MareNostrum Universe Research Articles

Upper bounds on matter-antimatter admixture from gamma-ray observations of colliding clusters of galaxies with the Fermi Large Area Telescope

We examined whether significant constraints on matter-antimatter admixture can be derived from gamma-ray observations of colliding clusters of galaxies with Fermi-LAT. We selected ten known systems of colliding clusters of galaxies for the analysis and computed the upper bounds on matter-antimatter admixture in these systems, which range from $7\ifmmode\times\else\texttimes\fi{}1{0}^{\ensuremath{-}9}$ to $2\ifmmode\times\else\texttimes\fi{}1{0}^{\ensuremath{-}6}$. This allowed us to exclude a symmetric universe on scales of order $\ensuremath{\sim}20\text{ }\text{ }\mathrm{Mpc}$ at the confidence level of 99.9%. Adopting the number of systems of colliding galaxy clusters from the Marenostrum Universe cosmological simulation, we checked if the Fermi-LAT second source catalog contains a sufficient number of gamma-ray sources to provide us with the required number of sources possibly associated with $p\overline{p}$ annihilation from cluster-anticluster collisions. We found that a matter-antimatter-symmetric universe is strongly ruled out on scales of order $\ensuremath{\sim}20\text{ }\text{ }\mathrm{Mpc}$ if a matter-antimatter admixture in these bullet-like systems is of $f\ensuremath{\gtrsim}1{0}^{\ensuremath{-}5}$, and on scales of order $\ensuremath{\sim}400\text{ }\text{ }\mathrm{Mpc}$ if $f\ensuremath{\gtrsim}1{0}^{\ensuremath{-}4}$.

Ram pressure statistics for bent tail radio galaxies

In this paper we use the MareNostrum Universe Simulation, a large scale, hydrodynamic, non-radiative simulation in combination with a simple abundance matching approach to determine the ram pressure statistics for bent radio sources (BRSs). The abundance matching approach allows us to determine the locations of all galaxies with stellar masses $> 10^{11} MSol$ in the simulation volume. Assuming ram pressure exceeding a critical value causes bent morphology, we compute the ratio of all galaxies exceeding the ram pressure limit (RPEX galaxies) relative to all galaxies in our sample. According to our model 50% of the RPEX galaxies at $z = 0$ are found in clusters with masses larger than $10^{14.5}MSol$ the other half resides in lower mass clusters. Therefore, the appearance of bent tail morphology alone does not put tight constraints on the host cluster mass. In low mass clusters, $M < 10^{14}MSol$, RPEX galaxies are confined to the central 500 kpc whereas in clusters of $> 10^{15}Msol$ they can be found at distances up to 1.5Mpc. Only clusters with masses $> 10^{15}MSol $ are likely to host more than one BRS. Both criteria may prove useful in the search for distant, high mass clusters.

How many radio relics await discovery?

Upcoming radio telescopes will allow to study the radio sky at low frequencies with unprecedent sensitivity and resolution. New surveys are expected to discover a large number of new radio sources. Here we investigate the abundance of radio relics, i.e. steep-spectrum diffuse radio emission coming from the periphery of galaxy clusters, which are believed to trace shock waves induced by cluster mergers. With the advent of comprehensive relic samples a framework is needed to analyze statistically the relic abundance. To this end, we introduce the probability to find a relic located in a galaxy cluster with given mass and redshift allowing us to relate the halo mass function of the Universe with the radio relic number counts. Up to date about 45 relics have been reported and we compile the resulting counts, N(>S_1.4). In principle, the parameters of the distribution could be determined using a sufficiently large relic sample. However, since the number of known relics is still small for that purpose we use the MareNostrum Universe simulation to determine the relic radio power scaling with cluster mass and redshift. Our model is able to reproduce the recently found tentative evidence for an increase in the fraction of clusters hosting relics, both with X-ray luminosity and redshift, using an X-ray flux limited cluster sample. Moreover, we find that a considerable fraction of faint relics (S_1.4 < ~10 mJy) reside in clusters with an X-ray flux below ~3e-12 erg/s/cm^2. Finally, we estimate the number of radio relics which await discovery by future low frequency surveys proposed for LOFAR and WSRT. We estimate that the WODAN survey proposed for WSRT may discover 900 relics and that the LOFAR-Tier 1-120 MHz survey may discover about 2500 relics. However, the actual number of newly discovered relics will crucially depend on the existence of sufficiently complete galaxy cluster catalogues.

Comparison of an X-ray-selected sample of massive lensing clusters with the MareNostrum Universe ΛCDM simulation

A long-standing problem of strong lensing by galaxy clusters regards the observed high rate of giant gravitational arcs as compared to the predictions in the framework of the "standard" cosmological model. Recently, few other inconsistencies between theoretical expectations and observations have been claimed which regard the large size of the Einstein rings and the high concentrations of few clusters with strong lensing features. All of these problems consistently indicate that observed galaxy clusters may be gravitational lenses stronger than expected. We use clusters extracted from the MareNostrum Universe to build up mock catalogs of galaxy clusters selected through their X-ray flux. We use these objects to estimate the probability distributions of lensing cross sections, Einstein rings, and concentrations for the sample of 12 MACS clusters at $z>0.5$ presented in Ebeling et al. (2007) and discussed in Zitrin et al. (2010). We find that simulated clusters produce $\sim 50%$ less arcs than observed clusters do. The medians of the distributions of the Einstein ring sizes differ by $\sim 25%$ between simulations and observations. We estimate that, due to cluster triaxiality and orientation biases affecting the lenses with the largest cross sections, the concentrations of the individual MACS clusters inferred from the lensing analysis should be up to a factor of $\sim 2$ larger than expected from the $\Lambda$CDM model. The arc statistics, the Einstein ring, and the concentration problems in strong lensing clusters are mitigated but not solved on the basis of our analysis. Nevertheless, due to the lack of redshifts for most of the multiple image systems used for modeling the MACS clusters, the results of this work will need to be verified with additional data. The upcoming CLASH program will provide an ideal sample for extending our comparison (abridged).

BULLET CLUSTERS IN THE MARENOSTRUM UNIVERSE

We estimate the expected distribution of displacements between the dark matter and gas cores in simulated clusters. We use the MareNostrum Universe, one of the largest non radiative, SPH Lambda CDM cosmological simulations. We find that projected 2-D displacements between dark matter and gas, equal or larger than the observed in the Bullet Cluster, are expected in 1% to 2% of the clusters with masses larger than 10^{14} Msun. The 2-D displacement distribution is roughly the same between redshifts 0<z<0.5 when multiplied by a factor of (1+z)^{-1/2}. We conclude that the separations between dark matter and gas as observed in the bullet cluster can be easily found in a Lambda CDM universe. Furthermore we find that the displacement distribution is not very sensitive to the normalization of the power spectrum. Upcoming surveys could extend the measurements of these displacements between dark matter and gas into large samples of hundreds of clusters, providing a potential test for Lambda CDM.

Strong lensing in the MARENOSTRUM UNIVERSE

<i>Context. <i/>Strong lensing is one of the most direct probes of mass distribution in the inner regions of galaxy clusters. It can be used to constrain the density profiles and to measure the mass of the lenses. Moreover, the abundance of strong lensing events can be used to constrain structure formation and cosmological parameters through the so-called “arc-statistics” approach. However, several issues related to the use of strong lensing clusters in cosmological applications are still controversial, leading to the suspicion that several biases may affect this very peculiar class of objects.<i>Aims. <i/>With this study we aim a better understanding of the properties of galaxy clusters that can potentially act as strong lenses.<i>Methods. <i/>We do so by investigating the properties of a large sample of galaxy clusters extracted from the <i>N<i/>-body/hydrodynamical simulation MareNostrum Universe. We perform ray-tracing simulations with each of them and identify those objects capable of producing strong lensing effects. We explore the correlation between the cross section for lensing and many properties of clusters, such as mass, three-dimensional and projected shapes, their concentrations, the X-ray luminosity, and the dynamical activity.<i>Results. <i/>We quantify the minimal cluster mass required for producing both multiple images and large distortions. While we do not measure a significant excess of triaxiality in strong lensing clusters, we find that the probability of strong alignments between the major axes of the lenses and the line of sight is a growing function of the lensing cross section. In projection, the strong lenses appear rounder within <i>R<i/><sub>200<sub/>, but we find that their cores tend to be more elliptical as the lensing cross section increases. As a result of the orientation bias, we also find that the cluster concentrations estimated from the projected density profiles tend to be biased high. The X-ray luminosity of strong lensing clusters tend to be higher than for normal lenses of similar mass and redshift. This is particularly significant for the least massive lenses. Finally, we find that the strongest lenses generally exhibit an excess of kinetic energy within the virial radius, thus indicating that they are more dynamically active than the usual clusters.<i>Conclusions. <i/>We conclude that strong lensing clusters are a very peculiar class of objects, affected by many selection biases that need to be properly modeled when using them to study the inner structure of galaxy clusters or to constrain the cosmological parameters.

Strong lensing in the MARENOSTRUM UNIVERSE

The strong lensing events that are observed in compact clusters of galaxies can, both statistically and individually, return important clues about the structural properties of the most massive structures in the Universe. Substantial work is ongoing in order to understand the degree of similarity between the lensing cluster population and the population of clusters as a whole, with members of the former being likely more massive, compact, and substructured than members of the latter. In this work we exploit synthetic clusters extracted from the MareNostrum Universe cosmological simulation in order to estimate the correlation between the strong lensing efficiency and other bulk properties of lensing clusters, such as the virial mass and the bolometric X-ray luminosity. We found that a positive correlation exist between all these quantities, with the substantial scatter being smaller for the luminosity-cross section relation. We additionally used the relation between the lensing efficiency and the virial mass in order to construct a synthetic optical depth that agrees well with the true one, while being extremely faster to be evaluated. We finally estimated what fraction of the total giant arc abundance is recovered when galaxy clusters are selected according to their dynamical activity or their X-ray luminosity. Our results show that there is a high probability for high-redshift strong lensing clusters to be substantially far away from dynamical equilibrium, and that 30–40% of the total amount of giant arcs are lost if looking only at very X-ray luminous objects.

Fractal analysis of the dark matter and gas distributions in the Mare-Nostrum universe

We develop a method of multifractal analysis of N-bodycosmological simulations that improves on the customary counts-in-cells methodby taking special care of the effects of discreteness and large scalehomogeneity. The analysis of the Mare-Nostrum simulation with our methodprovides strong evidence of self-similar multifractal distributions of darkmatter and gas, with a halo mass function that is of Press-Schechter type buthas a power-law exponent -2, as corresponds to a multifractal. Furthermore,our analysis shows that the dark matter and gas distributions areindistinguishable as multifractals. To determine if there is any gas biasing,we calculate the cross-correlation coefficient, with negative but inconclusiveresults. Hence, we develop an effective Bayesian analysis connected withinformation theory, which clearly demonstrates that the gas is biased in along range of scales, up to the scale of homogeneity. However, entropicmeasures related to the Bayesian analysis show that this gas bias is small (ina precise sense) and is such that the fractal singularities of bothdistributions coincide and are identical. We conclude that this commonmultifractal cosmic web structure is determined by the dynamics and isindependent of the initial conditions.

Evolution of dark-matter halos in numerical models

The properties of gravitationally bound clouds (halos) of dark matter derived via numerical simulations of the distribution of dark matter in the Universe are investigated. The analysis makes use of a catalog of halos obtained in the European “MareNostrum Universe” project, which has achieved a better balance between resolution and representativeness than catalogs used earlier for similar studies. This has made it possible to refine the main tendencies displayed by the evolution of the halo masses and the angular velocities and density profiles of the halos. The results are compared with the newest available observational data and with known results obtained earlier in numerical simulations with lower resolution and using smaller samples of halos, making it possible to trace the influence of these factors on the results obtained. Disagreements between observations and numerical models obtained in earlier studies are confirmed, and possible ways to explain them discussed.

The Sunyaev–Zeldovich effect in superclusters of galaxies using gasdynamical simulations: the case of Corona Borealis

[Abridged] We study the thermal and kinetic Sunyaev-Zel'dovich (SZ) effect associated with superclusters of galaxies using the MareNostrum Universe SPH simulation. We consider superclusters similar to the Corona Borealis Supercluster (CrB-SC). This paper is motivated by the detection at 33GHz of a strong temperature decrement in the CMB towards the core of this supercluster. Multifrequency observations with VSA and MITO suggest the existence of a thermal SZ effect component in the spectrum of this cold spot, which would account for roughly 25% of the total observed decrement. We identify nine regions containing superclusters similar to CrB-SC, obtain the associated SZ maps and calculate the probability of finding such SZ signals arising from hot gas within the supercluster. Our results show that WHIM produces a thermal SZ effect much smaller than the observed value. Neither can summing the contribution of small clusters and galaxy groups in the region explain the amplitude of the SZ signal. When we take into account the actual posterior distribution from the observations, the probability that WHIM can cause a thermal SZ signal like the one observed is <1%, rising up to a 3.2% when the contribution of small clusters and galaxy groups is included. If the simulations provide a suitable description of the gas physics, then we conclude that the thermal SZ component of the CrB spot most probably arises from an unknown galaxy cluster along the line of sight. The simulations also show that the kinetic SZ signal associated with the supercluster cannot provide an explanation for the remaining 75% of the observed cold spot in CrB.

Diffuse radio emission from clusters in the MareNostrum Universe simulation

Large-scale diffuse radio emission is observed in some clusters of galaxies. There is ample of evidence that the emission has its origin in synchrotron losses of relativistic electrons that have been accelerated in cluster mergers. In a cosmological simulation, we estimate the radio emission of structure formation shocks as follows: introducing a novel approach to identify strong shock fronts in an smoothed particle hydrodynamics (SPH) simulation, we determine the Mach number as well as the downstream density and temperature in the MareNostrum Universe simulation which has 2 × 10243 particles in a 500 h−1Mpc box. Then, we estimate the radio emission using the formalism derived in Hoeft & Brüggen to produce artificial radio maps of massive clusters and to derive a luminosity function of diffuse radio sources. Several of our clusters show radio objects with similar morphology to observed large-scale radio relics, whereas about half of the clusters show only very little radio emission. In agreement with observational findings, the maximum diffuse radio emission of our clusters depends strongly on their X-ray temperature. We find that the so-called accretion shocks cause only very little radio emission. We conclude that a moderate efficiency of shock acceleration, namely ξe= 0.005, and moderate magnetic fields in the region of the relics, namely 0.07–0.8 μG are sufficient to reproduce the number density and luminosity of radio relics.

Spin alignment of dark matter haloes in the shells of the largest voids

Using the high-resolution cosmological N-body simulation MareNostrum universe we study the orientation of shape and angular momentum of galaxy-size dark matter haloes around large voids. We find that haloes located on the shells of the largest cosmic voids have angular momenta that tend to be preferentially perpendicular to the direction that joins the centre of the halo and the centre of the void. This alignment has been found in spiral galaxies around voids using galaxy redshift surveys. We measure for the first time the strength of this alignment, showing how it falls off with increasing distance to the centre of the void. We also confirm the correlation between the intensity of this alignment and the halo mass. The analysis of the orientation of the halo main axes confirms the results of previous works. Moreover, we find a similar alignment for the baryonic matter inside dark matter haloes, which is much stronger in their inner parts.

Is WMAP3 Normalization Compatible with the X-Ray Cluster Abundance?

We present the mass and X-ray temperature functions derived from a sample of more than 15,000 galaxy clusters of the MareNostrum Universe cosmological SPH simulations. In these simulations, we follow structure formation in a cubic volume of 500/h Mpc on a side assuming cosmological parameters consistent with either the first or third year WMAP data and gaussian initial conditions. We compare our numerical predictions with the most recent observational estimates of the cluster X-ray temperature functions and find that the low normalization cosmological model inferred from the 3 year WMAP data results is barely compatible with the present epoch X-ray cluster abundances. We can only reconcile the simulations with the observational data if we assume a normalization of the Mass-Temperature relation which is a factor of 2.5--3 smaller than our non-radiative simulations predict. This deviation seems to be too large to be accounted by the effects of star formation or cooling in the ICM, not taken into account in these simulations.

Shape, Spin, and Baryon Fraction of Clusters in the MareNostrum Universe

The MareNostrum Universe is one of the largest cosmological smoothed particle hydrodynamics simulations done so far. It consists of 10243 dark and 10243 gas particles in a box of 500 h-1 Mpc on a side. Here we study the shapes and spins of the dark matter and gas components of the 10,000 most massive objects extracted from the simulation as well as the gas fraction in those objects. We find that the shapes of objects tend to be prolate both in the dark matter and gas. There is a clear dependence of shape on halo mass, the more massive ones being less spherical than the less massive objects. The gas distribution is nevertheless much more spherical than the dark matter, although the triaxiality parameters of gas and dark matter differ only by a few percent, and it increases with cluster mass. The spin parameters of gas and dark matter can be well fitted by a lognormal distribution function. On average, the spin of gas is 1.4 times larger than the spin of dark matter. We find a similar behavior for the spins at higher redshifts, with a slight decrease of the spin ratios to 1.16 at z = 1. The cosmic normalized baryon fraction in the entire cluster sample ranges from Yb = 0.94 at z = 1 to Yb = 0.92 at z = 0. At both redshifts we find a slight, but statistically significant, decrease of Yb with cluster mass.