Publications (150)
 [Show abstract] [Hide abstract] ABSTRACT: We present cosmological parameter constraints from a tomographic weak gravitational lensing analysis of ~450deg$^2$ of imaging data from the Kilo Degree Survey (KiDS). For a flat $\Lambda$CDM cosmology with a prior on $H_0$ that encompasses the most recent direct measurements, we find $S_8\equiv\sigma_8\sqrt{\Omega_{\rm m}/0.3}=0.745\pm0.039$. This result is in good agreement with other low redshift probes of large scale structure, including recent cosmic shear results, along with prePlanck cosmic microwave background constraints. A $2.3$$\sigma$ tension in $S_8$ and `substantial discordance' in the full parameter space is found with respect to the Planck 2015 results. We use shear measurements for nearly 15 million galaxies, determined with a new improved `selfcalibrating' version of $lens$fit validated using an extensive suite of image simulations. Fourband $ugri$ photometric redshifts are calibrated directly with deep spectroscopic surveys. The redshift calibration is confirmed using two independent techniques based on angular crosscorrelations and the properties of the photometric redshift probability distributions. Our covariance matrix is determined using an analytical approach, verified numerically with large mock galaxy catalogues. We account for uncertainties in the modelling of intrinsic galaxy alignments and the impact of baryon feedback on the shape of the nonlinear matter power spectrum, in addition to the small residual uncertainties in the shear and redshift calibration. The cosmology analysis was performed blind. Our highlevel data products, including shear correlation functions, covariance matrices, redshift distributions, and Monte Carlo Markov Chains are available at http://kids.strw.leidenuniv.nl.
 [Show abstract] [Hide abstract] ABSTRACT: Euclid is a European Space Agency medium class mission selected for launch in 2020 within the Cosmic Vision 2015 2025 program. The main goal of Euclid is to understand the origin of the accelerated expansion of the universe. Euclid will explore the expansion history of the universe and the evolution of cosmic structures by measuring shapes and redshifts of galaxies as well as the distribution of clusters of galaxies over a large fraction of the sky. Although the main driver for Euclid is the nature of dark energy, Euclid science covers a vast range of topics, from cosmology to galaxy evolution to planetary research. In this review we focus on cosmology and fundamental physics, with a strong emphasis on science beyond the current standard models. We discuss five broad topics: dark energy and modified gravity, dark matter, initial conditions, basic assumptions and questions of methodology in the data analysis. This review has been planned and carried out within Euclid's Theory Working Group and is meant to provide a guide to the scientific themes that will underlie the activity of the group during the preparation of the Euclid mission.
 [Show abstract] [Hide abstract] ABSTRACT: We study the spherical evolution model for voids in $\Lambda$CDM, where the evolution of voids is governed by dark energy at an earlier time than that for the whole universe or in overdensities. We show that the presence of dark energy suppresses the growth of peculiar velocities, causing void shellcrossing to occur at progressively later epochs as $\Omega_{\Lambda}$ increases. We apply the spherical model to evolve the initial conditions of Nbody simulated voids and compare the resulting final void profiles. We find that the model is successful in tracking the evolution of voids with radii greater than $30 h^{1} \rm Mpc$, implying that void profiles could be used to constrain dark energy. We find that the initial peculiar velocities of voids play a significant role in shaping their evolution. Excluding the peculiar velocity in the evolution model delays the time of shell crossing.
 [Show abstract] [Hide abstract] ABSTRACT: Galaxies and their dark matter haloes are part of a complex network of mass structures, collectively called the cosmic web. Using the tidal tensor prescription these structures can be classified into four cosmic environments: voids, sheets, filaments and knots. As the cosmic web may influence the formation and evolution of dark matter haloes and the galaxies they host, we aim to study the effect of these cosmic environments on the average mass of galactic haloes. To this end we measure the galaxygalaxy lensing profile of 91,195 galaxies, within 0.039 < z < 0.263, from the spectroscopic Galaxy And Mass Assembly (GAMA) survey, using ~100 square degrees of overlapping data from the KiloDegree Survey (KiDS). In each of the four cosmic environments we model the contributions from group centrals, satellites and neighbouring groups to the stacked galaxygalaxy lensing profiles. After correcting the lens samples for differences in the stellar mass distribution, we find no dependence of the average halo mass of central galaxies on their cosmic environment. We do find a significant increase in the average contribution of neighbouring groups to the lensing profile in increasingly dense cosmic environments. We show, however, that the observed effect can be entirely attributed to the galaxy density at much smaller scales (within 4 Mpc/h), which is correlated with the density of the cosmic environments. Within our current uncertainties we find no direct dependence of galaxy halo mass on their cosmic environment.
 [Show abstract] [Hide abstract] ABSTRACT: We measure the crosscorrelation signature between the Planck CMB lensing map and the weak lensing observations from both the Redsequence Cluster Lensing Survey (RCSLenS) and the CanadaFranceHawai Telescope Lensing Survey (CFHTLenS). In addition to a Fourier analysis, we include the first configurationspace detection, based on the estimators 〈κCMBκgal〉 and 〈κCMBγt〉. Combining 747.2 deg2 from both surveys, we find a detection significance that exceeds 4.2σ in both Fourier and configurationspace analyses. Scaling the predictions by a free parameter A, we obtain $A^{\rm Planck}_{\rm CFHT}= 0.68\pm 0.31$ and $A^{\rm Planck}_{\rm RCS}= 1.31\pm 0.33$. In preparation for the next generation of measurements similar to these, we quantify the impact of different analysis choices on these results. First, since none of these estimators probes the exact same dynamical range, we improve our detection by combining them. Second, we carry out a detailed investigation on the effect of apodization, zeropadding and mask multiplication, validated on a suite of highresolution simulations, and find that the latter produces the largest systematic bias in the cosmological interpretation. Finally, we show that residual contamination from intrinsic alignment and the effect of photometric redshift error are both largely degenerate with the characteristic signal from massive neutrinos, however the signature of baryon feedback might be easier to distinguish. The three lensing dataset are publicly available.

Article: The stellartohalo mass relation of GAMA galaxies from 100 square degrees of KiDS weak lensing data
[Show abstract] [Hide abstract] ABSTRACT: We study the stellartohalo mass relation of central galaxies in the range 9.7 < log 10(M*/h− 2 M⊙) < 11.7 and z < 0.4, obtained from a combined analysis of the Kilo Degree Survey (KiDS) and the Galaxy And Mass Assembly (GAMA) survey. We use ∼100 deg2 of KiDS data to study the lensing signal around galaxies for which spectroscopic redshifts and stellar masses were determined by GAMA. We show that lensing alone results in poor constraints on the stellartohalo mass relation due to a degeneracy between the satellite fraction and the halo mass, which is lifted when we simultaneously fit the stellar mass function. At M* > 5 × 1010 h− 2 M⊙, the stellar mass increases with halo mass as ${\sim }M_{\rm h}^{0.25}$. The ratio of dark matter to stellar mass has a minimum at a halo mass of 8 × 1011 h−1 M⊙ with a value of $M_{\rm h}/M_{\ast }=56_{10}^{+16}$ [h]. We also use the GAMA group catalogue to select centrals and satellites in groups with five or more members, which trace regions in space where the local matter density is higher than average, and determine for the first time the stellartohalo mass relation in these denser environments. We find no significant differences compared to the relation from the full sample, which suggests that the stellartohalo mass relation does not vary strongly with local density. Furthermore, we find that the stellartohalo mass relation of central galaxies can also be obtained by modelling the lensing signal and stellar mass function of satellite galaxies only, which shows that the assumptions to model the satellite contribution in the halo model do not significantly bias the stellartohalo mass relation. Finally, we show that the combination of weak lensing with the stellar mass function can be used to test the purity of group catalogues.  [Show abstract] [Hide abstract] ABSTRACT: We present the Redsequence Cluster Lensing Survey (RCSLenS), an application of the methods developed for the Canada France Hawaii Telescope Lensing Survey (CFHTLenS) to the ~785deg$^2$, multiband imaging data of the Redsequence Cluster Survey 2 (RCS2). This project represents the largest public, subarcsecond seeing, multiband survey to date that is suited for weak gravitational lensing measurements. With a careful assessment of systematic errors in shape measurements and photometric redshifts we extend the use of this data set to allow crosscorrelation analyses between weak lensing observables and other data sets. We describe the imaging data, the data reduction, masking, multicolour photometry, photometric redshifts, shape measurements, tests for systematic errors, and a blinding scheme to allow for more objective measurements. In total we analyse 761 pointings with rband coverage, which constitutes our lensing sample. Residual largescale Bmode systematics prevent the use of this shear catalogue for cosmic shear science. The effective number density of lensing sources over an unmasked area of 571.7deg$^2$ and down to a magnitude limit of r~24.5 is 8.1 galaxies per arcmin$^2$ (weighted: 5.5 arcmin$^{2}$) distributed over 14 patches on the sky. Photometric redshifts based on 4band griz data are available for 513 pointings covering an unmasked area of 383.5 deg$^2$ We present weak lensing mass reconstructions of some example clusters as well as the full survey representing the largest areas that have been mapped in this way. All our data products are publicly available through CADC at http://www.cadcccda.hiaiha.nrccnrc.gc.ca/en/community/rcslens/query.html in a format very similar to the CFHTLenS data release.
 [Show abstract] [Hide abstract] ABSTRACT: We present an accurate nonlinear matter power spectrum prediction scheme for a variety of extensions to the standard cosmological paradigm, which uses the tuned halo model previously developed in Mead et al. (2015b). We consider dark energy models that are both minimally and nonminimally coupled, massive neutrinos and modified gravitational forces with chameleon and Vainshtein screening mechanisms. In all cases we compare halomodel power spectra to measurements from highresolution simulations. We show that the tuned halo model method can predict the nonlinear matter power spectrum measured from simulations of parameterised w(a) dark energy models at the few per cent level for k < 10 hMpc−1, and we present theoretically motivated extensions to cover nonminimally coupled scalar fields, massive neutrinos and Vainshtein screened modified gravity models that result in few per cent accurate power spectra for k < 10 hMpc−1. For chameleon screened models we achieve only 10 per cent accuracy for the same range of scales. Finally, we use our halo model to investigate degeneracies between different extensions to the standard cosmological model, finding that the impact of baryonic feedback on the nonlinear matter power spectrum can be considered independently of modified gravity or massive neutrino extensions. In contrast, considering the impact of modified gravity and massive neutrinos independently results in biased estimates of power at the level of 5 per cent at scales k > 0.5 hMpc−1. An updated version of our publicly available hmcode can be found at https://github.com/alexandermead/hmcode.
 [Show abstract] [Hide abstract] ABSTRACT: We investigate the impact of astrophysical systematics on cosmic shear cosmological parameter constraints from the CanadaFranceHawaii Telescope Lensing Survey (CFHTLenS), and the concordance with cosmic microwave background measurements by Planck. We present updated CFHTLenS cosmic shear tomography measurements extended to degree scales using a covariance calibrated by a new suite of Nbody simulations. We analyze these measurements with a new model fitting pipeline, accounting for key systematic uncertainties arising from intrinsic galaxy alignments, baryonic effects in the nonlinear matter power spectrum, and photometric redshift uncertainties. We examine the impact of the systematic degrees of freedom on the cosmological parameter constraints, both independently and jointly. When the systematic uncertainties are considered independently, the intrinsic alignment amplitude is the only degree of freedom that is substantially preferred by the data. When the systematic uncertainties are considered jointly, there is no consistently strong preference in favor of the more complex models. We quantify the level of concordance between the CFHTLenS and Planck datasets by employing two distinct data concordance tests, grounded in Bayesian evidence and information theory. We find that the two data concordance tests largely agree with one another, and that the level of concordance between the CFHTLenS and Planck datasets is sensitive to the exact details of the systematic uncertainties included in our analysis, ranging from decisive discordance to substantial concordance as the treatment of the systematic uncertainties becomes more conservative. The least conservative scenario is the one most favored by the cosmic shear data, but it is also the one that shows the greatest degree of discordance with Planck. The data and analysis code are public at https://github.com/sjoudaki/cfhtlens_revisited

Article: Cluster mass profile reconstruction with size and flux magnification on the HST STAGES survey
[Show abstract] [Hide abstract] ABSTRACT: We present the first measurement of individual cluster mass estimates using weak lensing size and flux magnification. Using data from the HST STAGES (Space Telescope A901/902 Galaxy Evolution Survey) survey of the A901/902 supercluster we detect the four known groups in the supercluster at high significance using magnification alone. We discuss the application of a fully Bayesian inference analysis, and investigate a broad range of potential systematics in the application of the method. We compare our results to a previous weak lensing shear analysis of the same field finding the recovered signaltonoise of our magnificationonly analysis to range from 45 to 110 per cent of the signaltonoise in the shearonly analysis. On a casebycase basis we find consistent magnification and shear constraints on cluster virial radius, and finding that for the full sample, magnification constraints to be a factor 0.77 ± 0.18 lower than the shear measurements.  [Show abstract] [Hide abstract] ABSTRACT: We present a reanalysis of the CFHTLenS weak gravitational lensing survey using Complete Orthogonal Sets of E/Bmode Integrals, known as COSEBIs. COSEBIs provide a complete set of functions to efficiently separate Emodes from Bmodes and hence allow for robust and stringent tests for systematic errors in the data. This analysis reveals significant Bmodes on large angular scales that were not previously seen using the standard E/B decomposition analyses. We find that the significance of the Bmodes is enhanced when the data is split by galaxy type and analysed in tomographic redshift bins. Adding tomographic bins to the analysis increases the number of COSEBIs modes, which results in a less accurate estimation of the covariance matrix from a set of simulations. We therefore also present the first compressed COSEBIs analysis of survey data, where the COSEBIs modes are optimally combined based on their sensitivity to cosmological parameters. In this tomographic CCOSEBIs analysis we find the Bmodes to be consistent with zero when the full range of angular scales are considered.
 [Show abstract] [Hide abstract] ABSTRACT: We determine the accuracy of galaxy redshift distributions as estimated from photometric redshift probability distributions $p(z)$. Our method utilises measurements of the angular crosscorrelation between photometric galaxies and an overlapping sample of galaxies with spectroscopic redshifts. We describe the redshift leakage from a galaxy photometric redshift bin $j$ into a spectroscopic redshift bin $i$ using the sum of the $p(z)$ for the galaxies residing in bin $j$. We can then predict the angular crosscorrelation between photometric and spectroscopic galaxies due to intrinsic galaxy clustering when $i \neq j$ as a function of the measured angular crosscorrelation when $i=j$. We also account for enhanced clustering arising from lensing magnification using a halo model. The comparison of this prediction with the measured signal provides a consistency check on the validity of using the summed $p(z)$ to determine galaxy redshift distributions in cosmological analyses, as advocated by the CanadaFranceHawaii Telescope Lensing Survey (CFHTLenS). We present an analysis of the photometric redshifts measured by CFHTLenS, which overlaps the Baryon Oscillation Spectroscopic Survey (BOSS). We also analyse the Redsequence Cluster Lensing Survey (RCSLenS), which overlaps both BOSS and the WiggleZ Dark Energy Survey. We find that the summed $p(z)$ from both surveys are generally biased with respect to the true underlying distributions. If unaccounted for, this bias would lead to errors in cosmological parameter estimation from CFHTLenS by less than $\sim 4\%$. For photometric redshift bins which spatially overlap in 3D with our spectroscopic sample, we determine redshift bias corrections which can be used in future cosmological analyses that rely on accurate galaxy redshift distributions.
 [Show abstract] [Hide abstract] ABSTRACT: We present measurements of the galaxy bias b and the galaxy–matter crosscorrelation coefficient r for the Baryon Oscillation Spectroscopic Survey LOWZ luminous red galaxy sample. Using a new statistical weak lensing analysis of the Red Cluster Sequence Lensing Survey (RCSLenS), we find the bias properties of this sample to be higher than previously reported with $b=2.45_{0.05}^{+0.05}$ and $r=1.64_{0.16}^{+0.17}$ on scales between 3 and 20 arcmin. We repeat the measurement for angular scales of 20 arcmin ≤ ϑ ≤ 70 arcmin, which yields $b=2.39_{0.07}^{+0.07}$ and $r=1.24_{0.25}^{+0.26}$. This is the first application of a data compression analysis using a complete set of discrete estimators for galaxy–galaxy lensing and galaxy clustering. As cosmological data sets grow, our new method of data compression will become increasingly important in order to interpret joint weak lensing and galaxy clustering measurements and to estimate the data covariance. In future studies, this formalism can be used as a tool to study the largescale structure of the Universe to yield a precise determination of cosmological parameters.
 [Show abstract] [Hide abstract] ABSTRACT: In this paper we present results of applying the shearratio method to the RCSLenS data. The method takes the ratio of the mean of the weak lensing tangential shear signal about galaxy clusters, averaged over all clusters of the same redshift, in multiple background redshift bins. In taking a ratio the massdependency of the shear signal is cancelledout leaving a statistic that is dependent on the geometric part of the lensing kernel only. We apply this method to 535 clusters and measure a cosmologyindependent distanceredshift relation to redshifts z~1. In combination with Planck data the method lifts the degeneracies in the CMB measurements, resulting in cosmological parameter constraints of OmegaM=0.31 +/ 0.10 and w0 = 1.02 +/ 0.37, for a flat wCDM cosmology.
 [Show abstract] [Hide abstract] ABSTRACT: We present a complete derivation of the observationally motivated definition of the modified gravity statistic $E_G$. Using this expression, we investigate how variations to theory and survey parameters may introduce uncertainty in the general relativistic prediction of $E_G$. We forecast errors on $E_G$ for measurements using two combinations of upcoming surveys, and find that theoretical uncertainties may dominate for a futuristic measurement. Finally, we compute predictions of $E_G$ under modifications to general relativity in the quasistatic regime, and comment on the pros and cons of using $E_G$ to test gravity with future surveys.
 [Show abstract] [Hide abstract] ABSTRACT: Applying a transformation to a nonGaussian field can enhance the information content of the resulting power spectrum, by reducing the correlations between Fourier modes. In the context of weak gravitational lensing, it has been shown that this gain in information content is significantly compromised by the presence of shape noise. We apply clipping to mock convergence fields, a technique which is known to be robust in the presence of noise and has been successfully applied to galaxy number density fields. When analysed in isolation the resulting convergence power spectrum returns degraded constraints on cosmological parameters. However, substantial gains can be achieved by performing a combined analysis of the power spectra derived from both the original and transformed fields. Even in the presence of realistic levels of shape noise, we demonstrate that this approach is capable of reducing the area of likelihood contours within the Ωm − σ8 plane by more than a factor of 3.
 [Show abstract] [Hide abstract] ABSTRACT: We present weak lensing constraints on the ellipticity of galaxyscale matter haloes and the galaxyhalo misalignment. Using data from the Canada–France–Hawaii Telescope Lensing Survey (CFHTLenS), we measure the weightedaverage ratio of the aligned projected ellipticity components of galaxy matter haloes and their embedded galaxies, fh, split by galaxy type. We then compare our observations to measurements taken from the Millennium Simulation, assuming different models of galaxyhalo misalignment. Using the Millennium Simulation, we verify that the statistical estimator used removes contamination from cosmic shear. We also detect an additional signal in the simulation, which we interpret as the impact of intrinsic shape–shear alignments between the lenses and their largescale structure environment. These alignments are likely to have caused some of the previous observational constraints on fh to be biased high. From CFHTLenS, we find fh = −0.04 ± 0.25 for earlytype galaxies, which is consistent with current models for the galaxyhalo misalignment predicting fh ≃ 0.20. For latetype galaxies we measure $f_\mathrm{h}=0.69_{0.36}^{+0.37}$ from CFHTLenS. This can be compared to the simulated results which yield fh ≃ 0.02 for misaligned latetype models.
 [Show abstract] [Hide abstract] ABSTRACT: The unknown nature of ‘dark energy’ motivates continued cosmological tests of largescale gravitational physics. We present a new consistency check based on the relative amplitude of nonrelativistic galaxy peculiar motions, measured via redshiftspace distortion, and the relativistic deflection of light by those same galaxies traced by galaxy–galaxy lensing. We take advantage of the latest generation of deep, overlapping imaging and spectroscopic data sets, combining the Red Cluster Sequence Lensing Survey, the Canada–France–Hawaii Telescope Lensing Survey, the WiggleZ Dark Energy Survey and the Baryon Oscillation Spectroscopic Survey. We quantify the results using the ‘gravitational slip’ statistic EG, which we estimate as 0.48 ± 0.10 at z = 0.32 and 0.30 ± 0.07 at z = 0.57, the latter constituting the highest redshift at which this quantity has been determined. These measurements are consistent with the predictions of General Relativity, for a perturbed Friedmann–Robertson–Walker metric in a Universe dominated by a cosmological constant, which are EG = 0.41 and 0.36 at these respective redshifts. The combination of redshiftspace distortion and gravitational lensing data from current and future galaxy surveys will offer increasingly stringent tests of fundamental cosmology.

Article: The masses of satellites in GAMA galaxy groups from 100 square degrees of KiDS weak lensing data
[Show abstract] [Hide abstract] ABSTRACT: We use the first 100 deg2 of overlap between the KiloDegree Survey and the Galaxy And Mass Assembly survey to determine the average galaxy halo mass of ∼10 000 spectroscopically confirmed satellite galaxies in massive (M > 1013 h−1 M⊙) galaxy groups. Separating the sample as a function of projected distance to the group centre, we jointly model the satellites and their host groups with Navarro–Frenk–White density profiles, fully accounting for the data covariance. The probed satellite galaxies in these groups have total masses log 〈Msub/(h−1 M⊙)〉 ≈ 11.7–12.2 consistent across groupcentric distance within the errorbars. Given their typical stellar masses, log 〈M⋆, sat/(h−2 M⊙)〉 ∼ 10.5, such total masses imply stellar mass fractions of 〈M⋆, sat〉/〈Msub〉 ≈ 0.04 h−1. The average subhalo hosting these satellite galaxies has a mass Msub ∼ 0.015Mhost independent of host halo mass, in broad agreement with the expectations of structure formation in a Λ cold dark matter universe. 
Article: Dark matter halo properties of GAMA galaxy groups from 100 square degrees of KiDS weak lensing data
[Show abstract] [Hide abstract] ABSTRACT: The KiloDegree Survey is an optical widefield survey designed to map the matter distribution in the Universe using weak gravitational lensing. In this paper, we use these data to measure the density profiles and masses of a sample of ∼1400 spectroscopically identified galaxy groups and clusters from the Galaxy And Mass Assembly survey. We detect a highly significant signal (signaltonoiseratio ∼120), allowing us to study the properties of dark matter haloes over one and a half order of magnitude in mass, from M ∼ 1013–1014.5 h−1 M⊙. We interpret the results for various subsamples of groups using a halo model framework which accounts for the miscentring of the brightest cluster galaxy (used as the tracer of the group centre) with respect to the centre of the group's dark matter halo. We find that the density profiles of the haloes are well described by an NFW profile with concentrations that agree with predictions from numerical simulations. In addition, we constrain scaling relations between the mass and a number of observable group properties. We find that the mass scales with the total rband luminosity as a power law with slope 1.16 ± 0.13 (1σ) and with the group velocity dispersion as a power law with slope 1.89 ± 0.27 (1σ). Finally, we demonstrate the potential of weak lensing studies of groups to discriminate between models of baryonic feedback at group scales by comparing our results with the predictions from the CosmoOverWhelmingly Large Simulations project, ruling out models without AGN feedback.
Publication Stats
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Institutions

20002015

The University of Edinburgh
 Institute for Astronomy (IfA)
Edinburgh, Scotland, United Kingdom


20102014

Scottish Universities Physics Alliance
Glasgow, Scotland, United Kingdom


2012

The Ohio State University
 Department of Physics
Columbus, Ohio, United States


20072008

Pierre and Marie Curie University  Paris 6
 Institut d'astrophysique de Paris
Paris, IledeFrance, France 
Institut d'astrophysique de Paris
Lutetia Parisorum, ÎledeFrance, France


20062008

University of British Columbia  Vancouver
 Department of Physics and Astronomy
Vancouver, British Columbia, Canada


20042008

Max Planck Institute for Astronomy
Heidelburg, BadenWürttemberg, Germany


20022005

University of Oxford
 Department of Physics
Oxford, England, United Kingdom
