arXiv subjects

A. E. Evrard

Publications and source records attributed to A. E. Evrard.

180 records · Page 10Linked to original sources

arXiv2008

The Effect of Gas Physics on the Halo Mass Function

Cosmological tests based on cluster counts require accurate calibration of the space density of massive halos, but most calibrations to date have ignored complex gas physics associated with halo baryons. We explore the sensitivity of the halo mass function to baryon physics using two pairs of gas-dynamic simulations that are likely to bracket the true behavior. Each pair consists of a baseline model involving only gravity and shock heating, and a refined physics model aimed at reproducing the observed scaling of the hot, intracluster gas phase. One pair consists of billion-particle re-simulations of the original 500 Mpc/h Millennium Simulation of Springel et al. (2005), run with the SPH code Gadget-2 and using a refined physics treatment approximated by preheating (PH) at high redshift. The other pair are high-resolution simulations from the adaptive-mesh refinement code ART, for which the refined treatment includes cooling, star formation, and supernova feedback (CSF). We find that, although the mass functions of the gravity-only (GO) treatments are consistent with the recent calibration of Tinker et al. (2008), both pairs of simulations with refined baryon physics show significant deviations. Relative to the GO case, the masses of ~10^{14} Msun/h halos in the PH and CSF treatments are shifted by averages of -15 \pm 1 percent and +12 \pm 5 percent, respectively. These mass shifts cause ~ 30% deviations in number density relative to the Tinker function, significantly larger than the 5% statistical uncertainty of that calibration.

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arXiv2008

The L_X--M relation of Clusters of Galaxies

We present a new measurement of the scaling relation between X-ray luminosity and total mass for 17,000 galaxy clusters in the maxBCG cluster sample. Stacking sub-samples within fixed ranges of optical richness, N_200, we measure the mean 0.1-2.4 keV X-ray luminosity, , from the ROSAT All-Sky Survey. The mean mass, , is measured from weak gravitational lensing of SDSS background galaxies (Johnston et al. 2007). For 9 <= N_200 < 200, the data are well fit by a power-law, /10^42 h^-2 erg/s = (12.6+1.4-1.3 (stat) +/- 1.6 (sys)) ( /10^14 h^-1 M_sun)^1.65+/-0.13. The slope agrees to within 10% with previous estimates based on X-ray selected catalogs, implying that the covariance in L_X and N_200 at fixed halo mass is not large. The luminosity intercent is 30%, or 2σ, lower than determined from the X-ray flux-limited sample of Reiprich & Bohringer (2002), assuming hydrostatic equilibrium. This difference could arise from a combination of Malmquist bias and/or systematic error in hydrostatic mass estimates, both of which are expected. The intercept agrees with that derived by Stanek et al. (2006) using a model for the statistical correspondence between clusters and halos in a WMAP3 cosmology with power spectrum normalization sigma_8 = 0.85. Similar exercises applied to future data sets will allow constraints on the covariance among optical and hot gas properties of clusters at fixed mass.

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arXiv2007

Red Sequence Cluster Finding in the Millennium Simulation

We investigate halo mass selection properties of red-sequence cluster finders using galaxy populations of the Millennium Simulation (MS). A clear red sequence exists for MS galaxies in massive halos at redshifts z < 1, and we use this knowledge to inform a cluster-finding algorithm applied to 500 Mpc/h projections of the simulated volume. At low redshift (z=0.4), we find that 90% of the clusters found have galaxy membership dominated by a single, real-space halo, and that 10% are blended systems for which no single halo contributes a majority of a cluster's membership. At z=1, the fraction of blends increases to 22%, as weaker redshift evolution in observed color extends the comoving length probed by a fixed range of color. Other factors contributing to the increased blending at high-z include broadening of the red sequence and confusion from a larger number of intermediate mass halos hosting bright red galaxies of magnitude similar to those in higher mass halos. Our method produces catalogs of cluster candidates whose halo mass selection function, p(M|\Ngal,z), is characterized by a bimodal log-normal model with a dominant component that reproduces well the real-space distribution, and a redshift-dependent tail that is broader and displaced by a factor ~2 lower in mass. We discuss implications for X-ray properties of optically selected clusters and offer ideas for improving both mock catalogs and cluster-finding in future surveys.

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arXiv2007

Virial Scaling of Massive Dark Matter Halos: Why Clusters Prefer a High Normalization Cosmology

We present a precise estimate of the bulk virial scaling relation of halos formed via hierarchical clustering in an ensemble of simulated cold dark matter cosmologies. The result is insensitive to cosmological parameters, the presence of a trace, dissipationless gas component, and numerical resolution down to a limit of ~1000 particles. The dark matter velocity dispersion scales with total mass as log(sigma_{DM}(M,z)) = log(1082.9 +- 4.0 \kms) + (0.3361 +- 0.0026) log(h(z)M_{200}/10^{15} Msun), with h(z) the dimensionless Hubble parameter. At fixed mass, the velocity dispersion likelihood is nearly log-normal, with scatter sigma_{ln sigma} = 0.0426 +- 0.015, except for a tail to higher dispersions containing 10% of the population that are merger transients. We combine this relation with the halo mass function in LCDM models, and show that a low normalization condition, S_8 = sigma_8(Omega_m/0.3)^{0.35} \= 0.69, favored by recent WMAP and SDSS analysis requires that galaxy and gas specific energies in rich clusters be 50% larger than that of the underlying dark matter. Such large energetic biases are in conflict with the current generation of direct simulations of cluster formation. A higher normalization, S_8 = 0.80, alleviates this tension and implies that the hot gas fraction within r_{500} is (0.71 +- 0.09) h_{70}^{-3/2} Omega_b/Ω_m, a value consistent with recent Sunyaev-Zel'dovich observations.

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arXiv2007

Effects of Selection and Covariance on X-ray Scaling Relations of Galaxy Clusters

We explore how the behavior of galaxy cluster scaling relations are affected by flux-limited selection biases and intrinsic covariance among observable properties. Our models presume log-normal covariance between luminosity (L) and temperature (T) at fixed mass (M), centered on evolving, power-law mean relations as a function of host halo mass. Selection can mimic evolution; the \lm and \lt relations from shallow X-ray flux-limited samples will deviate from mass-limited expectations at nearly all scales while the relations from deep surveys ($10^{-14} \cgsflux$) become complete, and therefore unbiased, at masses above $\sims 2 \times 10^{14} \hinv \msol$. We derive expressions for low-order moments of the luminosity distribution at fixed temperature, and show that the slope and scatter of the \lt relation observed in flux-limited samples is sensitive to the assumed \lt correlation coefficient. In addition, \lt covariance affects the redshift behavior of halo counts and mean luminosity in a manner that is nearly degenerate with intrinsic population evolution.

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arXiv2006

The X-Ray Luminosity--Mass Relation for Local Clusters of Galaxies

We investigate the relationship between soft \xray luminosity and mass for low redshift clusters of galaxies by comparing observed number counts to expectations of $Λ$CDM cosmologies. We use a three-parameter model for the conditional probability of luminosity given mass and epoch, described as a log-normal distribution of fixed width centered on a power-law scaling relation, $L \spropto M^p\rhoc^s(z)$. We use an ensemble of simulated clusters to argue that the observed, intrinsic variance in the temperature--luminosity relation is directly indicative of mass--luminosity variance, and derive $\sigm \se 0.43 \pm 0.06$ from HIFLUGCS data. Adding this to the likelihood analysis results in best-fit estimates $p \se 1.59 \pm 0.05$, $\lnlf \se 1.34 \pm 0.09$, and $\sigm \se 0.37 \pm 0.05$ for self-similar redshift evolution in a concordance ($Ω_m \se 0.3$, $Ω_Λ\se 0.7$, $σ_8 \se0.9$) universe. We show that the present-epoch intercept is very sensitive to power spectrum normalization, $\lnlf \spropto \sigate^{-4}$, and the slope is weakly sensitive to the matter density, $p \spropto Ω_m^{1/2}$. The intercept derived here is dimmer by a factor 2, and slope slightly steeper, than the L-M relation published using hydrostatic mass estimates of the HIFLUGCS sample. We show that this discrepancy is largely due to Malmquist bias of the \xray\ flux-limited sample. In light of new WMAP constraints, we discuss the interplay between parameters and sources of systematic error, and offer a compromise model with $Ω_m \se 0.24$, $σ_8 \se 0.85$, and somewhat lower scatter $\sigm \se 0.25$, in which hydrostatic mass estimates remain accurate to $\ssim 15%$. We stress the need for independent calibration of the L-M relation via weak gravitational lensing.

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arXiv2004

Shapes and Alignments of Galaxy Cluster Halos

We present distribution functions and spatial correlations of the shapes of dark matter halos derived from Hubble Volume simulations of a LambdaCDM universe. We measure both position and velocity shapes within spheres encompassing mean density 200 times the critical value, and calibrate small-N systematic errors using Poisson realizations of isothermal spheres and higher resolution simulations. For halos more massive than 3x10^{14} Msun/h, the shape distribution function peaks at (minor/major, intermediate/major) axial ratios of (0.64,0.76) in position, and is rounder in velocity, peaking at (0.72,0.82). Halo shapes are rounder at lower mass and/or redshift; the mean minor axis ratio in position follows (M,z) = c_{15,0} [1-α\ln(M/10^{15}Msun/h)] (1+z)^{-ε}, with c_{15,0}=0.631 \pm 0.001, α=0.023 \pm 0.002 and ε=0.086 \pm 0.004. Position and velocity principal axes are well aligned in direction, with median alignment angle $22^\circ$, and the axial ratios in these spaces are correlated in magnitude. We investigate mark correlations of halo pair orientations using two measures: a simple scalar product shows $\ge 1%$ alignment extending to $30 \hinv \mpc$ while a filamentary statistic exhibits non-random alignment extending to scales $\sims 200 \hinv \mpc$, ten times the sample two-point correlation length and well into the regime of negative two-point correlation. Cluster shapes are unaffected by the large-scale environment; the shape distribution of supercluster members is consistent with that of the general population.

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arXiv2002

Galaxy Clusters in Hubble Volume Simulations: Cosmological Constraints from Sky Survey Populations

We use giga-particle N-body simulations to study galaxy cluster populations in Hubble Volumes of LCDM (Omega_m=0.3, Omega_Lambda=0.7) and tCDM (Omega_m=1) world models. Mapping past light-cones of locations in the computational space, we create mock sky surveys of dark matter structure to z~1.4 over 10,000 sq deg and to z~0.5 over two full spheres. Calibrating the Jenkins mass function at z=0 with samples of ~1.5 million clusters, we show that the fit describes the sky survey counts to <~20% acccuracy over all redshifts for systems larger than poor groups (M>5e13 Msun/h). Fitting the observed local temperature function determines the ratio beta of specific thermal energies in dark matter and intracluster gas. We derive a scaling with power spectrum normalization beta \propto sigma8^{5/3}, and measure a 4% error on sigma8 arising from cosmic variance in temperature-limited cluster samples. Considering distant clusters, the LCDM model matches EMSS and RDCS X-ray-selected survey observations under economical assumptions for intracluster gas evolution. Using transformations of mass-limited cluster samples that mimic sigma8 variation, we explore SZ search expectations for a 10 sq deg survey complete above 10^{14} Msun/h. Cluster counts are shown to be extremely sensitive to sigma8 uncertainty while redshift statistics, such as the sample median, are much more stable. For LCDM, the characteristic temperature at fixed sky surface density is a weak function of redshift, implying an abundance of hot clusters at z>1. Assuming constant beta, four kT>8 keV clusters lie at z>2 and 40 kT>5 keV clusters lie at z>3 on the whole sky. Detection of Coma-sized clusters at z>1 violate LCDM at 95% confidence if their surface density exceeds 0.003 per sq deg, or 120 on the whole sky.

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arXiv2001

Simulations of Deep Pencil-Beam Redshift Surveys

We create mock pencil-beam redshift surveys from very large cosmological $N$-body simulations of two Cold Dark Matter cosmogonies, an Einstein-de Sitter model ($τ$CDM) and a flat model with $Ω_0 =0.3$ and a cosmological constant ($Λ$CDM). We use these to assess the significance of the apparent periodicity discovered by Broadhurst et al. (1990). Simulation particles are tagged as `galaxies' so as to reproduce observed present-day correlations. They are then identified along the past light-cones of hypothetical observers to create mock catalogues with the geometry and the distance distribution of the Broadhurst et al. data. We produce 1936 (2625) quasi-independent catalogues from our $τ$CDM ($Λ$CDM) simulation. A couple of large clumps in a catalogue can produce a high peak at low wavenumbers in the corresponding one-dimensional power spectrum, without any apparent large-scale periodicity in the original redshift histogram. Although the simulated redshift histograms frequently display regularly spaced clumps, the spacing of these clumps varies between catalogues and there is no `preferred' period over our many realisations. We find only a 0.72 (0.49) per cent chance that the highest peak in the power spectrum of a $τ$CDM ($Λ$CDM) catalogue has a peak-to-noise ratio higher than that in the Broadhurst et al. data. None of the simulated catalogues with such high peaks shows coherently spaced clumps with a significance as high as that of the real data. We conclude that in CDM universes, the kind of regularity observed by Broadhurst et al. has a priori probability well below $10^{-3}$.

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arXiv2000

Mass function of dark matter halos

We combine data from a number of N-body simulations to predict the abundance of dark halos in Cold Dark Matter universes over more than 4 orders of magnitude in mass. A comparison of different simulations suggests that the dominant uncertainty in our results is systematic and is smaller than 10--30% at all masses, depending on the halo definition used. In particular, our ``Hubble Volume'' simulations of \tcdm and \lcdm cosmologies allow the abundance of massive clusters to be predicted with uncertainties well below those expected in all currently planned observational surveys. We show that for a range of CDM cosmologies and for a suitable halo definition, the simulated mass function is almost independent of epoch, of cosmological parameters, and of initial power spectrum when expressed in appropriate variables. This universality is of exactly the kind predicted by the familiar Press-Schechter model, although this model predicts a mass function shape which differs from our numerical results, overestimating the abundance of ``typical'' halos and underestimating that of massive systems.

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arXiv2000

Public Release of N-body simulation and related data by the Virgo consortium

We are making available on the WWW a selection of the archived data from N-body simulations carried out by the Virgo consortium and related groups. This currently includes: (i) time-slice, lightcone and cluster data from the two $10^9$-particle Hubble volume simulations described by Evrard 1998; (ii) time-slice data from simulations of 4 different cold dark matter cosmological models with $256^3$ particles analysed by Jenkins et al 1998; (iii) Dark halo catalogs, merger trees and galaxy catalogs from the GIF project described by Kauffmann et al 1999. Basic software is supplied to read the data. The data can be accessed from: http://www.mpa-garching.mpg.de/Virgo/data_download.html

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arXiv2000

Clustering of Galaxy Clusters in CDM Universes

We use very large cosmological N--body simulations to obtain accurate predictions for the two-point correlations and power spectra of mass-limited samples of galaxy clusters. We consider two currently popular cold dark matter (CDM) cosmogonies, a critical density model ($τ$CDM) and a flat low density model with a cosmological constant ($Λ$CDM). Our simulations each use $10^9$ particles to follow the mass distribution within cubes of side $2h^{-1}$Gpc ($τ$CDM) and $3h^{-1}$Gpc ($Λ$CDM) with a force resolution better than $10^{-4}$ of the cube side. We investigate how the predicted cluster correlations increase for samples of increasing mass and decreasing abundance. Very similar behaviour is found in the two cases. The correlation length increases from $r_0=12$ -- 13$h^{-1}$Mpc for samples with mean separation $d_{\rm c}=30h^{-1}$Mpc to $r_0=22$-- 27$h^{-1}$Mpc for samples with $d_{\rm c}=100h^{-1}$Mpc. The lower value here corresponds to $τ$CDM and the upper to $Λ$CDM. The power spectra of these cluster samples are accurately parallel to those of the mass over more than a decade in scale. Both correlation lengths and power spectrum biases can be predicted to better than 10% using the simple model of Sheth, Mo & Tormen (2000). This prediction requires only the linear mass power spectrum and has no adjustable parameters. We compare our predictions with published results for the APM cluster sample. The observed variation of correlation length with richness agrees well with the models, particularly for $Λ$CDM. The observed power spectrum (for a cluster sample of mean separation $d_{\rm c}=31h^{-1}$Mpc) lies significantly above the predictions of both models.

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arXiv1999

The Santa Barbara cluster comparison project: a comparison of cosmological hydrodynamics solutions

We have simulated the formation of an X-ray cluster in a cold dark matter universe using 12 different codes. The codes span the range of numerical techniques and implementations currently in use, including SPH and grid methods with fixed, deformable or multilevel meshes. The goal of this comparison is to assess the reliability of cosmological gas dynamical simulations of clusters in the simplest astrophysically relevant case, that in which the gas is assumed to be non-radiative. We compare images of the cluster at different epochs, global properties such as mass, temperature and X-ray luminosity, and radial profiles of various dynamical and thermodynamical quantities. On the whole, the agreement among the various simulations is gratifying although a number of discrepancies exist. Agreement is best for properties of the dark matter and worst for the total X-ray luminosity. Even in this case, simulations that adequately resolve the core radius of the gas distribution predict total X-ray luminosities that agree to within a factor of two. Other quantities are reproduced to much higher accuracy. For example, the temperature and gas mass fraction within the virial radius agree to about 10%, and the ratio of specific kinetic to thermal energies of the gas agree to about 5%. Various factors contribute to the spread in calculated cluster properties, including differences in the internal timing of the simulations. Based on the overall consistency of results, we discuss a number of general properties of the cluster we have modelled.

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arXiv1999

The Effects of Clumping and Substructure on ICM Mass Measurements

We examine an ensemble of 48 simulated clusters to determine the effects of small-scale density fluctuations and large-scale substructure on X-ray measurements of the intracluster medium (ICM) mass. We measure RMS density fluctuations in the ICM which can be characterized by a mean mass-weighted clumping factor C = / ^2 between 1.3 and 1.4 within a density contrast of 500 times the critical density. These fluctuations arise from the cluster history of accretion shocks and major mergers, and their presence enhances the cluster's luminosity relative to the smooth case. We expect, therefore, that ICM mass measurements utilizing models which assume uniform density at a given radius carry a bias of order sqrt(C) = 1.16. We verify this result by performing ICM mass measurements on X-ray images of the simulations and finding the expected level of bias. The varied cluster morphologies in our ensemble also allow us to investigate the effects of departures from spherical symmetry on our measurements. We find that the presence of large-scale substructure does not further bias the resulting gas mass unless it is pronounced enough to produce a second peak in the image of at least 1% the maximum surface brightness. We analyze the subset of images with no secondary peaks and find a bias of 9% and a Gaussian random error of 4% in the derived mass.

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arXiv1999

A Multiphase Model for the Intracluster Medium

Constraints on the clustered mass density Ω_m of the universe derived from the mean intracluster gas fraction of X-ray clusters may be biased by a single-phase assumption for the thermodynamic structure of the intracluster medium (ICM). We propose a descriptive model for multiphase structure in which a spherically symmetric ICM contains isobaric density perturbations with radially dependent variance σ^2(r)=σ_c^2 (1+r^2/r_c^2)^{-\eps}. The model extends the work of Gunn & Thomas (1996) which assumed radially independent density fluctuations thoughout the ICM. Fixing the X-ray emission profile and emission weighted temperature, we explore two independently observable signatures of the model in the {σ_c,\eps} space. For bremsstrahlung dominated emission, the central Sunyaev--Zeldovich (SZ) decrement in the multiphase case is increased over the single-phase case and multiphase X-ray spectra in the range 0.1-20 keV are flatter in the continuum and exhibit stronger low energy emission lines than their single-phase counterpart. We quantify these effects for a fiducial 10^8 K cluster and demonstrate how the combination of SZ and X-ray spectroscopy can be used to identify a preferred location in the {σ_c,\eps} plane. From these parameters, the correct value of mean ICM gas fraction in the multiphase model results, allowing an unbiased estimate of Ω_m to be recovered. The consistency of recent determinations of the Hubble constant from SZ and X-ray observations with values determined by other methods suggests that biases in ICM gas fractions are small, \ltsim 20%.

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arXiv1997

Constraints on Ω_0 and Cluster Evolution Using the ROSAT LogN-LogS

We examine the likelihoods of different cosmological models and cluster evolutionary histories by comparing semi-analytical predictions of X-ray cluster number counts to observational data from the ROSAT satellite. We model cluster abundance as a function of mass and redshift using a Press-Schechter distribution, and assume the temperature T(M,z) and bolometric luminosity L_X(M,z) scale as power laws in mass and epoch, in order to construct expected counts as a function of X-ray flux. The L_X-M scaling is fixed using the local luminosity function while the degree of evolution in the X-ray luminosity with redshift L_X \propto (1+z)^s is left open, with s an interesting free parameter which we investigate. We examine open and flat cosmologies with initial, scale-free fluctuation spectra having indices n = 0, -1 and -2. An independent constraint arising from the slope of the luminosity-temperature relation strongly favors the n = -2 spectrum. The expected counts demonstrate a strong dependence on Ω_0 and s, with lesser dependence on λ_0 and n. Comparison with the observed counts reveals a "ridge" of acceptable models in the Ω_0 - s plane, roughly following the relation s = 6 Ω_0 and spanning low-density models with a small degree of evolution to Ω= 1 models with strong evolution. Models with moderate evolution are revealed to have a strong lower limit of Ω_0 \gtrsim 0.3, and low-evolution models imply that Ω_0 < 1 at a very high confidence level. We suggest observational tests for breaking the degeneracy along this ridge, and discuss implications for evolutionary histories of the intracluster medium.

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arXiv1995

GALAXY DYNAMICS IN CLUSTERS

We use high resolution simulations to study the formation and distribution of galaxies within a cluster which forms hierarchically. We follow both dark matter and baryonic gas which is subject to thermal pressure, shocks and radiative cooling. Galaxy formation is identified with the dissipative collapse of the gas into cold, compact knots. We examine two extreme representations of galaxies during subsequent cluster evolution --- one purely gaseous and the other purely stellar. The results are quite sensitive to this choice. Gas-galaxies merge efficiently with a dominant central object while star-galaxies merge less frequently. Thus, simulations in which galaxies remain gaseous appear to suffer an ``overmerging'' problem, but this problem is much less severe if the gas is allowed to turn into stars. We compare the kinematics of the galaxy population in these two representations to that of dark halos and of the underlying dark matter distribution. Galaxies in the stellar representation are positively biased (\ie over-represented in the cluster) both by number and by mass fraction. Both representations predict the galaxies to be more centrally concentrated than the dark matter, whereas the dark halo population is more extended. A modest velocity bias also exists in both representations, with the largest effect, $σ_{gal}/σ_{DM} \simeq 0.7$, found for the more massive star-galaxies. Phase diagrams show that the galaxy population has a substantial net inflow in the gas representation, while in the stellar case it is roughly in hydrostatic equilibrium. Virial mass estimators can underestimate the true cluster mass by up to a factor of 5. The discrepancy is largest if only the most massive galaxies are used, reflecting significant mass segregation.

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arXiv1995

COSMOLOGICAL CONSTRAINTS FROM CLUSTER X-RAY MORPHOLOGIES

We use a representative sample of 65 galaxy clusters observed with the Einstein IPC to constrain the range of cluster X-ray morphologies. We develop and apply quantitative and reproducible measures to constrain the intrinsic distributions of emission weighted centroid variation, axial ratio, orientation, and the radial fall--off. We then use the range of cluster X--ray morphologies to constrain three generic cosmological models (Omega=1, Omega_0=0.2, and Omega_0=0.2 & lambda_0=0.8). For each of these models, we evolve eight sets of Gaussian random initial conditions consistent with an effective power spectrum $P(k)\propto k^{-1}$ on cluster scales. Using this sample of 24 numerical cluster simulations, we compare the X--ray morphologies of the observed and simulated clusters. The comparisons indicate that: (i) cluster centroid variations, axial ratios, and radial fall-offs are sensitive to the underlying cosmological model, and (ii) galaxy clusters with the observed range of X--ray morphologies are very unlikely in low Omega_0 cosmologies. The analysis favors the Omega=1 model, though some discrepancies remain. We discuss the effects of changing the initial conditions and including additional physics in the simulations.

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