σ H is also found, and can be further tested by optical studies. In addition, we find that observationally R H scales non-linearly with the virial radius of the host cluster, and this immediately means that the fraction of the cluster volume which is radio emitting increases with cluster mass and thus that the non-thermal component in clusters is not self-similar.

Realistic simulations of gravitational lensing by galaxy clusters: extracting arc parameters from mock DUNE images

Abstract: We present a newly developed code that allows simulations of optical observations of galaxy fields with a variety of instruments. The code incorporates gravitational lensing effects and is targetted at simulating lensing by galaxy clusters. Our goal is to create the tools required for comparing theoretical expectations with observations to obtain a better understanding of how observational noise affects lensing applications such as mass estimates, studies on the internal properties of galaxy clusters and arc statistics. Starting from a set of input parameters, characterizing both the instruments and the observational conditions, the simulator provides a virtual observation of a patch of the sky. It includes several sources of noise such as photon-noise, sky background, seeing, and instrumental noise. Ray-tracing through simulated mass distributions accounts for gravitational lensing. Source morphologies are realistically simulated based on shapelet decompositions of galaxy images retrieved from the GOODS-ACS archive. According to their morphological class, spectral-energy-distributions are assigned to the source galaxies in order to reproduce observations of each galaxy in arbitrary photometric bands. We illustrate our techniques showing virtual observations of a galaxy-cluster core as it would be observed with the space telescope DUNE, which was recently proposed to ESA within its "Cosmic vision" programme. (Abridged)

Radial profile and lognormal fluctuations of the intracluster medium as the origin of systematic bias in spectroscopic temperature

Abstract: The origin of the recently reported systematic bias in the spectroscopic temperature of galaxy clusters is investigated using a cosmological hydrodynamic simulation. We find that the local inhomogeneities of the gas temperature and density, after being corrected for their global radial profiles, have a nearly universal distribution that resembles a lognormal function. Based on this lognormal approximation for the fluctuations in the intracluster medium, we develop an analytical model that explains the bias in the spectroscopic temperature. We conclude that the multiphase nature of the intracluster medium, due not only to the radial profiles but also to the local inhomogeneities, plays an essential role in producing the systematic bias.

The importance of merging activity for the kinetic polarization of the Sunyaev-Zel'dovich signal from galaxy clusters

Abstract: Context. The polarization sensitivity of the upcoming millimetric observatories will open new possibilities for studying the p roperties of galaxy clusters and for using them as powerful cosmological probes. For this reason it is necessary to investigate in det ail the characteristics of the polarization signals produced by their highly ionize d intra-cluster medium (ICM). This work is focussed on the polarization effect induced by the ICM bulk motions, the so-called kpSZ signal, which has an amplitude proportional to the optical depth and to the square of the tangential velocity. Aims. In particular we study how this polarization signal is affected by the internal dynamics of galaxy clusters and what is its dependence on the physical modelling adopted to describe the baryonic component. Methods. This is done by producing realistic kpSZ maps starting from the outputs of two different sets of high-resolution hydrodynamical N-body simulations. The first set (17 objects) follows only n on-radiative hydrodynamics, while for each of 9 objects of the second set we implement four different kinds of physical processes. Results. Our results shows that the kpSZ signal turns out to be a very sensitive probe of the dynamical status of galaxy clusters. We find that major merger events can amplify the signal up to one order of magnitude with respect to relaxed clusters, reaching amplit ude up to about 100 nK. This result implies that the internal ICM dynamics must be taken into account when evaluating this signal because simplicistic models, based on spherical rigid bodies, may provide wrong estimates. In particular, the selection of enough relaxed clusters s eems to be fundamental to obtain a robust measurement of the intrinsic quadrupole of the cosmic microwave background through polarization. Finally we find that the dependence on the physical modelling of the baryonic component is relevant only in the very inner regions of clusters.

The importance of the merging activity for the kinetic polarization of the Sunyaev-Zel'dovich signal from galaxy clusters

Abstract: The polarization sensitivity of the upcoming millimetric observatories will open new possibilities for studying the properties of galaxy clusters and for using them as powerful cosmological probes. For this reason it is necessary to investigate in detail the characteristics of the polarization signals produced by their highly ionized intra-cluster medium (ICM). This work is focussed on the polarization effect induced by the ICM bulk motions, the so-called kpSZ signal, which has an amplitude proportional to the optical depth and to the square of the tangential velocity. In particular we study how this polarization signal is affected by the internal dynamics of galaxy clusters and what is its dependence on the physical modelling adopted to describe the baryonic component. This is done by producing realistic kpSZ maps starting from the outputs of two different sets of high-resolution hydrodynamical N-body simulations. The first set (17 objects) follows only non-radiative hydrodynamics, while for each of 9 objects of the second set we implement four different kinds of physical processes. Our results shows that the kpSZ signal turns out to be a very sensitive probe of the dynamical status of galaxy clusters. We find that major merger events can amplify the signal up to one order of magnitude with respect to relaxed clusters, reaching amplitude up to about 100 nuK. This result implies that the internal ICM dynamics must be taken into account when evaluating this signal because simplicistic models, based on spherical rigid bodies, may provide wrong estimates. Finally we find that the dependence on the physical modelling of the baryonic component is relevant only in the very inner regions of clusters.

The Effect of Primordial Non--Gaussianity on the Topology of Large-Scale Structure

Abstract: We study the effect of primordial non-Gaussianity on the development of large-scale cosmic structure using high-resolution N-body simulations. In particular, we focus on the topological properties of the "cosmic web", quantitatively characterized by the Minkowski Functionals, for models with quadratic non-linearities with different values of the usual non-Gaussianity parameter fNL. In the weakly non-linear regime, we find that analytic formulae derived from perturbation theory agree with the numerical results within a few percent of the amplitude of each MF when |fNL|<1000. In the non-linear regime, the detailed behavior of the MFs as functions of threshold density deviates more strongly from the analytical curves, while the overall amplitude of the primordial non-Gaussian effect remains comparable to the perturbative prediction. When smaller-scale information is included, the influence of primordial non-Gaussianity becomes increasingly significant statistically due to decreasing sample variance. We find that the effect of the primordial non-Gaussianity with |fNL|=50 is comparable to the sample variance of mass density fields with a volume of 0.125(Gpc/h)^3 when they are smoothed by Gaussian filter at a scale of 5Mpc/h. The detectability of this effect in actual galaxy surveys will strongly depend upon residual uncertainties in cosmological parameters and galaxy biasing.

Observing metallicity in simulated clusters with X-MAS2

Abstract: hervebourdin@roma2infnit 4 INAF, Osservatorio Astronomico di Bologna, via Ranzani 1, I-40127 Bologna, Italy; stefanoettori@boastroit 5 Dipartimento di Astronomia, Universita di Trieste, via Tiepolo 11, I-34131 Trieste, Italy; borgani@tsastroit 6 Max-Planck-Institut fur Astrophysik, Karl-Schwarzschild Strasse 1, D-85748 Garching bei Munchen, Germany; kdolag@mpa-garchingmpgde 7 CEA, DSM, DAPNIA, Service d’Astrophysique, CE Saclay, 91191, Gif-sur-Yvette Cedex, France; jsauvageot@ceafr 8 SISSA, via Beirut 4, 34014, Trieste, Italy; torna@sissait

Cosmic rays and Radio Halos in galaxy clusters : new constraints from radio observations

Abstract: Clusters of galaxies are sites of acceleration of charged particles and sources of non-thermal radiation. We report on new constraints on the population of cosmic rays in the Intra Cluster Medium (ICM) obtained via radio observations of a fairly large sample of massive, X-ray luminous, galaxy clusters in the redshift interval 0.2--0.4. The bulk of the observed galaxy clusters does not show any hint of Mpc scale synchrotron radio emission at the cluster center (Radio Halo). We obtained solid upper limits to the diffuse radio emission and discuss their implications for the models for the origin of Radio Halos. Our measurements allow us to derive also a limit to the content of cosmic ray protons in the ICM. Assuming spectral indices of these protons delta =2.1-2.4 and microG level magnetic fields, as from Rotation Measures, these limits are one order of magnitude deeper than present EGRET upper limits, while they are less stringent for steeper spectra and lower magnetic fields.