arXiv subjects

Henk Hoekstra

Publications and source records attributed to Henk Hoekstra.

At least 181 records · Page 10Linked to original sources

arXiv2010

Spectroscopic Confirmation of Three Red-Sequence Selected Galaxy Clusters at z=0.87, 1.16 and 1.21 from the SpARCS Survey

The Spitzer Adaptation of the Red-sequence Cluster Survey (SpARCS) is a z'-passband imaging survey of the 50 deg^2 Spitzer SWIRE Legacy fields, designed with the primary aim of creating the first large, homogeneously selected sample of massive clusters at z>1. SpARCS uses an infrared adaptation of the two-filter cluster red-sequence technique. In this paper we report Keck/LRIS spectroscopic confirmation of two new exceptionally rich galaxy clusters, SpARCS J161315+564930 at z=0.871+/-0.002, with 14 high-confidence members and a rest-frame velocity dispersion of sigma_v= 1230+/-320 km s^-1, and SpARCS J161641+554513 at z=1.161+/-0.003, with seven high-confidence members (including one AGN) and a rest-frame velocity dispersion of sigma_v=950+/-330 km s^-1. We also report confirmation of a third new system, SpARCS J161037+552417 at z=1.210+/-0.002, with seven high-confidence members and a rest-frame velocity dispersion of sigma_v=410+/-300 km s^-1. These three new spectroscopically confirmed clusters further demonstrate the efficiency and effectiveness of two-filter imaging for detecting bona fide galaxy clusters at high redshift. We conclude by demonstrating that prospects are good for the current generation of surveys aiming to estimate cluster redshifts and masses at z>~1 directly from optical-infrared imaging.

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arXiv2009

Large-scale Structure and Dynamics of the Most X-ray Luminous Galaxy Cluster Known -- RX J1347-1145

We present photometric, spectroscopic and weak lensing analysis of the large-scale structure and dynamics of the most X-ray luminous galaxy cluster known, RX J1347-1145, at z=0.451. We spectroscopically confirmed 47 new members with LDSS3 on the Magellan telescope. Together with previously known members, we measure a new velocity dispersion of 1163(+/-97) km/s. The mass inferred from our velocity dispersion is M200 = 1.16^{+0.32}_{-0.27}x10^{15} solar mass, with r200=1.85Mpc, under the assumption of a singular isothermal sphere. We also present a weak lensing analysis using deep CFHT data on this cluster, and find a deprojected mass of 1.47^{+0.46}_{-0.43}x10^{15} solar mass within r200, in excellent agreement with our dynamical estimate. Thus, our new dynamical mass estimate is consistent with that from weak lensing and X-ray studies in the literature, resolving a previously claimed discrepancy. We photometrically detect and spectroscopically confirm another massive cluster with sigma=780(+/-100) km/s and M200=3.4^{+1.4}_{-1.1}x10^{14} solar mass ~7Mpc south-west of RX J1347-1145, which we refer to as RXJ1347-SW. Our spectroscopic survey reveals a possible excess of galaxies in velocity space in the region between RX J1347-1145 and RXJ1347-SW; comparing with simulations, this excess appears consistent with that expected from a large filamentary structure traced by galaxies connecting these two clusters.

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arXiv2009

Handbook for the GREAT08 Challenge: An image analysis competition for cosmological lensing

The GRavitational lEnsing Accuracy Testing 2008 (GREAT08) Challenge focuses on a problem that is of crucial importance for future observations in cosmology. The shapes of distant galaxies can be used to determine the properties of dark energy and the nature of gravity, because light from those galaxies is bent by gravity from the intervening dark matter. The observed galaxy images appear distorted, although only slightly, and their shapes must be precisely disentangled from the effects of pixelisation, convolution and noise. The worldwide gravitational lensing community has made significant progress in techniques to measure these distortions via the Shear TEsting Program (STEP). Via STEP, we have run challenges within our own community, and come to recognise that this particular image analysis problem is ideally matched to experts in statistical inference, inverse problems and computational learning. Thus, in order to continue the progress seen in recent years, we are seeking an infusion of new ideas from these communities. This document details the GREAT08 Challenge for potential participants. Please visit http://www.great08challenge.info for the latest information.

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arXiv2009

Sersiclets - A Matched Filter Extension of Shapelets for Weak Lensing Studies

The precision study of dark matter using weak lensing by large scale structure is strongly constrained by the accuracy with which one can measure galaxy shapes. Several methods have been devised but none have demonstrated the ability to reach the level of precision required by future weak lensing surveys. In this Letter we explore new avenues to the existing Shapelets approach, combining a priori knowledge of the galaxy profile with the power of orthogonal basis function decomposition. This Letter discusses the new issues raised by this matched filter approach and proposes promising alternatives to shape measurement techniques. In particular it appears that the use of a matched filter (e.g. Sersic profile) restricted to elliptical radial fitting functions resolves several well known Shapelet issues.

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arXiv2008

The relation between stellar mass and weak lensing signal around galaxies: Implications for MOND

We study the amplitude of the weak gravitational lensing signal as a function of stellar mass around a sample of relatively isolated galaxies. This selection of lenses simplifies the interpretation of the observations, which consist of data from the Red- sequence Cluster Survey and the Sloan Digital Sky Survey. We find that the amplitude of the lensing signal as a function of stellar mass is well described by a power law with a best fit slope α= 0.74 \pm 0.08. This result is inconsistent with Modified Newtonian Dynamics, which predicts α= 0.5 (we find α> 0.5 with 99.7% confidence). As a related test, we determine the MOND mass-to-light ratio as a function of luminosity. Our results require dark matter for the most luminous galaxies (L >=10^11 L_sun). We rule out an extended halo of gas or active neutrinos as a way of reconciling our findings with MOND. Although we focus on a single alternative gravity model, we note that our results provide an important test for any alternative theory of gravity.

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arXiv2008

Spectroscopic Confirmation of Two Massive Red-Sequence-Selected Galaxy Clusters at z ~ 1.2 in the SpARCS-North Cluster Survey

The Spitzer Adaptation of the Red-sequence Cluster Survey (SpARCS) is a deep z'-band imaging survey covering the Spitzer SWIRE Legacy fields designed to create the first large homogeneously-selected sample of massive clusters at z > 1 using an infrared adaptation of the cluster red-sequence method. We present an overview of the northern component of the survey which has been observed with CFHT/MegaCam and covers 28.3 deg^2. The southern component of the survey was observed with CTIO/MOSAICII, covers 13.6 deg^2, and is summarized in a companion paper by Wilson et al. (2008). We also present spectroscopic confirmation of two rich cluster candidates at z ~ 1.2. Based on Nod-and-Shuffle spectroscopy from GMOS-N on Gemini there are 17 and 28 confirmed cluster members in SpARCS J163435+402151 and SpARCS J163852+403843 which have spectroscopic redshifts of 1.1798 and 1.1963, respectively. The clusters have velocity dispersions of 490 +/- 140 km/s and 650 +/- 160 km/s, respectively which imply masses (M200) of (1.0 +/- 0.9) x 10^{14} M_{solar} and (2.4 +/- 1.8) x 10^{14} M_{solar}. Confirmation of these candidates as bona fide massive clusters demonstrates that two-filter imaging is an effective, yet observationally efficient, method for selecting clusters at z > 1.

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arXiv2008

Spectroscopic Confirmation of a Massive Red-Sequence-Selected Galaxy Cluster at z = 1.34 in the SpARCS-South Cluster Survey

The Spitzer Adaptation of the Red-sequence Cluster Survey (SpARCS) is a z'-passband imaging survey, consisting of deep (z' ~ 24 AB) observations made from both hemispheres using the CFHT 3.6m and CTIO 4m telescopes. The survey was designed with the primary aim of detecting galaxy clusters at z >~ 1. In tandem with pre-existing 3.6um observations from the Spitzer Space Telescope SWIRE Legacy Survey, SpARCS detects clusters using an infrared adaptation of the two-filter red-sequence cluster technique. The total effective area of the SpARCS cluster survey is 41.9 deg^2. In this paper, we provide an overview of the 13.6 deg^2 Southern CTIO/MOSAICII observations. The 28.3 deg^2 Northern CFHT/MegaCam observations are summarized in a companion paper by Muzzin et al. (2008). In this paper, we also report spectroscopic confirmation of SpARCS J003550-431224, a very rich galaxy cluster at z = 1.335, discovered in the ELAIS-S1 field. To date, this is the highest spectroscopically confirmed redshift for a galaxy cluster discovered using the red-sequence technique. Based on nine confirmed members, SpARCS J003550-431224 has a preliminary velocity dispersion of 1050 +/- 230 km/s. With its proven capability for efficient cluster detection, SpARCS is a demonstration that we have entered an era of large, homogeneously-selected z > 1 cluster surveys.

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arXiv2008

The X-ray Cluster Normalization of the Matter Power Spectrum

The number density of galaxy clusters provides tight statistical constraints on the matter fluctuation power spectrum normalization, traditionally phrased in terms of sigma_8, the root mean square mass fluctuation in spheres with radius 8 h^-1 Mpc. We present constraints on sigma_8 and the total matter density Omega_m0 from local cluster counts as a function of X-ray temperature, taking care to incorporate and minimize systematic errors that plagued previous work with this method. In particular, we present new determinations of the cluster luminosity - temperature and mass - temperature relations, including their intrinsic scatter, and a determination of the Jenkins mass function parameters for the same mass definition as the mass - temperature calibration. Marginalizing over the 12 uninteresting parameters associated with this method, we find that the local cluster temperature function implies sigma_8 (Omega_m0/0.32)^alpha = 0.86+/-0.04 with alpha = 0.30 (0.41) for Omega_m0 < 0.32 (Omega_mo > 0.32) (68% confidence for two parameters). This result agrees with a wide range of recent independent determinations, and we find no evidence of any additional sources of systematic error for the X-ray cluster temperature function determination of the matter power spectrum normalization. The joint WMAP5 + cluster constraints are: Omega_m0 = 0.30+0.03/-0.02 and sigma_8 = 0.85+0.04/-0.02 (68% confidence for two parameters).

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arXiv2008

Weak Gravitational Lensing and its Cosmological Applications

Weak gravitational lensing is a unique probe of the dark side of the universe: it provides a direct way to map the distribution of dark matter around galaxies, clusters of galaxies and on cosmological scales. Furthermore, the measurement of lensing induced distortions of the shapes of distant galaxies is a powerful probe of dark energy. In this review we describe how lensing measurements are made and interpreted. We discuss various systematic effects that can hamper progress and how they may be overcome. We review some of the recent results in weak lensing by galaxies, galaxy clusters and cosmic shear and discuss the prospects for dark energy measurements from planned surveys.

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arXiv2007

Chandra X-ray Observations of the 0.6 < z < 1.1 Red-Sequence Cluster Survey Sample

We present the results of Chandra observations of 13 optically-selected clusters with 0.6 3; though 3 were not observed long enough to support detailed analysis. Surface brightness profiles are fit to beta-models. Integrated spectra are extracted within R(2500), and Tx and Lx information is obtained. We derive gas and total masses within R(2500) and R(500). Cosmologically corrected scaling relations are investigated, and we find the RCS clusters to be consistent with self-similar scaling expectations. However discrepancies exist between the RCS sample and lower-z X-ray selected samples for relationships involving Lx, with the higher-z RCS clusters having lower Lx for a given Tx. In addition, we find that gas mass fractions within R(2500) for the high-z RCS sample are lower than expected by a factor of ~2. This suggests that the central entropy of these high-z objects has been elevated by processes such as pre-heating, mergers, and/or AGN outbursts, that their gas is still infalling, or that they contain comparatively more baryonic matter in the form of stars. Finally, relationships between red-sequence optical richness (Bgc) and X-ray properties are fit to the data. For systems with measured Tx, we find that optical richness correlates with both Tx and mass, having a scatter of ~30% with mass for both X-ray and optically-selected clusters. However we also find that X-ray luminosity is not well correlated with richness, and that several of our sample appear to be significantly X-ray faint.

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arXiv2007

Multiwavelength Mass Comparisons of the z~0.3 CNOC Cluster Sample

Results are presented from a detailed analysis of optical and X-ray observations of moderate-redshift galaxy clusters from the Canadian Network for Observational Cosmology (CNOC) subsample of the EMSS. The combination of extensive optical and deep X-ray observations of these clusters make them ideal candidates for multiwavelength mass comparison studies. X-ray surface brightness profiles of 14 clusters with 0.17<z<0.55 are constructed from Chandra observations and fit to single and double beta-models. Spatially resolved temperature analysis is performed, indicating that five of the clusters in this sample exhibit temperature gradients within their inner 60-200 kpc. Integrated spectra extracted within R_2500 provide temperature, abundance, and luminosity information. Under assumptions of hydrostatic equilibrium and spherical symmetry, we derive gas and total masses within R_2500 and R_200. We find an average gas mass fraction within R_200 of 0.136 +/- 0.004, resulting in Omega_m=0.28 +/- 0.01 (formal error). We also derive dynamical masses for these clusters to R_200. We find no systematic bias between X-ray and dynamical methods across the sample, with an average M(dyn)/M(X-ray) = 0.97 +/- 0.05. We also compare X-ray masses to weak lensing mass estimates of a subset of our sample, resulting in a weighted average of M(lens)/M(X-ray) of 0.99 +/- 0.07. We investigate X-ray scaling relationships and find powerlaw slopes which are slightly steeper than the predictions of self-similar models, with an E(z)^(-1) Lx-Tx slope of 2.4 +/- 0.2 and an E(z) M_2500-Tx slope of 1.7 +/- 0.1. Relationships between red-sequence optical richness (B_gc,red) and global cluster X-ray properties (Tx, Lx and M_2500) are also examined and fitted.

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arXiv2007

A Multiwavelength Analysis of the Strong Lensing Cluster RCS 022434-0002.5 at z=0.778

We present the results of two (101 ks total) Chandra observations of the z=0.778 optically selected lensing cluster RCS022434-0002.5, along with weak lensing and dynamical analyses of this object. An X-ray spectrum extracted within R(2500) (362 h(70)^(-1) kpc) results in an integrated cluster temperature of 5.1 (+0.9,-0.5) keV. The surface brightness profile of RCS022434-0002.5 indicates the presence of a slight excess of emission in the core. A hardness ratio image of this object reveals that this central emission is primarily produced by soft X-rays. Further investigation yields a cluster cooling time of 3.3 times 10^9 years, which is less than half of the age of the universe at this redshift given the current LCDM cosmology. A weak lensing analysis is performed using HST images, and our weak lensing mass estimate is found to be in good agreement with the X-ray determined mass of the cluster. Spectroscopic analysis reveals that RCS022434-0002.5 has a velocity dispersion of 900 +/- 180 km/s, consistent with its X-ray temperature. The core gas mass fraction of RCS022434-0002.5 is, however, found to be three times lower than expected universal values. The radial distribution of X-ray point sources within R(200) of this cluster peaks at ~0.7 R(200), possibly indicating that the cluster potential is influencing AGN activity at that radius. Correlations between X-ray and radio (VLA) point source positions are also examined.

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arXiv2007

Cosmological Constraints From the 100 Square Degree Weak Lensing Survey

We present a cosmic shear analysis of the 100 square degree weak lensing survey, combining data from the CFHTLS-Wide, RCS, VIRMOS-DESCART and GaBoDS surveys. Spanning ~100 square degrees, with a median source redshift z~0.78, this combined survey allows us to place tight joint constraints on the matter density parameter Omega_m, and the amplitude of the matter power spectrum sigma_8, finding sigma_8*(Omega_m/0.24)^0.59 = 0.84+/-0.05. Tables of the measured shear correlation function and the calculated covariance matrix for each survey are included. The accuracy of our results is a marked improvement on previous work owing to three important differences in our analysis; we correctly account for cosmic variance errors by including a non-Gaussian contribution estimated from numerical simulations; we correct the measured shear for a calibration bias as estimated from simulated data; we model the redshift distribution, n(z), of each survey from the largest deep photometric redshift catalogue currently available from the CFHTLS-Deep. This catalogue is randomly sampled to reproduce the magnitude distribution of each survey with the resulting survey dependent n(z) parametrised using two different models. While our results are consistent for the n(z) models tested, we find that our cosmological parameter constraints depend weakly (at the 5% level) on the inclusion or exclusion of galaxies with low confidence photometric redshift estimates (z>1.5). These high redshift galaxies are relatively few in number but contribute a significant weak lensing signal. It will therefore be important for future weak lensing surveys to obtain near-infra-red data to reliably determine the number of high redshift galaxies in cosmic shear analyses.

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arXiv2007

The Masses and Shapes of Dark Matter Halos from Galaxy-Galaxy Lensing in the CFHTLS

We present the first galaxy-galaxy weak lensing results using early data from the Canada-France-Hawaii Telescope Legacy Survey (CFHTLS). These results are based on ~22 sq. deg. of i' data. From this data, we estimate the average velocity dispersion for an L* galaxy at a redshift of 0.3 to be 137 +- 11 km/s, with a virial mass, M_{200}, of 1.1 +- 0.2 \times 10^{12} h^{-1} Msun and a rest frame R-band mass-to-light ratio of 173 +- 34 h Msun/Lsun. We also investigate various possible sources of systematic error in detail. Additionally, we separate our lens sample into two sub-samples, divided by apparent magnitude, thus average redshift. From this early data we do not detect significant evolution in galaxy dark matter halo mass-to-light ratios from a redshift of 0.45 to 0.27. Finally, we test for non-spherical galaxy dark matter halos. Our results favor a dark matter halo with an ellipticity of ~0.3 at the 2-sigma level when averaged over all galaxies. If the sample of foreground lens galaxies is selected to favor ellipticals, the mean halo ellipticity and significance of this result increase.

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arXiv2007

Comparison of weak lensing masses and X-ray properties of galaxy clusters

We present measurements of the masses of 20 X-ray luminous clusters of galaxies at intermediate redshifts, determined from a weak lensing analysis of deep archival R-band data obtained using the Canada-France-Hawaii-Telescope. Compared to previous work, our analysis accounts for a number of effects that are typically ignored, but can lead to small biases, or incorrect error estimates. We derive masses that are essentially model independent and find that they agree well with measurements of the velocity dispersion of cluster galaxies and with the results of X-ray studies. Assuming a power law between the lensing mass and the X-ray temperature, M_2500 T^alpha, we find a best fit slope of alpha=1.34^{+0.30}_{-0.28}. This slope agrees with self-similar cluster models and studies based on X-ray data alone. For a cluster with a temperature of kT=5keV we obtain a mass M_{2500}=(1.4+-0.2)\times 10^{14}h^{-1}Msun in fair agreement with recent Chandra and XMM studies.

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arXiv2006

The Shear TEsting Programme 2: Factors affecting high precision weak lensing analyses

The Shear TEsting Programme (STEP) is a collaborative project to improve the accuracy and reliability of weak lensing measurement, in preparation for the next generation of wide-field surveys. We review sixteen current and emerging shear measurement methods in a common language, and assess their performance by running them (blindly) on simulated images that contain a known shear signal. We determine the common features of algorithms that most successfully recover the input parameters. We achieve previously unattained discriminatory precision in our analysis, via a combination of more extensive simulations, and pairs of galaxy images that have been rotated with respect to each other, thus removing noise from their intrinsic ellipticities. The robustness of our simulation approach is also confirmed by testing the relative calibration of methods on real data. Weak lensing measurement has improved since the first STEP paper. Several methods now consistently achieve better than 2% precision, and are still being developed. However, the simulations can now distinguish all methods from perfect performance. Our main concern continues to be the potential for a multiplicative shear calibration bias: not least because this can not be internally calibrated with real data. We determine which galaxy populations are responsible and, by adjusting the simulated observing conditions, we also investigate the effects of instrumental and atmospheric parameters. We have isolated several previously unrecognised aspects of galaxy shape measurement, in which focussed development could provide further progress towards the sub-percent level of precision desired for future surveys. [ABRIDGED]

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arXiv2006

Redshift and Shear Calibration: Impact on Cosmic Shear Studies and Survey Design

The cosmological interpretation of weak lensing by large-scale structures requires knowledge of the redshift distribution of the source galaxies. Current lensing surveys are often calibrated using external redshift samples which span a significantly smaller sky area in comparison to the lensing survey, and are thus subject to sample variance. Some future lensing surveys are expected to be calibrated in the same way, in particular the fainter galaxy populations where the entire color coverage, and hence photometric redshift estimate, could be challenging to obtain. Using N-body simulations, we study the impact of this sample variance on cosmic shear analysis and show that, to first approximation, it behaves like a shear calibration error 1+/-epsilon. Using the Hubble Deep Field as a redshift calibration survey could therefore be a problem for current lensing surveys. We discuss the impact of the redshift distribution sampling error and a shear calibration error on the design of future lensing surveys, and find that a lensing survey of area Theta square degrees and limiting magnitude m_lim}, has a minimum shear and redshift calibration accuracy requirements given by epsilon = epsilon_0 10^{beta(m_lim-24.5)} / sqrt(Theta/ 200). Above that limit, lensing surveys would not reach their full potential. Using the galaxy number counts from the Hubble Ultra-Deep Field, we find (epsilon_0,beta)=(0.015,-0.18) and (epsilon_0,beta)=(0.011,-0.23) for ground and space based surveys respectively. Lensing surveys with no or limited redshift information and/or poor shear calibration accuracy will loose their potential to analyse the cosmic shear signal in the sub-degree angular scales, and therefore complete photometric redshift coverage should be a top priority for future lensing surveys.

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arXiv2006

Clusters of Galaxies at 1 < z < 2 : The Spitzer Adaptation of the Red-Sequence Cluster Survey

As the densest galaxy environments in the universe, clusters are vital to our understanding of the role that environment plays in galaxy formation and evolution. Unfortunately, the evolution of high-redshift cluster galaxies is poorly understood because of the ``cluster desert'' that exists at 1 < z < 2. The SpARCS collaboration is currently carrying out a 1-passband (z') imaging survey which, when combined with the pre-existing 50 square degree 3.6 micron Spitzer SWIRE Legacy Survey data, will efficiently detect hundreds of clusters in the cluster desert using an infrared application of the well-proven cluster red-sequence technique. We have already tested this 1-color (z' - [3.6]) approach using a 6 square degree ``pilot patch'' and shown it to be extremely successful at detecting clusters at 1 < z < 2. The clusters discovered in this project will be the first large sample of ``nascent'' galaxy clusters which connect the star-forming proto-cluster regions at z > 2 to the quiescent population at z < 1. The existing seven-passband Spitzer data (3.6, 4.5, 5.8, 8.0, 24, 70, 160 micron) will allow us to make the first measurements of the evolution of the cluster red-sequence, IR luminosity function, and the mid-IR dust-obscured star-formation rate for 1 < z < 2 clusters.

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