arXiv subjects

Henk Hoekstra

Publications and source records attributed to Henk Hoekstra.

At least 199 records · Page 11Linked to original sources

arXiv2006

Cosmological Constraints from the Red-Sequence Cluster Survey

[abridged] We present a first cosmological analysis of a refined cluster catalog from the Red-Sequence Cluster Survey (RCS). The input cluster sample is derived from 72.07 square degrees of imaging data [...] The catalog contains 956 clusters over 0.35<z<0.95, limited by cluster richness and richness error. The calibration of the survey images has been extensively cross-checked against publicly available Sloan Digital Sky Survey imaging [...] We analyze the cluster sample via a general self-calibration technique including scatter in the mass-richness relation [...]. We fit simultaneously for Omega_M and sigma_8, and four parameters describing the calibration of cluster richness to mass, its evolution with redshift, and scatter in the richness-mass relation. The principal goal of this general analysis is to establish the consistency (or lack thereof) between the fitted parameters (both cosmological and cluster mass observables) and available results on both from independent measures. From an unconstrained analysis, Omega_M and sigma_8 are 0.31+0.11-0.10 and 0.67+0.18-0.13 respectively. An analysis including Gaussian priors on the slope and zeropoint of the mass-richness relation gives very similar results: 0.30+0.12-0.11 and 0.70+0.27-0.15. Both analyses are in good agreement with the current literature. The parameters describing the mass-richness relation in the unconstrained fit are also eminently reasonable and agree with existing follow-up data on both the RCS-1 and other cluster samples. Our results directly demonstrate that future surveys (optical and otherwise), with much larger samples of clusters, can give constraints competitive with other probes of cosmology.

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arXiv2006

Tracking quintessence by cosmic shear - Constraints from VIRMOS-Descart and CFHTLS and future prospects

Dark energy can be investigated in two complementary ways, by considering either general parameterizations or physically well-defined models. Following the second route, we explore the constraints on quintessence models where the acceleration is driven by a slow-rolling scalar field. The analysis focuses on cosmic shear, combined with supernovae Ia and CMB data. Using a Boltzmann code including quintessence models and the computation of weak lensing observables, we determine several two-point shear statistics. The non-linear regime is described by two different mappings. The likelihood analysis is based on a grid method. The data include the "gold set" of supernovae Ia, the WMAP-1 year data and the VIRMOS-Descart and CFHTLS-deep and -wide data for weak lensing. This is the first analysis of high-energy motivated dark energy models that uses weak lensing data. We explore larger angular scales, using a synthetic realization of the complete CFHTLS-wide survey as well as next space-based missions surveys. Two classes of cosmological parameters are discussed: i) those accounting for quintessence affect mainly geometrical factors; ii) cosmological parameters specifying the primordial universe strongly depend on the description of the non-linear regime. This dependence is addressed using wide surveys, by discarding the smaller angular scales to reduce the dependence on the non-linear regime. Special care is payed to the comparison of these physical models with parameterizations of the equation of state. For a flat universe and a quintessence inverse power law potential with slope alpha, we obtain alpha < 1 and Omega_Q=0.75^{+0.03}_{-0.04} at 95% confidence level, whereas alpha=2^{+18}_{-2}, Omega_Q=0.74^{+0.03}_{-0.05} when including supergravity corrections.

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arXiv2006

The Shear TEsting Programme 1: Weak lensing analysis of simulated ground-based observations

The Shear TEsting Programme, STEP, is a collaborative project to improve the accuracy and reliability of all weak lensing measurements in preparation for the next generation of wide-field surveys. In this first STEP paper we present the results of a blind analysis of simulated ground-based observations of relatively simple galaxy morphologies. The most successful methods are shown to achieve percent level accuracy. From the cosmic shear pipelines that have been used to constrain cosmology, we find weak lensing shear measured to an accuracy that is within the statistical errors of current weak lensing analyses, with shear measurements accurate to better than 7%. The dominant source of measurement error is shown to arise from calibration uncertainties where the measured shear is over or under-estimated by a constant multiplicative factor. This is of concern as calibration errors cannot be detected through standard diagnostic tests. The measured calibration errors appear to result from stellar contamination, false object detection, the shear measurement method itself, selection bias and/or the use of biased weights. Additive systematics (false detections of shear) resulting from residual point-spread function anisotropy are, in most cases, reduced to below an equivalent shear of 0.001, an order of magnitude below cosmic shear distortions on the scales probed by current surveys. Our results provide a snapshot view of the accuracy of current ground-based weak lensing methods and a benchmark upon which we can improve. To this end we provide descriptions of each method tested and include details of the eight different implementations of the commonly used Kaiser, Squires and Broadhurst (1995) method (KSB+) to aid the improvement of future KSB+ analyses.

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arXiv2005

Weak lensing constraints on galaxy halos

Weak gravitational lensing has become an important tool to study the properties of dark matter halos around galaxies, thanks to the advent of large panoramic cameras on 4m class telescopes. This area of research has been developing rapidly in the past few years, and in these proceedings we present some results based on the Red-Sequence Cluster Survey, thus highlighting what can be achieved with current data sets. We present results on the measurement of virial masses as a function of luminosity and the extent of dark matter halos. Much larger surveys are underway or planned, which will result in an impressive improvement in the accuracy of the measurements. However, the interpretation of future results will rely more and more on comparison with numerical simulations, thus providing direct tests of galaxy formation models.

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arXiv2005

Mass-to-Light Ratios of Galaxy Groups from Weak Lensing

We present the findings of our weak lensing study of a sample of 116 CNOC2 galaxy groups. The lensing signal is used to estimate the mass-to-light ratio of these galaxy groups. The best fit isothermal sphere model to our lensing data has an Einstein radius of 0.88"+/-0.12", which corresponds to a shear-weighted velocity dispersion of 245+/-18 km/s. The mean mass-to-light ratio within 1 h^-1 Mpc is 185+/-28 h times solar in the B-band and is independent of radius from the group center. The signal-to-noise ratio of the shear measurement is sufficient to split the sample into subsets of "poor" and "rich" galaxy groups. The poor galaxy groups were found to have an average velocity dispersion of 193+/-38 km/s and a mass-to-light ratio of 134+/-26 h times solar in the B-band, while the rich galaxy groups have a velocity dispersion of 270+/-39 km/s and a mass-to-light ratio of 278+/-42 h times solar in the B-band, similar to the mass-to-light ratio of clusters. This steep increase in the mass-to-light ratio as a function of mass, suggests that the mass scale of ~10^13 solar masses is where the transition between the actively star-forming field environment and the passively-evolving cluster environment occurs. This is the first such detection from weak lensing.

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arXiv2005

An algorithm to detect blends with eclipsing binaries in planet transit searches

We present an algorithm that can detect blends of bright stars with fainter, un-associated eclipsing binaries. Such systems contaminate searches for transiting planets, in particular in crowded fields where blends are common. Spectroscopic follow-up observations on large aperture telescopes have been used to reject these blends, but the results are not always conclusive. Our approach exploits the fact that a blend with a eclipsing binary changes its shape during eclipse. We analyze original imaging data from the Optical Gravitational Lensing Experiment (OGLE), which were used to discover planet transit candidates. Adopting a technique developed in weak gravitational lensing to carefully correct for the point spread function which varies both with time and across the field, we demonstrate that ellipticities can be measured with great accuracy using an ensemble of images. Applied to OGLE-TR-3 and OGLE-TR-56, two of the planetary transit candidates, we show that both systems are blended with fainter stars, as are most other stars in the OGLE fields. Moreover, while we do not detect shape change when TR-56 undergoes transits, TR-3 exhibits a significant shape change during eclipses. We therefore conclude that TR-3 is indeed a blend with an eclipsing binary, as has been suggested from other lines of evidence. The probability that its shape change is caused by residual systematics is found to be less than 0.6%. Our technique incurs no follow-up cost and requires little human interaction. As such it could become part of the data pipeline for any planetary transit search to minimize contamination by blends. We briefly discuss its relevance for the Kepler mission and for binary star detection.

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arXiv2004

Large scale systematic signals in weak lensing surveys

We use numerical simulations to model the effect of seeing and extinction modulations on weak lensing surveys. We find that systematic fluctuations in the shear amplitude and source depth can give rise to changes in the $E$-mode signal and to varying amplitudes of large scale $B$-modes. Exquisite control of such systematics will be required as we approach the era of precision cosmology with weak lensing.

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arXiv2004

Properties of galaxy dark matter halos from weak lensing

We present the results of a study of weak lensing by galaxies based on 45.5 deg$^2$ of $R_C$ band imaging data from the Red-Sequence Cluster Survey (RCS). We present the first weak lensing detection of the flattening of galaxy dark matter halos. We use a simple model in which the ellipticity of the halo is $f$ times the observed ellipticity of the lens. We find a best fit value of $f=0.77^{+0.18}_{-0.21}$, suggesting that the dark matter halos are somewhat rounder than the light distribution. The fact that we detect a significant flattening implies that the halos are well aligned with the light distribution. Given the average ellipticity of the lenses, this implies a halo ellipticity of $ =0.33^{+0.07}_{-0.09}$, in fair agreement with results from numerical simulations of CDM. This result provides strong support for the existence of dark matter, as an isotropic lensing signal is excluded with 99.5% confidence. We also study the average mass profile around the lenses, using a maximum likelihood analysis. We consider two models for the halo mass profile: a truncated isothermal sphere (TIS) and an NFW profile. We adopt observationally motivated scaling relations between the lens luminosity and the velocity dispersion and the extent of the halo. The best fit NFW model yields a mass $M_{200}=(8.4\pm0.7\pm0.4)\times 10^{11} h^{-1} M_\odot$ and a scale radius $r_s=16.2^{+3.6}_{-2.9} h^{-1}$ kpc. This value for the scale radius is in excellent agreement with predictions from numerical simulations for a halo of this mass.

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arXiv2003

Mapping the dark matter using weak lensing

Weak gravitational lensing of distant galaxies by foreground structures has proven to be a powerful tool to study the mass distribution in the universe. The advent of panoramic cameras on 4m class telescope has led to a first generation of surveys that already compete with large redshift surveys in terms of the accuracy with which cosmological parameters can be determined. The next surveys, which already have started taking data, will provide another major step forward. At the current level, systematics appear under control, and it is expected that weak lensing will develop into a key tool in the era of precision cosmology, provided we improve our knowledge of the non-linear matter power spectrum and the source redshift distribution. In this review we will briefly describe the principles of weak lensing and discuss the results of recent cosmic shear surveys. We show how the combination of weak lensing and cosmic microwave background measurements can provide tight constraints on cosmological parameters. We also demonstrate the usefulness of weak lensing in studies of the relation between the galaxy distribution and the underlying dark matter distribution (``galaxy biasing''), which can provide important constraints on models of galaxy formation. Finally, we discuss new and upcoming large cosmic shear surveys.

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arXiv2003

Properties of galaxy dark matter halos from weak lensing

We present the results of a study of the average mass profile around galaxies using weak gravitational lensing. We use 45.5 deg^2 of R_C band imaging data from the Red-Sequence Cluster Survey (RCS) and define a sample of 1.2\times 10^5 lenses with 19.5 =0.20^{+0.04}_{-0.05}, in fair agreement with results from numerical simulations of CDM. Alternative theories of gravity (without dark matter) predict an isotropic lensing signal, which is excluded with 99.5% confidence. Hence, our results provide strong support for the existence of dark matter.

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arXiv2003

The effect of imperfect corrections of PSF anisotropy on cosmic shear measurements

Current measurements of the weak lensing signal induced by large scale structure provide useful constraints on a range of cosmological parameters. However, the ultimate succes of this technique depends on the accuracy with which one can correct for the effect of the Point Spread Function (PSF). In this paper we examine the accuracy of the PSF anisotropy correction using images of fields with a large number of stars. The ellipticity correlation function of the residuals is studied to quantify the effect of imperfect corrections for PSF anisotropy on cosmic shear studies. These imperfections occur on the chip scale and consequently the systematic signal decreases rapidly with increasing angular scale. Separation of the signal into ``E'' (curl-free) and ``B'' (curl) components can help to identify the presence of residual systematics, but in general, the amplitude of the ``B''-mode is different from that of the ``E''-mode. The study of fields with many stars can be beneficial in finding a proper description of the variation of PSF anisotropy, and consequently help to significantly improve the accuracy with which the cosmic shear signal can be measured. We show that with such an approach it is feasible that the accuracy of future cosmic shear studies is limited by the statistical noise introduced by the intrinsic shapes of the sources. In particular, the prospects for accurate measurements of the cosmic shear signal on scales larger than ~10 arcminutes are excellent.

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arXiv2003

The Incidence of Strong-Lensing Clusters in the Red-Sequence Cluster Survey

The incidence of giant arcs due to strong-lensing clusters of galaxies is known to be discrepant with current theoretical expectations. This result derives from a comparison of several cluster samples to predictions in the framework of the currently favored $Λ$CDM cosmology, and one possible explanation for the discrepancy is that this cosmological model is not correct. In this paper we discuss the incidence of giant arcs in the Red-Sequence Cluster Survey (RCS), which again shows significant disagreement with theoretical predictions. We briefly describe a total of eight strong lens systems, seven of which are discussed here for the first time. Based on the details of these systems, in particular on the ratio of single to multiple arc systems, we argue that it may be possible to explain this discrepancy in the currently favored cosmology, by modifying the details of the lenses themselves. Specifically, the high incidence of multiple arc systems and their overall high redshift suggests that a sub-population of the global cluster population is responsible for much of the observed lensing. The lack of lensing clusters at $z<0.64$ in the RCS indicates that a property associated with clusters at early times results in the boosted lensing cross sections; likely a combination of ellipticity and elongation along the line of sight, substructure, and changes in the cluster mass profiles is responsible. Cluster mass, which should evolve to globally higher values toward lower redshifts, is clearly not the most significant consideration for the formation of giant arcs.

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arXiv2003

Joint CMB and Weak Lensing Analysis; Physically Motivated Constraints on Cosmological Parameters

We use Cosmic Microwave Background (CMB) observations together with the Red-sequence Cluster Survey (RCS) weak lensing results to derive constraints on a range of cosmological parameters. This particular choice of observations is motivated by their robust physical interpretation and complementarity. Our combined analysis, including a weak nucleosynthesis constraint, yields accurate determinations of a number of parameters including the amplitude of fluctuations sigma_8 = 0.89+/-0.05 and matter density Omega_m =0.30+/-0.03. We also find a value for the Hubble parameter of H_0 =70+/-3km/s/Mpc, in good agreement with the Hubble Space Telescope (HST) key--project result. We conclude that the combination of CMB and weak lensing data provides some of the most powerful constraints available in cosmology today.

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arXiv2002

How well can we determine cluster mass profiles from weak lensing?

Weak gravitational lensing provides a direct way to study the mass distribution of clusters of galaxies at large radii. Unfortunately, large scale structure along the line of sight also contributes to the lensing signal, and consequently affects the measurements. We quantify the effect of distant uncorrelated large scale structure on the inferred mass profile of clusters as measured from weak lensing. We consider NFW profiles, and find that large scale structure is a major source of uncertainty for most practical situations, when a model, with the mass M_200 and the concentration parameter c as free parameters, is fit to the observations. We find that the best constraints are found for clusters at intermediate redshifts (z~0.3). For a cluster at z=0.3, optimal results are obtained when the lensing signal is measured out to 10-15 arcminutes. Measurements at larger radii do not improve the accuracy with which the profile can be determined, contrary to what is expected when the contribution from large scale structure is ignored. The true uncertainties in M_200 and the concentration parameter c are ~2 times larger than when distant large scale structure is not included in the error budget.

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arXiv2002

Weak lensing study of galaxy biasing

We combine weak lensing measurements from the Red-Sequence Cluster Survey and the VIRMOS-DESCART survey, and present the first direct measurements of the bias parameter b and the galaxy-mass cross-correlation coefficient r on scales ranging from 0.2 to 9.3 h_{50}^{-1} Mpc at a lens redshift z~0.35. We find strong evidence that both b and r change with scale for our sample of lens galaxies (19.5<R_C<21), which have luminosities around L_*. For the currently favored cosmology (Ω_m=0.3,Ω_Λ=0.7), we find b=0.71^+0.06_-0.04 on a scale of 1-2 Mpc, increasing to ~1 on larger scales. The value of r has only minimal dependence on the assumed cosmology. The variation of r with scale is very similar to that of b, and reaches a minimum value of r=0.57^+0.08_-0.07 (at 1 Mpc). This suggests significant stochastic biasing and/or non-linear biasing. On scales larger than ~4 Mpc the value of r is consistent with a value of 1. In addition we use RCS data alone to measure the ratio b/r on scale ranging from 0.15 to 12.5 Mpc, and find that the ratio varies somewhat with scale. We obtain an average value of b/r=1.090+-0.035, in good agreement with previous estimates. A (future) careful comparison of our results with models of galaxy formation can provide unique constraints, as r is linked intimately to the details of galaxy formation.

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arXiv2002

Constraints on Omega_m and sigma_8 from weak lensing in RCS fields

We have analysed 53 square degrees of imaging data from the Red-Sequence Cluster Survey (RCS), and measured the excess correlations in the shapes of galaxies on scales out to ~1.5 degrees. We separate the signal into an ``E''- (lensing) and ``B''-mode (systematics), which allows us to study residual systematics. On scales larger than 10 arcminutes, we find no ``B''-mode. On smaller scales we find a small, but significant ``B''-mode. This signal is also present when we select a sample of bright galaxies. These galaxies are rather insensitive to observational distortions, and we therefore conclude that the oberved ``B''-mode is likely to be caused by intrinsic alignments. We therefore limit the cosmic shear analysis to galaxies with 22 0.1+0.16Ω_m$ (95% confidence). Comparison of the RCS results with three other recent cosmic shear measurements shows excellent agreement. The current weak lensing results are also in good agreement with CMB measurements, when we allow the reionization optical depth tau and the spectral index n_s to vary. We present a simple demonstration of how the weak lensing results can be used as a prior in the parameter estimation from CMB measurements to derive constraints on the reionization optical depth tau. (abridged)

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arXiv2002

Current status of weak gravitational lensing

Weak gravitational lensing of distant galaxies by foreground structures has proven to be a powerful tool to study the mass distribution in the universe. Nowadays, attention has shifted from clusters of galaxies to the statistical properties of the large scale structures and the halos of (field) galaxies. These applications have become feasible with the advent of panoramic cameras on 4m class telescopes. In this review we will give an overview of recent advances in this fast evolving field of astronomy. We start with a discussion of the recent measurements of weak lensing by large scale structure (``cosmic shear''), which can be used to constrain cosmological parameters. We also show how weak lensing can be used to measure the relation between galaxies and dark matter (galaxy biasing) directly. Another area that benefitted greatly from the current data is the weak lensing by galaxies (galaxy-galaxy lensing). Weak lensing provides a unique probe of the gravitational potential on large scales. Hence, in the context of dark matter, it can provide constrains on the extent and shapes of dark matter halos. Furthermore, it can test alternative theories of gravity (without dark matter). The best constraint comes from the first detection of the anisotropy of the lensing signal around lens galaxies, which suggest that the dark matter halos are flattened. An isotropic signal, as predicted by MOND, is excluded with 99% confidence.

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arXiv2002

A measurement of weak lensing by large scale structure in RCS fields

We have analysed ~24 square degrees of R_C-band imaging data from the Red-Sequence Cluster Survey (RCS), and measured the excess correlations between galaxy ellipticities on scales ranging from 1 to 30 arcminutes. We have used data from two different telescopes: ~16.4 square degrees of CFHT data and ~7.6 square degrees of CTIO 4-meter data, distributed over 13 widely separated patches. For the first time, a direct comparison can be made of the lensing signal measured using different instruments, which provides an important test of the weak lensing analysis itself. The measurements obtained from the two telescopes agree well. For the lensing analysis we use galaxies down to a limiting magnitude of R_C=24, for which the redshift distribution is known relatively well. This allows us to constrain some cosmological parameters. For the currently favored $Λ$CDM model $(Ω_m=0.3, Ω_Λ=0.7, Γ=0.21)$ we obtain $σ_8=0.81^{+0.14}_{-0.19}$ (95% confidence), in agreement with the results from Van Waerbeke et al. (2001) which used fainter galaxies (and consequently higher redshift galaxies). The good agreement between these two very different weak lensing studies demonstrates that weak lensing is a useful tool in observational cosmology.

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