Search

We use semi-analytic modelling of the galaxy-cluster population and its strong lensing efficiency to explore how the expected abundance of large gravitational arcs on the sky depends on $\sigma_8$. Our models take all effects into account that have been shown to affect strong cluster lensing substantially, in particular cluster asymmetry, substructure, merging, and variations in the central density concentrations. We show that the optical depth for long and thin arcs increases by approximately one order of magnitude when $\sigma_8$increases from 0.7 to 0.9, owing to a constructive combination of several effects. Models with high $\sigma_8$are also several orders of magnitude more efficient in producing arcs at intermediate and high redshifts. Finally, we use realistic source number counts to quantitatively predict the total number of arcs brighter than several magnitude limits in the Rand Ibands. We confirm that, while $\sigma_8$~ 0.9 may come close to the known abundance of arcs, even $\sigma_8$~ 0.8 falls short by almost an order of magnitude in reproducing known counts. We conclude that, should $\sigma_8$~ 0.8 be confirmed, we would fail to understand the strong-lensing efficiency of the galaxy cluster population, and in particular the abundance of arcs in high-redshift clusters. We argue that early-dark energy or non-Gaussian density fluctuations may indicate one way out of this problem.

Aims. We calibrate the number density, completeness, reliability, and the lower mass limit of galaxy-cluster detections through their thermal SZ signal and compare them to X-ray cluster detections.

Context.The polarization sensitivity of 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 focused on the polarization effect induced by the ICM bulk motion, the so-called kpSZ signal, which has an amplitude proportional to the optical depth and to the square of the tangential velocity.

We use simulated merger trees of galaxy-cluster halos to study the effect of the halo concentration distribution on strong lensing and X-ray emission. Its log-normal shape typically found in simulations favors outliers with high concentration. Since, at fixed mass, more concentrated halos tend to be more efficient lenses, the scatter in the concentration increases the strong-lensing optical depth by $\la$$50\%$. Within cluster samples, mass and concentration have counteracting effects on strong lensing and X-ray emission because the concentration decreases for increasing mass. Selecting clusters by concentration thus has no effect on the lensing cross section. The most efficiently lensing and hottest clusters are typically the leastconcentrated in samples with a broad mass range. Among cluster samples with a narrow mass range, however, the most strongly lensing and X-ray brightest clusters are typically $10\%$to $25\%$more concentrated.

We study the reliability of dark-matter halo detections with three different linear filters applied to weak-lensing data. We use ray-tracing in the multiple lens-plane approximation through a large cosmological simulation to construct realizations of cosmic lensing by large-scale structures between redshifts zero and two. We apply the filters mentioned above to detect peaks in the weak-lensing signal and compare them with the true population of dark matter halos present in the simulation. We confirm the stability and performance of a filter optimised for suppressing the contamination by large-scale structure. It allows the reliable detection of dark-matter halos with masses above a few times $10^{13}\,h^{-1}\,M_\odot$with a fraction of spurious detections below ~$10\%$. For sources at redshift two, 50% of the halos more massive than ~$7\times10^{13}\,h^{-1}\,M_\odot$are detected, and completeness is reached at ~$2\times10^{14}\,h^{-1}\,M_\odot$.

Context.We apply the linear filter for the weak-lensing signal of dark-matter halos developed in Maturi et al. (2005, A&A, 442, 851) to the cosmic-shear data extracted from the Garching-Bonn-Deep-Survey (GaBoDS).

Aims.We investigate how ellipticity, asymmetries and substructures separately affect the ability of galaxy clusters to produce strong lensing events, i.e. gravitational arcs, and how they influence the arc morphologies and fluxes. This is important for studies aiming, for example, at constraining cosmological parameters from statistical lensing, or at determining the inner structure of galaxy clusters through gravitational arcs.

We propose a method for recovering the two-dimensional gravitational potential of galaxy clusters which combines data from weak and strong gravitational lensing. A first estimate of the potential from weak lensing is improved at the approximate locations of critical curves. The method can be fully linearised and does not rely on the existence and identification of multiple images. We use simulations to show that it recovers the surface-mass density profiles and distributions very accurately,even if critical curves are only partially known and if their location is realistically uncertain. We further describe how arcs at different redshifts can be combined, and how deviations from weak lensing can be included.

Aims.We study the clustering properties of Distant Red Galaxies (DRGs) to test whether they are the progenitors of local massive galaxies.

Strong gravitational lensing by irregular mass distributions, such as galaxy clusters, is generally not quantified well by cross sections of analytic mass models. Computationally expensive ray-tracing methods have so far been necessary for accurate cross-section calculations. We describe a fast, semi-analytic method here that is based on surface integrals over high-magnification regions in the lens plane and demonstrate that it yields reliable cross sections even for complex, asymmetric mass distributions. The method is faster than ray-tracing simulations by factors of ~30 and thus suitable for large cosmological simulations, saving large amounts of computing time. We apply this method to a sample of galaxy cluster-sized dark matter haloes with simulated merger trees and show that cluster mergers approximately double the strong-lensing optical depth for lens redshifts $z_\mathrm{l}\gtrsim0.5$and sources near $z_\mathrm{s} = 2$. We believe that this result hints at one possibility for understanding the recently detected high arcs abundance in clusters at moderate and high redshifts, and is thus worth studying further.

We construct a linear filter optimised for detecting dark-matter halos in weak-lensing data. The filter assumes a mean radial profile of the halo shear pattern and modifies that shape by the noise power spectrum. Aiming at separating dark-matter halos from spurious peaks caused by large-scale structure lensing, we model the noise as being composed of weak lensing by large-scale structures and Poisson noise from random galaxy positions and intrinsic ellipticities. Optimal filtering against the noise requires the optimal filter scale to be smaller than typical halo sizes. Although a perfect separation of halos from spurious large-scale structure peaks is strictly impossible, we use numerical simulations to demonstrate that our filter produces substantially more sensitive, reliable and stable results than the conventionally used aperture-mass statistic.

We adapt a non-linear filter proposed by Hu (2001) for detecting lensing of the CMB by large-scale structures to recover surface-density profiles of galaxy clusters from their localised, weak gravitational lensing effect on CMB fields. Shifting the band-pass of the filter to smaller scales, and normalising it such as to reproduce the convergence rather than the deflection angle, we find that the mean density profile of a sample of 100 clusters can be recovered to better than 10% from well within the scale radius to almost the virial radius. The kinetic Sunyaev-Zel'dovich effect is shown to be a negligible source of error. We test the filter applying it to data simulated using the characteristics of the Atacama Cosmology Telescope(ACT), showing that it will be possible to recover mean cluster profiles outside a radius of $1'$corresponding to ACT's angular resolution.

The detection of an excess of soft X-ray or Extreme Ultraviolet (EUV) radiation, above the thermal contribution from the hot intracluster medium (ICM), has been a controversial subject ever since the initial discovery of this phenomenon. We use a large-scale hydrodynamical simulation of a concordance ΛCDM model, to investigate the possible thermal origin of such an excess in a set of 20 simulated clusters having temperatures in the range 1-7 keV. Simulated clusters are analysed by mimicking the observational procedure applied to ROSAT-PSPC data, which for the first time showed evidence for the soft X-ray excess: we compare the low-energy (e.g. [ 0.2-1]  keV) part of the spectrum of each cluster with that predicted for a plasma having temperature and metallicity as computed after weighting by the emissivity in a harder band (e.g., [ 1-2]  keV). For distances to the cluster center $0.4< R/R_{\rm vir}< 0.7$we detect a significant excess in most of the simulated clusters, whose relative amount changes from cluster to cluster and, for the same cluster, by changing the projection direction. In about 30 per cent of the cases the soft X-ray flux is measured to be at least 50 per cent larger than predicted by the one-temperature plasma model. We find that this excess is generated in most cases within the virialized regions of the cluster. It mainly comes from low-entropy and high-density gas associated with merging sub-halos, rather than from diffuse warm gas. Only in a few cases does the excess arise from fore/background groups observed in projection, while no evidence is found for a significant contribution from gas lying within large-scale filaments. We compute the distribution of the relative soft excess, as a function of the distance to the cluster center , and compare it with the observational result by Bonamente et al. ([CITE]) for the Coma cluster. As in the observations, we find that the relative excess increases with the distance from the cluster center, with no significant excess detected for $R<0.4R_{\rm vir}$. However, an excess as large as that reported for the Coma cluster at scales $0.4\la R/R_{\rm vir}\la 0.7$is found to be rather unusual in our set of simulated clusters.

We study the concentration parameters, their mass dependence and redshift evolution, of dark-matter halos in different dark-energy cosmologies with constant and time-variable equation of state, and compare them with “standard” ΛCDM and OCDM models. We find that previously proposed algorithms for predicting halo concentrations can be well adapted to dark-energy models. When centred on the analytically expected values, halo concentrations show a log-normal distribution with a uniform standard deviation of ~0.2. The dependence of averaged halo concentrations on mass and redshift permits a simple fit of the form $(1+z)\,c=c_0\,(M/M_0)^\alpha$, with $\alpha\approx-0.1$throughout. We find that the cluster concentration depends on the dark energy equation of state at the cluster formation redshift zcollthrough the linear growth factor $D_+(z_{\rm coll})$. As a simple correction accounting for dark-energy cosmologies, we propose scaling c0from ΛCDM with the ratio of linear growth factors, $c_0\rightarrow c_0\,D_+(z_{\rm coll})/D_{+,~\Lambda{\rm CDM}}(z_{\rm coll})$.

We investigate how the probability of the formation of giant arcs in galaxy clusters is expected to change in cosmological models dominated by dark energy with an equation of state $p=w\rho c^2$compared to cosmological-constant or open models. To do so, we use a simple analytic model for arc cross sections based on the Navarro-Frenk-White density profile which we demonstrate reproduces essential features of numerically determined arc cross sections. Since analytic lens models are known to be inadequate for accurate absolute quantifications of arc probabilities, we use them only for studying changes relative to cosmological-constant models. Our main results are (1) the order of magnitude difference between the arc probabilities in low density, spatially flat and open CDM models found numerically is reproduced by our analytic model, and (2) dark-energy cosmologies with $w>-1$increase the arc optical depth by at most a factor of two and are thus unlikely to reconcile arc statistics with spatially flat cosmological models with low matter density.

We study the clustering properties of galaxy clusters expected to be observed by various forthcoming surveys both in the X-ray and sub-mm regimes by the thermal Sunyaev-Zel'dovich effect. Several different background cosmological models are assumed, including the concordance ?CDM and various cosmologies with dynamical evolution of the dark energy. Particular attention is paid to models with a significant contribution of dark energy at early times which affects the process of structure formation. Past light cone and selection effects in cluster catalogs are carefully modeled by realistic scaling relations between cluster mass and observables and by properly taking into account the selection functions of the different instruments. The results show that early dark-energy models are expected to produce significantly lower values of effective bias and both spatial and angular correlation amplitudes with respect to the standard ?CDM model. Among the cluster catalogs studied in this work, it turns out that those based on eRosita, Planck, and South Pole Telescope observations are the most promising for distinguishing between various dark-energy models.

Aims. 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 targeted at simulating lensing by galaxy clusters. Our goal is to create the tools required for comparing theoretical expectations with observations to better understand of how observational noise affects lensing applications such as mass estimates, studies of the internal properties of galaxy clusters and arc statistics.Methods. 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 to reproduce observations of each galaxy in arbitrary photometric bands.Results. 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? program. We analyze the simulated images using methods applicable to real observations and measure the properties of gravitational arcs. In particular, we focus on the determination of their length, width, and curvature radius.Conclusions. We find that arc properties strongly depend on several properties of the sources. In particular, our results show that compact, faint, or low surface-brightness galaxies that are barely detectable are more easily distorted as arcs with large length-to-width ratios. We conclude that realistic lensing simulations can be obtained with the method proposed here. They will be essential for evaluating and improving the analysis techniques currently used for cosmological interpretations of cluster lensing.

Aims. We calibrate the number density, completeness, reliability, and the lower mass limit of galaxy-cluster detections through their thermal SZ?signal and compare them to X-ray cluster detections.Methods. We simulate maps of the thermal SZ?effect and the X-ray emission from light cones constructed in a large, hydrodynamical, cosmological simulation volume, including realistic noise contributions. The maps are convolved with linear, optimised, single- and multi-band filters to identify local peaks and their signal-to-noise ratios. The resulting peak catalogues are then compared to the halo population in the simulation volume to identify true and spurious detections.Results. Multi-band filtering improves the statistics of SZ cluster detections considerably compared to single-band filtering. Observations with the characteristics of ACT detect clusters with masses M ? 6-9?1013?M?/h, quite independent of redshift, reach 50% completeness at ~1014?M?/h and 100% completeness at ~2?1014?M?/h. Samples are contaminated by a few spurious detections, but they are only a small percentage of all detections. This is broadly comparable to X-ray cluster detections with XMM-Newton with 100?ks exposure time in the soft band, except that the mass limit for X-ray detections increases much more steeply with redshift than for SZ?detections. A comparison of true and filtered signals in the SZ?and X-ray maps confirms that the filters introduce at most a negligible bias.

Context.A long-standing problem of strong-lensing by galaxy clusters is the observed high rate of giant gravitational arcs that are not predicted in the framework of the “standard” cosmological model. This is known as the “arc statistics problem”. Recently, several other inconsistencies between the theoretical expectations and observations have been claimed regarding the large size of the Einstein rings and the high concentrations of few clusters with strong-lensing features. All these problems consistently indicate that observed galaxy clusters may be stronger gravitational lenses than expected.Aims.We aim at better understanding these problems by comparing the lensing properties of well defined cluster samples with those of a large set of numerically simulated objects.Methods.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 a sample of 12 MACS clusters at z> 0.5 from the literature.Results.We find that three clusters in the MACS sample have lensing cross sections and Einstein ring sizes larger than any simulated cluster in the MareNostrum Universe. We use the lensing cross sections of simulated and real clusters to estimate the number of giant arcs that should arise from lensed sources at z= 2. We find that simulated clusters produce ~50% less arcs than observed clusters do. The medians of the distributions of the Einstein ring sizes differ by ~25% between simulations and observations. We estimate that the concentrations of the individual MACS clusters inferred from the lensing analysis should be up to a factor of ~2 larger than expected from the ΛCDM model because of cluster triaxiality and orientation biases that affect the lenses with the largest cross sections. In particular, we predict that for ~20% of the clusters in the MACS sample the lensing-derived concentrations should be higher than expected by more than ~40%. Conclusions.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, owing 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.

We present the results of our analyses of the X-ray emission and of the strong lensing systems in the galaxy cluster Abell 611 [z= 0.288]. This cluster is an optimal candidate for a comparison of the mass reconstructions obtained through X-ray and lensing techniques, because of its very relaxed dynamical appearance and its exceptional strong lensing system. We infer the X-ray mass estimate deriving the density and temperature profile of the intra-cluster medium within the radius r≃ 700 kpc through a non-parametric approach, taking advantage of the high spatial resolution of a Chandra observation. Assuming that the cluster is in hydrostatic equilibrium and adopting a matter density profile, we can recover the total mass distribution of Abell 611 via the X-ray data. Moreover, we derive the total projected mass in the central regions of Abell 611 performing a parametric analysis of its strong lensing features through the publicly available analysis software Lenstool. As a final step we compare the results obtained with both methods. We derive a good agreement between the X-ray and strong lensing total mass estimates in the central regions where the strong lensing constraints are present (i.e. within the radius r≃ 100 kpc), while a marginal disagreement is found between the two mass estimates when extrapolating the strong lensing results in the outer spatial range. We suggest that in this case the X-ray/strong lensing mass disagreement can be explained by an incorrect estimate of the relative contributions of the baryonic component and of the dark matter, caused by the intrinsic degeneracy between the different mass components in the strong lensing analysis. We discuss the effect of some possible systematic errors that influence both mass estimates. We find a slight dependence of the measurements of the X-ray temperatures (and therefore of the X-ray total masses) on the background adopted in the spectral analysis, with total deviations on the value of M200of the order of the 1σstatistical error. The strong lensing mass results are instead sensitive to the parameterisation of the galactic halo mass in the central regions, in particular to the modelling of the brightest cluster galaxy (BCG) baryonic component, which induces a significant scatter in the strong lensing mass results.