In this thesis I present the first gravitational lensing results from the Southern Cosmology Survey (SCS). I provide a preliminary study of an automated pipeline analysis of a large survey, in preparation for larger surveys. Future large-area sky surveys, such as Pan-STARRS-1 (PS1), have similar characteristics to the SCS data and will require full automation of the processing. Therefore, this data set provides an ideal test case to highlight the problems which will be faced by such surveys. To analyse the large SCS dataset, I develop an automated weak lensing pipeline based on the KSB. This pipeline has been rigorously verified using simulations and data which I detail here. Results are shown from a weak lensing analysis of 152 optically-selected clusters in 56 square degrees. I fit universal Navarro, Frenk and White (NFW) profiles to measure cluster masses, and use the relatively large area of the survey to test the universal shape of cluster profiles using stacking of the tangential shears. I present the first lensing mass measurements of Sunyaev-Zel’dovich (SZ) selected clusters. It has been long thought that SZ surveys would be a powerful way to detect galaxy clusters for cosmological studies. Simulations show that the SZ detection is independent of redshift and that the threshold corresponds very closely to a threshold in mass. It was, however, not guaranteed that the first blind SZ experiments would detect mass. Using optical imaging from the SCS, I present lensing masses for three clusters selected by their SZ emission in the South Pole Telescope survey (SPT). I confirm that the SZ selection procedure is successful in detecting mass concentrations and find that the SZ clusters have amongst the largest masses, as high as 15x1014M . Consequently I can confirm that the first installment of SZ detections has detected large mass concentrations. Using the best fit masses for all the clusters, I analytically calculate the expected SZ integrated Y parameter. Finally, the scaling relation of Reyes et al. (2008) of lensing Mlens 200 against optical L200 is tested over the redshift range z = 0:1 - 0:3 and extended to z = 0:3 - 0:8. While there is some discrepancy in the lower redshift-range, we agree with Reyes et al (2008) in the higherredshift sample if we assume no evolution of the scaling relation. To test the tangential shear profile of these clusters, 98 clusters are stacked. We find that by allowing the model to vary from an NFW, a very good fit can be found with a higher normalisation of the shears and a lower concentration. This study supports that of Mandelbaum et al. (2008) who show that that massive halos have a lower concentration than expected. Like the SCS, new large area surveys such as PS1 are not very deep, and it is crucial to understand not only how to analyse this size of dataset, but also the sort of results one could expect to achieve. I show in this thesis that 2D mass reconstructions can be done on data of this quality, and large galaxy clusters successfully reconstructed. With a number density of n ~ 9 it is possible to detect the most massive clusters with lensing, but it is difficult. With the lower number density of n ~ 6 or lower expected from PS1 it will prove very difficult to detect individual clusters. However, PS1 will survey a massive area, and so the stacking analysis should work extremely well, and it should be possible to further test the shape of the cluster profiles with stacking as I demonstrated here with the smaller SCS dataset.

We present a stacked weak-lensing analysis of 27 richness selected galaxy clusters at 0.40 <= z <= 0.62 in the COnstrain Dark Energy with X-ray galaxy clusters (CODEX) survey. The fields were observed in five bands with the Canada-France-Hawaii Telescope (CFHT). We measure the stacked surface mass density profile with a 14 sigma significance in the radial range 0.1 < R Mpc h(-1) < 2.5. The profile is well described by the halo model, with the main halo term following a Navarro-Frenk-White profile (NFW) profile and including the off-centring effect. We select the background sample using a conservative colour-magnitude method to reduce the potential systematic errors and contamination by cluster member galaxies. We perform a Bayesian analysis for the stacked profile and constrain the best-fitting NFW parameters M-200c = 6.6(- 0.8)(+1.0) x 10(14) h(-1)M(circle dot) and c(200c) = 3.7(+0.7) (-0.6). The off-centring effect was modelled based on previous observational results found for redMaPPer Sloan Digital Sky Survey clusters. Our constraints on M(200)c and c(200)c allow us to investigate the consistency with numerical predictions and select a concentration-mass relation to describe the high richness CODEX sample. Comparing our best-fitting values forM(200c) and c(200c) with other observational surveys at different redshifts, we find no evidence for evolution in the concentration-mass relation, though it could be mitigated by particular selection functions. Similar to previous studies investigating the X-ray luminosity-mass relation, our data suggest a lower evolution than expected from self-similarity.

Matter near a gravitational lens galaxy or projected along the line of sight (LOS) can affect strong lensing observables by more than contemporary measurement errors. We simulate lens fields with realistic threedimensional mass configurations (self-consistently including voids), and then fit mock lensing observables with increasingly complex lens models to quantify biases and uncertainties associated with different ways of treating the lens environment (ENV) and LOS. We identify the combination of mass, projected offset, and redshift that determines the importance of a perturbing galaxy for lensing. Foreground structures have a stronger effect on the lens potential than background structures, due to nonlinear effects in the foreground and downweighting in the background. There is dramatic variation in the net strength of ENV/LOS effects across different lens fields; modeling fields individually yields stronger priors for H-0 than ray tracing through N-body simulations. Models that ignore mass outside the lens yield poor fits and biased results. Adding external shear can account for tidal stretching from galaxies at redshifts z >= z(lens), but it requires corrections for external convergence and cannot reproduce nonlinear effects from foreground galaxies. Using the tidal approximation is reasonable for most perturbers as long as nonlinear redshift effects are included. Even then, the scatter in H0 is limited by the lens profile degeneracy. Asymmetric image configurations produced by highly elliptical lens galaxies are less sensitive to the lens profile degeneracy, so they offer appealing targets for precision lensing analyses in future surveys like LSST and Euclid.

We present cosmological constraints from the Dark Energy Survey (DES) using a combined analysis of angular clustering of red galaxies and their cross-correlation with weak gravitational lensing of background galaxies. We use a 139 deg(2) contiguous patch of DES data from the Science Verification (SV) period of observations. Using large-scale measurements, we constrain the matter density of the Universe as Omega(m) = 0.31 +/- 0.09 and the clustering amplitude of the matter power spectrum as sigma(8) = 0.74 +/- 0.13 after marginalizing over seven nuisance parameters and three additional cosmological parameters. This translates into S-8 = sigma(8)(Omega(m)/0.3)(0.16) = 0.74 +/- 0.12 for our fiducial lens redshift bin at 0.35 < z < 0.5, while S-8 = 0.78 +/- 0.09 using two bins over the range 0.2 < z < 0.5. We study the robustness of the results under changes in the data vectors, modelling and systematics treatment, including photometric redshift and shear calibration uncertainties, and find consistency in the derived cosmological parameters. We show that our results are consistent with previous cosmological analyses from DES and other data sets and conclude with a joint analysis of DES angular clustering and galaxy-galaxy lensing with Planck Cosmic Microwave Background data, baryon accoustic oscillations and Supernova Type Ia measurements.

We report the serendipitous discovery of HSC J142449-005322, a double source plane lens system in the Hyper Suprime-Cam Subaru Strategic Program. We dub the system Eye of Horus. The lens galaxy is a very massive early-type galaxy with stellar mass of similar to 7 x 10(11) M-circle dot located at z(L) = 0.795. The system exhibits two arcs/rings with clearly different colors, including several knots. We have performed spectroscopic follow-up observations of the system with FIRE on Magellan. The outer ring is confirmed at z(S2) = 1.988 with multiple emission lines, while the inner arc and counterimage is confirmed at z(S1) = 1.302. This makes it the first double source plane system with spectroscopic redshifts of both sources. Interestingly, redshifts of two of the knots embedded in the outer ring are found to be offset by Delta z = 0.002 from the other knots, suggesting that the outer ring consists of at least two distinct components in the source plane. We perform lens modeling with two independent codes and successfully reproduce the main features of the system. However, two of the lensed sources separated by similar to 0.7 arcsec cannot be reproduced by a smooth potential, and the addition of substructure to the lens potential is required to reproduce them. Higher-resolution imaging of the system will help decipher the origin of this lensing feature and potentially detect the substructure.

Cosmic voids are usually identified in spectroscopic galaxy surveys, where 3D information about the large-scale structure of the Universe is available. Although an increasing amount of photometric data is being produced, its potential for void studies is limited since photometric redshifts induce line-of-sight position errors of >= 50 Mpc h(-1)which can render many voids undetectable. We present a new void finder designed for photometric surveys, validate it using simulations, and apply it to the high-quality photo-z redMaGiC galaxy sample of the DES Science Verification data. The algorithm works by projecting galaxies into 2D slices and finding voids in the smoothed 2D galaxy density field of the slice. Fixing the line-of-sight size of the slices to be at least twice the photo-z scatter, the number of voids found in simulated spectroscopic and photometric galaxy catalogues is within 20 per cent for all transverse void sizes, and indistinguishable for the largest voids (R-v >= 70 Mpc h(-1)). The positions, radii, and projected galaxy profiles of photometric voids also accurately match the spectroscopic void sample. Applying the algorithm to the DES-SV data in the redshift range 0.2 < z < 0.8, we identify 87 voids with comoving radii spanning the range 18-120 Mpc h(-1), and carry out a stacked weak lensing measurement. With a significance of 4.4 sigma, the lensing measurement confirms that the voids are truly underdense in the matter field and hence not a product of Poisson noise, tracer density effects or systematics in the data. It also demonstrates, for the first time in real data, the viability of void lensing studies in photometric surveys.

Gravitational lensing studies the distortions of a distant galaxy’s observed size, shape or flux due to the tidal bending of photons by matter between the source and observer. Such distortions can be used to infer knowledge on the mass distribution of the intervening matter, such as the dark matter halos in which clusters of individual galaxies may reside, or on cosmology through the statistics of the matter density of large scale structure and geometrical factors. In particular, gravitational lensing has the advantage that it is insensitive to the nature of the lensing matter. However, contamination of the signal by correlations between galaxy shape or size and local environment complicate a lensing analysis. Further, measurement of traditional lensing estimators is made more difficult by limitations on observations, in the form of atmospheric distortions or optical limits of the telescope itself. As a result, there has been a large effort within the lensing community to develop methods to either reduce or remove these contaminants, motivated largely by stringent science requirements for current and forthcoming surveys such as CFHTLenS, DES, LSST, HSC, Euclid and others. With the wealth of data from these wide-field surveys, it is more important than ever to understand the full range of independent probes of cosmology at our disposal. In particular, it is desirable to understand how each probe may be used, individually and in conjunction, to maximise the information of a lensing analysis and minimise or mitigate the systematics of each. With this in mind, I investigate the use of galaxy clustering measurements using photometric redshift information, including a contribution from flux magnification, as a probe of cosmology. I present cosmological forecasts when clustering data alone are used, and when clustering is combined with a cosmic shear analysis. I consider two types of clustering analysis: firstly, clustering with only redshift auto-correlations in tomographic redshift bins; secondly, clustering using all available redshift bin correlations. Finally, I consider how inferred cosmological parameters may be biased using each analysis when flux magnification is neglected. Results are presented for a Stage–III ground-based survey, and a Stage–IV space-based survey modelled with photometric redshift errors, and values for the slope of the luminosity function inferred from CFHTLenS catalogues. I find that combining clustering information with shear gives significant improvement on cosmological parameter constraints, with the largest improvement found when all redshift bins are included in the analysis. The addition of galaxy-galaxy lensing gives further improvement, with a full combined analysis improving constraints on dark energy parameters by a factor of > 3. The presence of flux magnification in a clustering analysis does not significantly affect the precision of cosmological constraints when combined with cosmic shear and galaxy-galaxy lensing. However if magnification is neglected, inferred cosmological parameter values are biased, with biases in some cosmological parameters found to be larger than statistical errors. We find that a combination of clustering, cosmic shear and galaxy-galaxy lensing can provide a significant reduction in statistical errors from each analysis individually, however care must be taken to measure and model flux magnification. Finally, I consider how measurements of galaxy size and flux may be used to constrain the dark matter profile of a foreground lens, such as galaxy- or galaxy-cluster-dark matter halos. I present a method of constructing probability distributions for halo profile free parameters using Bayes’ Theorem, provided the intrinsic size-magnitude distribution may be measured from data. I investigate the use of this method on mock clusters, with an aim of investigating the precision and accuracy of returned parameter constraints under certain conditions. As part of this analysis, I quantify the size and significance of inaccuracies in the dark matter reconstruction as a result of limitations in the data from which the sample and size-magnitude distribution is obtained. This method is applied to public data from the Space Telescope A901/902 Galaxy Evolution Survey (STAGES), and results are presented for the four STAGES clusters using measurements of source galaxy size and magnitude, and a combination of both. I find consistent results with existing shear measurements using measurements of galaxy magnitudes, but interesting inconsistent results when galaxy size measurements are used. The simplifying assumptions and limitations of the analysis are discussed, and extensions to the method presented.

O objetivo deste trabalho foi determinar a massa do aglomerado de galáxias RXC J1504-0248, localizado em z = 0.215, através da análise de lentes fracas, e comparar os resultados com aqueles obtidos em trabalhos anteriores através da análise de raios-X. Imagens do aglomerado foram obtidos nas bandas r\', g\' e i\' com o detector GMOS do telescópio Gemini Sul. A partir dessas imagens, contruímos um catálogo de objetos no campo usando o software Sextractor (SE) (Bertin e Arnouts, 1996). Este software também foi utilizado para classificá-los como galáxias ou estrelas. Foram identificadas 172 galáxias neste campo, que também foram detectadas no Data Release 7 do Sloan Digital Sky Survey (SDSS). Estas galáxias foram então usadas para obter uma calibração fotométrica das imagens, comparando as magnitudes instrumentais e do SDSS nas mesmas bandas. Após a calibração fotométrica, e através da comparação com imagens do CFHTLS obtidas em cores semelhantes, as galáxias foram classificadas como membros de cluster, foreground ou background, a partir de sua posição nos diagramas cor-cor e cor-magnitude. A reconstrução da massa do aglomerado através da análise de lentes gravitacionais foi realizada em duas etapas. Na primeira, foi utilizado o software IM2SHAPE, desenvolvido por Bridle et al. (1998), que modela os objetos, adicionando até três gaussianas, cada uma definida por seis parâmetros: as coordenadas do centro do objeto, x0 e y0, a elipticidade e, o ângulo de posição , o produto dos semi-eixos maior e menor ab, e a amplitude A. Inicialmente, o programa foi executado apenas para as estrelas do campo, com o objetivo de se obter uma estimativa da distribuição da PSF. A estimativa foi então utilizado como entrada para a análise das galáxias. Na segunda etapa, para estimar a massa do aglomerado foi utilizado o programa LENSENT, desenvolvido por Marshall et al. (2002), cujos parâmetros de entrada são a elipticidade das galáxias de fundo e seus erros. Na técnica de lentes gravitacionais fracas, a dependência radial da deformação das galáxias de fundo permite determinar o perfil de massa do aglomerado. Para estimar a massa, ajustamos um perfil de uma Esfera Isotérmica Singular (SIS, na sigla em inglês), e determinamos o valor da massa dentro de um raio de 3Mpc, 1.3 ± 0.6 x 10¹ Msol. O resultado é consistente com o obtido por Bohringer et al., 2005, 1.7 x 10¹ Msol, através da análise em raios-X. Comparando o mapa de distribuição de luminosidade e da emissão de raios-X concluímos que eles são muito semelhantes à distribuição superficial de massa, resultado que indica equilíbrio. / In this work we studied the galaxy cluster RXC J1504-0248, at z=0.215, from images in the bands r\', g\' and i\' obtained with Gemini South telescope. The photometric calibration was performed by comparison with field objects identified in the Sloan Digital Sky Survey (SDSS). From the analysis of color-color and color-magnitude diagrams, galaxies in the field were then divided into cluster members, background and foreground objects. We determined the PSF using the IM2SHAPE program \\cite{Bridle98}. These results enabled us to obtain the cluster projected mass distribution through a weak lensing analysis performed with the LENSENT program \\cite{Marshall02}. We also shown that the cluster luminosity distribution and the X-ray emission are consistent with the mass map. Using a SIS model, we estimated the mass of the cluster, obtaining 1.3 x 10¹ Msun, consistent with the mass obtained in a previous X-ray analysis, 1.7 x 10¹ Msun, by \\cite{Bohringer05}.

In this thesis, I develop and combine strong lensing and dynamical probes of the mass of early-type galaxies (ETGs) in order to improve our understanding of their dark and luminous mass structure and evolution. Firstly, I demonstrate that the dark matter halo of our nearest brightest cluster galaxy (BCG), M87, is centrally cored relative to the predictions of dark-matter-only models, and suggest an interpretation of this result in terms of dynamical heating due to the infall of satellite galaxies. Conversely, I find that the haloes of a sample of 12 field ETGs are strongly cusped, consistent with adiabatic contraction models due to the initial infall of gas. I suggest an explanation for these differences in which the increased rate of merging and accretion experienced by ETGs in dense environments leads to increased amounts of halo heating and expansion, such that the signature of the halo's initial contraction is erased in BCGs but retained in more isolated systems. Secondly, I find evidence that the stellar-mass-to-light ratio declines with increasing radius in both field and cluster ETGs. With M87, I show that the strength of this gradient cannot be explained by trends in stellar metallicity or age if the stellar initial mass function (IMF) is spatially uniform, but that an IMF which becomes increasing bottom-heavy towards the galaxy centre can fully reproduce the inference on the stellar mass. Finally, I use the sizes, stellar masses and luminous structures of two samples of massive ETGs at redshift z ~ 0.6 to set constraints on the mechanisms of ETG growth. I find that ETGs in dense cluster environments already lie on the local size-mass relation at this redshift, contrary to their isolated counterparts, and suggest that this may be evidence for their accelerated growth at early times due to the higher incidence of merger events in clusters. I also show that massive compact ETGs at this redshift are composed of a compact, red, spheroidal core surrounded by a more extended, diffuse, bluer envelope, which may be a structural imprint of their ongoing inside-out growth. Overall, the studies presented in this thesis suggest a coherent scenario for ETG evolution which is dominated by hierarchical processes.

Galaxy clusters are the most massive collapsed structures in the Universe and their properties offer a crucial insight into the formation of structure. High quality observational data is forthcoming with ongoing and upcoming surveys, but simulations are needed to provide robust theoretical predictions for comparison, as well mock data for testing observational techniques. Numerical simulations are now able to accurately model a range of astrophysical processes. This is highlighted in the BAHAMAS and MACSIS simulations, which have successfully reproduced the observed scaling relations of galaxy clusters. We use these simulations to quantify the impact baryons have on the mass distribution within galaxy clusters, as well as the bias in X-ray and weak lensing mass estimates. It is shown that baryons have only a minor affect on the spins, shape and density profiles of galaxy clusters and they have no significant impact on the bias in weak lensing mass estimates. When using spectroscopic temperatures and densities, the X-ray hydrostatic mass bias decreases as a function of mass, leading to a bias of ~40% for clusters with M_500 > 10^15 solar masses. In the penultimate chapter, we use the EAGLE and C-EAGLE simulations to construct more realistic mock cluster observations. The EAGLE simulations have been shown to successfully reproduce the properties of field galaxies and they are complemented by the C-EAGLE project, which extends this work to the cluster scale. We use these simulations to construct a cluster lightcone that accounts for the impact of uncorrelated large scale structure on cluster observables, including weak lensing mass estimates, the Sunyaev-Zel'dovich parameter and X-ray luminosity.