This thesis is concerned with developing new methods for performing weak gravitational lensing with the aim of addressing specific systematic effects in weak lensing surveys. The first of these effects is the multiplicative biases which arise as a result of isotropic smearing. This smearing may be due to atmospheric seeing or an instrumental PSF. Isotropic smearing circularizes a galaxy image and leads to a systematic under-estimate of the modulus of the observed ellipticity. The orientation of the observed galaxy is, however, unaffected. We exploit this property by formulating a weak lensing shear estimator that requires measurements of galaxy position angles only, thereby avoiding the contribution from this systematic. We demonstrate the method on simulations and the CFHTLenS data by reconstructing convergence maps and comparing the results with the standard full ellipticity based approach. We show that the difference between the reconstructed maps for the two approaches is consistent with noise in all of the tests performed. We then apply the technique to the GREAT3 challenge data using three distinct methods to measure the position angles of the galaxies. For all three methods, we find that the position angle-only approach yields shear estimates with a performance comparable with current well established shape based techniques. The second effect addressed arises from the intrinsic alignment of the source galaxies. This alignment mimics a shear signal, and hence biases estimates of the shear. To mitigate this effect, we develop three shear estimators that include polarization information from radio observations as a tracer of a galaxy’s intrinsic orientation. In addition to the shear estimator, we also develop estimators for the intrinsic alignment signal. We test these estimators by successfully reconstructing the shear and intrinsic alignment auto and cross-power spectra across three overlapping redshift bins.

This thesis will present a new cosmic shear analysis pipeline SUNGLASS (Simulated UNiverses for Gravitational Lensing Analysis and Shear Surveys). SUNGLASS is a pipeline that rapidly generates simulated universes for weak lensing and cosmic shear analysis. The pipeline forms suites of cosmological N-body simulations and performs tomographic cosmic shear analysis using a novel line-of-sight integration through the simulations while saving the particle lightcone information. Galaxy shear and convergence catalogues with realistic 3-D galaxy redshift distributions are produced for the purposes of testing weak lensing analysis techniques and generating covariance matrices for data analysis and cosmological parameter estimation. This thesis presents a suite of fast medium-resolution simulations with shear and convergence maps for a generic 100 square degree survey out to a redshift of z = 1.5, with angular power spectra agreeing with the theoretical expectations to better than a few percent accuracy up to ℓ = 103 for all source redshifts up to z = 1.5 and wavenumbers up to ℓ = 2000 for source redshifts z ≥ 1.1. A two-parameter Gaussian likelihood analysis of Ωm and σ8 is also performed on the suite of simulations for a 2-D weak lensing survey, demonstrating that the cosmological parameters are recovered from the simulations and the covariance matrices are stable for data analysis, with negligible bias. An investigation into the accuracy of traditional Fisher matrix calculations is presented. Fisher Information Matrix methods are commonly used in cosmology to estimate the accuracy that cosmological parameters can be measured with a given experiment, and to optimise the design of experiments. However, the standard approach usually assumes both data and parameter estimates are Gaussian-distributed. Further, for survey forecasts and optimisation it is usually assumed the power-spectra covariance matrix is diagonal in Fourier-space. But in the low-redshift Universe, non-linear mode-coupling will tend to correlate small-scale power, moving information from lower to higher-order moments of the field. This movement of information will change the predictions of cosmological parameter accuracy. In this thesis, the loss of information is quantified by comparing näıve Gaussian Fisher matrix forecasts with a Maximum Likelihood parameter estimation analysis of the suite of mock weak lensing catalogues derived from the SUNGLASS pipeline, for 2-D and tomographic shear analyses of a Euclid-like survey. In both cases the 68% confidence area of the Ωm − σ8 plane is found to increase by a factor 5. However, the marginal errors increase by just 20 to 40%. A new method is proposed to model the effects of non-linear shear-power mode-coupling in the Fisher Matrix by approximating the shear-power distribution as a multivariate Gaussian with a covariance matrix derived from the mock weak lensing survey. The findings in this thesis show that this approximation can reproduce the 68% confidence regions of the full Maximum Likelihood analysis in the Ωm − σ8 plane to high accuracy for both 2-D and tomographic weak lensing surveys. Finally, three multi-parameter analyses of (Ωm, σ8, ns), (Ωm, σ8, ns, ΩΛ)and (Ωm, σ8, h, ns, w0, wa) are performed to compare the Gaussian and non-linear mode-coupled Fisher matrix contours. The multi-parameter volumes of the 1σ error contours for the six-parameter non-linear Fisher analysis are consistently larger than for the Gaussian case, and the shape of the 68% confidence volume is modified. These results strongly suggest that future Fisher Matrix estimates of cosmological parameter accuracies should include mode-coupling effects.

Weak gravitational lensing is now well established as a powerful cosmological probe, particularly for studying large scale structure growth in the Universe. The vast majority of weak lensing experiments to date use optical and near infrared observations which are well suited to the requirements in source densities and shape analysis. In this thesis we outline the prospects associated with weak lensing surveys from radio observations. This can offer key advantages to optical counterpart studies such as the well defined observing beam pattern of a radio telescope and a window into a much broader observed redshift distribution. In addition to the prospect of radio weak lensing surveys alone, combining with optical counterparts in a cross-correlation study has been shown to mitigate uncorrelated systematics, further motivating the case for radio based weak lensing studies. The correlation of galaxy shapes through multi-wavelength observations will affect the noise on the cosmological power spectrum in cross-correlation analysis. We use radio and optical observations of the COSMOS field with the VLA and HST respectively, accompanied with simulations for calibration in order to measure shape correlations between wavelength regimes. Although we do not detect a correlation between optical and radio shapes, a lower limit on the intrinsic astrophysical scatter was placed at >0.212pi (or 38.2 degrees), through a Monte Carlo simulation of source catalogues with the measured uncertainties. The SuperCLASS experiment aims to measure a weak lensing signal with radio observations from a super-cluster field. We introduce the radio data, collected with the e-MERLIN and JVLA, and the reduction steps taken. Assisted by simulations, we have designed a shape measurement pipeline (SuperTRAP) which performs additional phase rotation and averaging steps to extract visibility sets on a source by source basis followed by image plane shape analysis. A series of staged tests of increasing complexity are outlined here and evaluated by the shape recovery bias and efficiency. Finally we present the optical counterpart observations and shape analysis for the SuperCLASS field, with data collected by the Subaru Suprime-Cam. Observational systematics are measured to form representative PSF models in each CCD exposure and the subsequent shape analysis from the I band photometry is presented. Shear analysis from the measured power spectrum shows good agreement with theoretical predictions. From the measured shear power spectrum we detect a strong signal in the E-mode band powers, equivalent to a 9.31sigma detection. Our measurements from the B-mode and E-B cross band powers suggest negligible contamination from systematics. The optical analysis presented here will provide the counterpart analysis to the radio for future cross-correlation studies.

We investigate the uses of gravitational lensing for analysing the dark matter haloes around galaxies, comparing galaxies within groups and clusters to those in the field. We consider two cases: when only photometric redshift data is available, and when spectroscopic redshift data is available for a sufficiently large sample of galaxies. For the case of data with photometric redshifts, we analyse the CFHTLenS dataset. This dataset is derived from the CFHTLS-Wide survey, and encompasses 154 deg^2 of high-quality shape data. Using the photometric redshifts to estimate local density, we divide the sample of lens galaxies with stellar masses in the range 10^9 Msun to 10^10.5 Msun into those likely to lie in high-density environments (HDE) and those likely to lie in low-density environments (LDE). Through comparison with galaxy catalogues extracted from the Millennium Simulation, we show that the sample of HDE galaxies should primarily (~61%) consist of satellite galaxies in groups, while the sample of LDE galaxies should consist of mostly (~87%) non-satellite (field and central) galaxies. Comparing the lensing signals around samples of HDE and LDE galaxies matched in stellar mass, we show that the subhaloes of HDE galaxies are less massive than those around LDE galaxies by a factor 0.65+/-0.12, significant at the 2.9 sigma level. A natural explanation is that the haloes of satellite galaxies are stripped through tidal effects in the group environment. Our results are consistent with a typical tidal truncation radius of ~40 kpc. For the case of data with spectroscopic redshifts, we analyse the GAMA-I and the ongoing GAMA-II surveys. We demonstrate the possibility of detecting tidal stripping of dark matter subhaloes within galaxy groups using weak gravitational lensing. We have run ray-tracing simulations on galaxy catalogues from the Millennium Simulation to generate mock shape catalogues. The ray-tracing catalogues assume a halo model for galaxies and groups, using various models with different distributions of mass between galaxy and group haloes to simulate different stages of group evolution. Using these mock catalogues, we forecast the lensing signals that will be detected around galaxy groups and satellite galaxies, as well as test two different methods for isolating the satellites' lensing signals. A key challenge is to determine the accuracy to which group centres can be identified. We show that with current and ongoing surveys, it will possible to detect stripping in groups of mass 10^12 Msun to 10^15 Msun.

The dark matter paradigm has been a great source of speculation in both the 20th and 21st Centuries. Since its proposed existence in 1933, the mounting evidence has led to this theoretical particle becoming one of the greatest mysteries of modern physics. However, despite its dominant presence in the Universe, little is known about its nature and how it behaves. In this thesis I critically analyse one particular property of dark matter: the self-coupling. The self-interacting dark matter paradigm hypothesises that dark matter is not collisionless as assumed in most cosmological simulations, and in-fact has some probability that it will scatter off itself. Such a self-coupling will resolve many discrepancies that exist between observations and theory, particularly on small, non-linear scales. Moreover, any detection of a self-interaction cross-section will place considerable limitations on the acceptable particle physics models of dark matter and hence has grown to become an important question. In this thesis I develop and implement a method to constrain the self-interaction cross-section of dark matter that exploits continually accreting and merging groups of galaxies as they fall into galaxy clusters. Utilising the ubiquitous nature of accreting substructure, I measure the offsets between dark matter and baryonic gas as they become separated due to their differing interaction properties. Studying this effect over a sample of events, I will be able to make the first ever statistical estimate of the cross-section of dark matter, while averaging over many different unknown merging scenarios. I begin my thesis by deriving an analytical description of sub-halo in-fall, allowing me to constrain dark matter self-interaction models directly from observations. In this study, I find that current archival data should be able to detect a difference in the dynamical behaviour of dark matter and standard model particles at 6σ, and measure the total interaction cross-section σDM/m with 68% confidence limits of ±1 cm2g-1. Having constructed a new method to derive constraints on the cross-section of dark matter I carry out a study into the potential systematics that may affect a measurement. I determine the accuracy of weak gravitational lensing, which is the distortion of light due to intervening mass, as a tool to estimate the positions of substructure in galaxy clusters. I find that the public Lenstool software can measure the position of individual 1:5 x 1013Mʘ peaks with ~ 0:3" systematic bias, as long as they are at least ~ 30" from the cluster centre. Finally, I develop a pipeline that can analyse a sample of inhomogeneous observations from The Hubble Space Telescope and the Chandra X-ray Observatory. By measuring the positions of dark matter, gas and galaxies for 68 individual merging events, from a total of 28 galaxy clusters, I detect a 7:4σ offset between gas and an unobserved dark mass. I make the first ever measurement of cross-section of dark matter from a sample of clusters finding σDM < 0:50cm2/g [95% CL], the best constraints to date. In addition to this I find that the brightest group galaxy in-fact tends to lead the dark matter halo during merging events. Although evidence for the existence of interacting dark matter, I conclude that the astrophysics of the BCG is complicated, and that this apparent directional bias should be considered in all galaxy cluster analyses. Moreover, I show that this technique is easily extendable for future surveys that have larger samples of galaxy clusters, with constraints of σDM < 0:001cm2/g potentially attainable.

Weak gravitational lensing has developed to be one of the most powerful tools for studying the (dark) matter distribution in the Universe. Most weak lensing studies thus far were con- ducted in the optical and near infrared. Measuring weak lensing in the radio though, provided it is feasible, can be very advantageous. One can exploit the well known and deterministic beam pattern of a radio telescope and the polarisation information in radio data to reduce shape biases and intrinsic alignment effects respectively. Combining the information from an optical and a radio survey can also help remove systematics from both datasets. This has motivated this study that uses archival radio and optical data to treat telescope systematics and measure an unbiased weak lensing signal using shape information derived from radio observations. Using simulations I have shown that an unbiased convergence cross power spectrum can be measured in the presence of the large scale (θ > 1◦) systematics detected in FIRST and SDSS. The method however amplifies the uncertainties by a factor ∼2.5 compared to the errors due to cosmic variance and noise due to galaxy intrinsic shape alone. Using the shape information from the two surveys I measure a Ckappakappa spectrum signal that is inconsistent with zero at the 2.7sigma. The placed constraints are consistent with the expected signal in the concordance cosmological model assuming recent estimates of the cosmological parameters from the Planck satellite and literature values for the median redshifts of SDSS and FIRST.Through simulations I also show that I can successfully remove position based small scale systematics (θ5). Using the deconvolved information for the resolved sources I calculate a FWHM median size and flux density of 0.5'' and 300μJy respectively. Comparing the source number density and RMS noise of the study with those of FIRST, I extrapolate to predict that the number density of sources at > 5sigma will be ∼5arcmin-2, assuming the target noise threshold for the survey is reached.

Dark matter and dark energy clearly emerged from recent cosmological surveys as key ingredients of the Universe. Understanding their physical nature might be a way to unlock some of the mysteries in particle physics and General Relativity. In this talk I will discuss how gravitational lensing will have a unique contribution in this endeavor. I will also discuss how future weak lensing surveys, primarily designed to study dark matter and dark energy, will enable the detailed analysis of the physical processes underlying structure formation such as galaxies and clusters of galaxies. Presented on April 10, 2008.

We investigate the uses of gravitational lensing for analysing the dark matter haloes around galaxies, comparing galaxies within groups and clusters to those in the field. We consider two cases: when only photometric redshift data is available, and when spectroscopic redshift data is available for a sufficiently large sample of galaxies. For the case of data with photometric redshifts, we analyse the CFHTLenS dataset. This dataset is derived from the CFHTLS-Wide survey, and encompasses 154 deg^2 of high-quality shape data. Using the photometric redshifts to estimate local density, we divide the sample of lens galaxies with stellar masses in the range 10^9 Msun to 10^10.5 Msun into those likely to lie in high-density environments (HDE) and those likely to lie in low-density environments (LDE). Through comparison with galaxy catalogues extracted from the Millennium Simulation, we show that the sample of HDE galaxies should primarily (~61%) consist of satellite galaxies in groups, while the sample of LDE galaxies should consist of mostly (~87%) non-satellite (field and central) galaxies. Comparing the lensing signals around samples of HDE and LDE galaxies matched in stellar mass, we show that the subhaloes of HDE galaxies are less massive than those around LDE galaxies by a factor 0.65+/-0.12, significant at the 2.9 sigma level. A natural explanation is that the haloes of satellite galaxies are stripped through tidal effects in the group environment. Our results are consistent with a typical tidal truncation radius of ~40 kpc. For the case of data with spectroscopic redshifts, we analyse the GAMA-I and the ongoing GAMA-II surveys. We demonstrate the possibility of detecting tidal stripping of dark matter subhaloes within galaxy groups using weak gravitational lensing. We have run ray-tracing simulations on galaxy catalogues from the Millennium Simulation to generate mock shape catalogues. The ray-tracing catalogues assume a halo model for galaxies and groups, using various models with different distributions of mass between galaxy and group haloes to simulate different stages of group evolution. Using these mock catalogues, we forecast the lensing signals that will be detected around galaxy groups and satellite galaxies, as well as test two different methods for isolating the satellites' lensing signals. A key challenge is to determine the accuracy to which group centres can be identified. We show that with current and ongoing surveys, it will possible to detect stripping in groups of mass 10^12 Msun to 10^15 Msun.

No description available.

Les relevés à venir comme Euclid, LSST et WFIRST vont nous ouvrir la perspective d’étudier l’univers profond. Pour ces grands relevés, l’astigmatisme cosmique correspond à une sonde indispensable pour étudier la nature de l’énergie noire et la matière noire. Compte tenu de la précision attendue par ces observations, nous devons faire des prédictions basées sur des simulations correspondant à l’état de l’art afin de quantifier avec précision la variance, les biais et les dégénérescences potentielles liés aux baryons. Dans ce contexte, ma thèse se focalise sur la construction d’estimateurs précis basés sur les observables de lentillage. La première partie de ma thèse consiste à caractériser la géométrie des grandes structures par astigmatisme cosmique (Gouin et al. 2017). Une décomposition multipolaire du signal est appliquée afin de quantifier la distribution azimutale de la matière noire, centrée sur les amas. Les propriétés statistiques de ces moments sont estimées à partir d’une simulation cosmologique. Les distorsions harmoniques calculées dans le voisinage des amas tracent la structure filamentaire. Un plus grand nombre de filaments semblent connectés aux amas de forte masse. Dans la dernière partie de ma thèse, je synthétise le signal d’astigmatisme cosmique dans le cône de lumière de la simulation Horizon AGN. Pour ce faire, je propage les rayons de lumière le long du cône dans l’approximation des plans de lentillage multiples. L’effet des baryons est significatif dans la statistique du cisaillement aux échelles angulaires inférieures à l’arc-minute. Le signal de cisaillement galaxie-galaxie est comparée aux observations récentes, et semble être en bon accord. / Upcoming weak lensing surveys such as Euclid, LSST and WFIRST will provide an unprecedented opportunity to investigate the dark Universe. Through these large scale surveys, gravitational lensing is an indispensable cosmological probe to investigate the dark energy and the dark matter. Due to the new level of accuracy in observations, we must perform cosmological predictions in state-of-art simulations, to precisely quantify its variances, biases and potential degeneracies coming from baryonic physics. In this context, my thesis focuses on the construction of accurate weak lensing observables. The first part of my PhD work characterises the geometry of large-scale structure through weak lensing (Gouin et al. 2017). I relied on multipolar decomposition of weak lensing signal to quantify the azimuthal distribution of dark matter centred on galaxy clusters. The statistical properties of these moments are estimated from a large N-body simulation. The harmonic distortions computed in the vicinity of clusters appear to trace the filamentary structure. Larger number of filaments seem to be connected to high-mass clusters.The detection level of this statistical estimator is estimated. In the last part of my thesis, I mock the weak gravitational lensing signal in the light-cone of the Horizon-AGN simulation (Gouin et al. 2019). To do so, I propagate light-rays along the light-cone in the multiple-lens-plane approximation. The impact of baryons is significant in cosmic shear statistics for angular scales below a few arcmins. In addition, the galaxy-galaxy lensing signal is compared to current observational measurements (Leauthaud et al. 2017), and seems in good agreement.