arXiv subjects

L. Moscardini

Publications and source records attributed to L. Moscardini.

At least 253 records · Page 14Linked to original sources

arXiv2005

The integrated bispectrum as a test of CMB non-Gaussianity: detection power and limits on f_NL with WMAP data

We propose a fast and efficient bispectrum statistic for Cosmic Microwave Background (CMB) temperature anisotropies to constrain the amplitude of the primordial non-Gaussian signal measured in terms of the non-linear coupling parameter f_NL. We show how the method can achieve a remarkable computational advantage by focussing on subsets of the multipole configurations, where the non-Gaussian signal is more concentrated. The detection power of the test, increases roughly linearly with the maximum multipole, as shown in the ideal case of an experiment without noise and gaps. The CPU-time scales as l_{max}^3 instead of l_{max}^5 for the full bispectrum which for Planck resolution l_{max} \sim 3000 means an improvement in speed of a factor 10^7 compared to the full bispectrum analysis with minor loss in precision. We find that the introduction of a galactic cut partially destroys the optimality of the configuration, which will then need to be dealt with in the future. We find for an ideal experiment with l_{max}=2000 that upper limits of f_{NL}<8 can be obtained at 1 sigma. For the case of the WMAP experiment, we would be able to put limits of |f_{NL}|<40 if no galactic cut were present. Using the real data with galactic cut, we obtain an estimate of -80<f_{NL}<80 and -160<f_{NL}<160 at 1 and 2 sigma respectively.

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arXiv2005

A fast method for computing strong-lensing cross sections: Application to merging clusters

Strong gravitational lensing by irregular mass distributions, such as galaxy clusters, is generally not well quantified 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 which 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 $\sim30$ 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 further studies.

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arXiv2005

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

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.

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arXiv2005

Primordial non-Gaussianity: local curvature method and statistical significance of constraints on f_NL from WMAP data

We test the consistency of estimates of the non-linear coupling constant f_{NL} using non-Gaussian CMB maps generated by the method described in (Liguori, Matarrese and Moscardini 2003). This procedure to obtain non-Gaussian maps differs significantly from the method used in previous works on estimation of f_{NL}. Nevertheless, using spherical wavelets, we find results in very good agreement with (Mukherjee and Wang 2004), showing that the two ways of generating primordial non-Gaussian maps give equivalent results. Moreover, we introduce a new method for estimating the non-linear coupling constant from CMB observations by using the local curvature of the temperature fluctuation field. We present both Bayesian credible regions (assuming a flat prior) and proper (frequentist) confidence intervals on f_{NL}, and discuss the relation between the two approaches. The Bayesian approach tends to yield lower error bars than the frequentist approach, suggesting that a careful analysis of the different interpretations is needed. Using this method, we estimate f_{NL}=-10^{+270}_{-260} at the 2σlevel (Bayesian) and f_{NL}=-10^{+310}_{-270} (frequentist). Moreover, we find that the wavelet and the local curvature approaches, which provide similar error bars, yield approximately uncorrelated estimates of f_{NL} and therefore, as advocated in (Cabella et al. 2004), the estimates may be combined to reduce the error bars. In this way, we obtain f_{NL}=-5\pm 85 and f_{NL}=-5\pm 175 at the 1σand 2σlevel respectively using the frequentist approach.

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arXiv2005

Effect of clustering on extragalactic source counts with low-resolution instruments

In the presence of strong clustering, low-resolution surveys measure the summed contributions of groups of sources within the beam. The counts of bright intensity peaks are therefore shifted to higher flux levels compared to the counts of individual sources detected with high-resolution instruments. If the beam-width corresponds a sizable fraction of the clustering size, as in the case of Planck/HFI, one actually detects the fluxes of clumps of sources. We argue that the distribution of clump luminosities can be modelled in terms of the two- and three-point correlation functions, and apply our formalism to the Planck/HFI 850 micron surveys. The effect on counts is found to be large and sensitive also to the evolution of the three-point correlation function; in the extreme case that the latter function is redshift-independent, the source confusion due to clustering keeps being important above the canonical 5*sigma detection limit. Detailed simulations confirm the reliability of our approach. As the ratio of the beam-width to the clustering angular size decreases, the observed fluxes approach those of the brightest sources in the beam and the clump formalism no longer applies. However, simulations show that also in the case of the Herschel/SPIRE 500 micron survey the enhancement of the bright source counts due to clustering is important.

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arXiv2004

An optimal filter for the detection of galaxy clusters through weak lensing

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.

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arXiv2004

Spectroscopic-Like Temperature of Clusters of Galaxies and Cosmological Implications

The thermal properties of hydrodynamical simulations of galaxy clusters are usually compared to observations by relying on the emission-weighted temperature T_ew, instead of on the spectroscopic X-ray temperature T_spec, which is obtained by actual observational data. Here we show that, if the intra-cluster medium is thermally complex, T_ew fails at reproducing T_spec. We propose a new formula, the spectroscopic-like temperature, T_sl, which approximates T_spec better than a few per cent. By analyzing a set of hydrodynamical simulations of galaxy clusters, we also find that T_sl is lower than T_ew by 20-30 per cent. As a consequence, the normalization of the M-T relation from the simulations is larger than the observed one by about 50 per cent. If masses in simulated clusters are estimated by following the same assumptions of hydrostatic equilibrium and beta-model gas density profile, as often done for observed clusters, then the M-T relation decreases by about 40 per cent, and significantly reduces its scatter. Based on 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 sigma_8 by 10-20 per cent. This may alleviate the tension between the value of sigma_8 inferred from the cluster number density and those from cosmic microwave background and large scale structure.

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arXiv2004

Measuring cluster peculiar velocities with the Sunyaev-Zeldovich effects: scaling relations and systematics

The fluctuations in the Cosmic Microwave Background (CMB) intensity due to the Sunyaev-Zeldovich (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 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 2x480^3 particles, of a cosmological volume 192 Mpc/h on a side of a flat Cold Dark Matter model with Omega_0=0.3 and Lambda=0.7. Our simulation includes radiative cooling, star formation and the effect of feedback and galactic winds from supernovae. We find that (1) our simulated clusters reproduce the observed scaling relations between X-ray and SZ properties; (2) bulk flows internal to the intracluster medium affect the velocity estimate by less than 200 km/s in 93 per cent of the cases; (3) 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 to the systematic errors that will affect upcoming measures of cluster peculiar velocities with the SZ effect.

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arXiv2004

Simulating the Soft X-ray excess in clusters of galaxies

The detection of 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 $Λ$CDM model, to investigate the possible thermal origin for 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 evidences for the soft X-ray excess. For cluster--centric distances $0.4< R/R_{\rm vir}< 0.7$ 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 cluster virialized regions. It is mainly contributed by low--entropy and high--density gas associated with merging sub--halos, rather than to diffuse warm gas. Only in a few cases the excess arises 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 cluster--centric distance, and compare it with the observational result by Bonamente et al. (2003) for the Coma cluster. Similar to observations, we find that the relative excess increases with the distance from the cluster center, with no significant excess detected for $R<0.4R_{\rm vir}$. (abridged)

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arXiv2004

Mismatch between X-ray and emission-weighted temperatures in galaxy clusters: cosmological implications

The thermal properties of hydrodynamical simulations of galaxy clusters are usually compared to observations by relying on the emission-weighted temperature T_ew, instead of on the spectroscopic X-ray temperature T_spec, which is obtained by actual observational data. In a recent paper Mazzotta et al. show that, if the cluster is thermally complex, T_ew fails at reproducing T_spec, and propose a new formula, the spectroscopic-like temperature, T_sl, which approximates T_spec better than a few per cent. By analyzing a set of hydrodynamical simulations of galaxy clusters, we find that T_sl is lower than T_ew by 20-30 per cent. As a consequence, the normalization of the M-T_sl relation from the simulations is larger than the observed one by about 50 per cent. If masses in simulated clusters are estimated by following the same assumptions of hydrostatic equilibrium and β--model gas density profile, as often done for observed clusters, then the M-T relation decreases by about 40 per cent, and significantly reduces its scatter. Based on 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 σ_8 by 10-20 per cent. This may alleviate the tension between the value of σ_8 inferred from the cluster number density and those from cosmic microwave background and large scale structure.

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arXiv2004

Gravitational lensing of the CMB by galaxy clusters

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.

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arXiv2004

Evolution at z>0.5 of the X-ray properties of simulated galaxy clusters: comparison with the observational constraints

(ABRIDGED) We analyze the X-ray properties of a sample of local and high redshift galaxy clusters extracted from a large cosmological hydrodynamical simulation. This simulation has been realized using the Tree+SPH code GADGET-2 for a LambdaCDM model. In our analysis, we consider only objects with T_ew >2 keV and adopt an approach that mimics observations, associating with each measurement an error comparable with recent observations and providing best-fit results via robust techniques. Within the clusters, baryons are distributed among (i) a cold neutral phase, with a relative contribution that increases from less than 1 to 3 per cent at higher redshift, (ii) stars which contribute with about 20 per cent and (iii) the X-ray emitting plasma that contributes by 80 (76) per cent at z=0 (1) to the total baryonic budget. A depletion of the cosmic baryon fraction of ~7 (at z=0) and 5 (at z=1) per cent is measured at the virial radius, R_vir, in good agreement with adiabatic hydrodynamical simulations. We confirm that, also at redshift >0.5, power-law relations hold between gas temperature, T, bolometric luminosity, L, central entropy, S, gas mass, M_gas, and total gravitating mass, M_tot and that these relations are steeper than predicted by simple gravitational collapse. A significant, negative evolution in the L-T and L-M_tot relations and positive evolution in the S-T relation are detected at 0.5 < z < 1 in this set of simulated galaxy clusters. This is partially consistent with recent analyses of the observed properties of z>0.5 X-ray galaxy clusters. By fixing the slope to the values predicted by simple gravitational collapse, we measure at high redshift normalizations lower by 10-40 per cent in the L-T, M_tot-T, M_gas-T, f_gas-T and L-M_tot relations than the observed estimates.

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arXiv2004

Beppo-SAX temperature maps of galaxy clusters in the Corona Borealis supercluster: A2061, A2067 and A2124

In this paper we present the analysis of Beppo-SAX observations of the cluster pairs A2061-A2067 and A2122-A2124, located in the Corona Borealis supercluster, which have been selected as candidate merging clusters. The aim of this work is to study the physics of the intracluster medium and to look for the possible presence of merging signatures. We derived the global temperatures and abundances and the temperature profiles and maps for these clusters. We do not find any significant evidence of interaction between the clusters forming the pairs, but we detect a candidate shock inside A2061. On the basis of the X-ray and optical properties of this cluster we propose a scenario in which a group is falling inside A2061. This interaction is in the phase in which the cores have not encountered yet and in which the formation of a shock is expected.

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arXiv2004

The Diffuse Light in Simulations of Galaxy Clusters

We study the properties of the diffuse light in galaxy clusters forming in a large hydrodynamical cosmological simulation of the Lambda-CDM cosmology. The simulation includes a model for radiative cooling, star formation in dense cold gas, and feedback by SN-II explosions. We select clusters having mass M>10^(14) h^(-1) Msun and study the spatial distribution of their star particles. While most stellar light is concentrated in gravitationally bound galaxies orbiting in the cluster potential, we find evidence for a substantial diffuse component, which may account for the extended halos of light observed around central cD galaxies. We find that more massive simulated clusters have a larger fraction of stars in the diffuse light than the less massive ones. The intracluster light is more centrally concentrated than the galaxy light, and the stars in the diffuse component are on average older than the stars in cluster galaxies, supporting the view that the diffuse light is not a random sampling of the stellar population in the cluster galaxies. We thus expect that at least ~10% of the stars in a cluster may be distributed as intracluster light, largely hidden thus far due to its very low surface brightness.

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arXiv2004

Evolution of dark-matter haloes in a variety of dark-energy cosmologies

High-resolution, numerical simulations of 17 cluster-sized dark-matter haloes in eight different cosmologies with and without dynamical dark energy confirm the picture that core halo densities are imprinted early during their formation by the mean cosmological density. Quite independent of cosmology, halo concentrations have a log-normal distribution with a scatter of ~0.2 about the mean. We propose a simple scaling relation for halo concentrations in dark-energy cosmologies.

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arXiv2004

Confusion noise at far-IR to millimeter wavelengths

We present detailed predictions for the confusion noise due to extragalactic sources in the far-IR/(sub)-millimeter channels of ESA/ISO, NASA/Spitzer, ESA/Herschel and ESA/Planck satellites, including the contribution from clustering of unresolved SCUBA galaxies. Clustering is found to increase the confusion noise, compared to the case of purely Poisson fluctuations, by 10-15% for the lowest frequency (i.e. lowest angular resolution) Spitzer and Herschel channels, by 25-35% for the 175 micron ISOPHOT channel, and to dominate in the case of Planck/HFI channels at nu>143GHz. Although our calculations make use of a specific evolutionary model (Granato et al. 2004), the results are strongly constrained by the observed counts and by data on the redshift distribution of SCUBA sources, and therefore are not expected to be heavily model dependent. The main uncertainty arises from the poor observational definition of the source clustering properties. Two models have been used for the latter: a power-law with constant slope and a redshift-independent comoving correlation length,r_0, and the standard theoretical model for clustering evolution in a LambdaCDM universe, with a redshift-dependent bias factor. In both cases, the clustering amplitude has been normalized to yield a unit angular correlation function at theta_0=1-2 arcsec for 850 micron sources fainter than 2 mJy, consistent with the results by Peacock et al. (2000). This normalization yields, for the first model, r_0=8.3$ Mpc/h, and, for the second model, an effective mass of dark matter haloes in which these sources reside of M_halo=1.8*10^{13} M_sun/h. These results are consistent with independent estimates for SCUBA galaxies and for other, likely related, sources.

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arXiv2004

Comparing the temperatures of galaxy clusters from hydro-N-body simulations to Chandra and XMM-Newton observations

Theoretical studies of the physical processes guiding the formation and evolution of galaxies and galaxy clusters in the X-ray are mainly based on the results of numerical hydrodynamical N-body simulations, which in turn are often directly compared to X-ray observations. Although trivial in principle, these comparisons are not always simple. We demonstrate that the projected spectroscopic temperature of thermally complex clusters obtained from X-ray observations is always lower than the emission-weighed temperature, which is widely used in the analysis of numerical simulations. We show that this temperature bias is mainly related to the fact that the emission-weighted temperature does not reflect the actual spectral properties of the observed source. This has important implications for the study of thermal structures in clusters, especially when strong temperature gradients, like shock fronts, are present. Because of this bias, in real observations shock fronts appear much weaker than what is predicted by emission-weighted temperature maps, and may even not be detected. This may explain why, although numerical simulations predict that shock fronts are a quite common feature in clusters of galaxies, to date there are very few observations of objects in which they are clearly seen. To fix this problem we propose a new formula, the spectroscopic-like temperature function, and show that, for temperature larger than 3 keV, it approximates the spectroscopic temperature better than few per cent, making simulations more directly comparable to observations.

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arXiv2003

Simulating Chandra observations of galaxy clusters

Numerical hydro-N-body simulations are very important tools for making theoretical predictions for the formation of galaxy clusters. They show that the atmospheres of clusters of galaxies have quite complex angular and thermal structures. The full understanding of the physical processes behind these features can be only achieved by direct comparison of observations to hydro-N-body simulations. Although simple in principle, these comparisons are not always trivial. In fact, real data are convolved with the instrument response which may substantially influence the apparent properties of the studied features. To overcome this problem we build the software package X-MAS devoted to simulate X-ray observations of galaxy clusters obtained from hydro-N-body simulations. In this paper we present how this software package works and discuss its application to the simulation of Chandra ACIS-S3 observations. We compare some of the main physical properties of the input data to the ones derived from simulated observations after performing a standard imaging and spectral analysis. We show that, if the thermal structure of the cluster along a particular line of sight is quite complex, the projected spectroscopic temperature obtained from the observation is significantly lower than the emission-weighed value inferred directly from hydrodynamical simulation. This implies that much attention must be paid in the theoretical interpretation of observational temperatures.

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