This thesis presents a practical guide to applying shear measurements as a cosmological tool. We first present one of two scienceready galaxy shape catalogues from Year 1 of the Dark Energy Survey (DES Y1), which covers 1500 square degrees in four bands griz, with a median redshift of 0.59. We describe the shape measurement process implemented by the DES Y1 im3shape catalogue, which contains 21.9M highquality rband bulge/disc fits. In Chapter 3 a new suite of image simulations, referred to as hoopoe, are presented. The hoopoe dataset is tailored to DES Y1 and includes realistic blending, spatial masks and variation in the point spread function. We derive shear corrections, which we show are robust to changes in calibration method, galaxy binning and variance within the simulated dataset. Sources of systematic uncertainty in the simulationbased shear calibration are discussed, leading to a final estimate of the 1 sigma uncertainties in the residual multiplicative bias after calibration of 0.025. Chapter 4 describes an extension of the analysis on the hoopoe simulations into a detailed investigation of the impact of galaxy neighbours on shape measurement and shear cosmology. Four mechanisms by which neighbours can have a nonnegligible influence on shear measurement are identified. These effects, if ignored, would contribute a net multiplicative bias of m ~ 0.03  0.09 in DES Y1, though the precise impact will depend on both the measurement code and the selection cuts applied. We use the cosmological inference pipeline of DES Y1 to explore the cosmological implications of neighbour bias and show that omitting blending from the calibration simulation for DES Y1 would bias the inferred clustering amplitude S8 = sigma_8 (Omega_m /0.3)^0.5 by 1.5 sigma towards low values. Finally, we use the hoopoe simulations to test the effect of neighbourinduced spatial correlations in the multiplicative bias. We find the cosmological impact to be subdominant to statistical error at the current level of precision. Another major uncertainty in shear cosmology is the accuracy of our ensemble redshift distributions. Chapter 5 presents a numerical investigation into the combined constraining power of cosmic shear, galaxy clustering and their crosscorrelation in DES Y1, and the potential for internal calibration of redshift errors. Introducing a moderate uniform bias into the redshift distributions used to model the weak lensing (WL) galaxies is shown to produce a > 2 sigma bias in S8. We demonstrate that this cosmological bias can be eliminated by marginalising over redshift error nuisance parameters. Strikingly, the cosmological constraint of the combined dataset is largely undiminished by the loss of prior information on the WL distributions. We demonstrate that this implicit selfcalibration is the result of complementary degeneracy directions in the combined data. In Chapter 6 we present the preliminary results of an investigation into galaxy intrinsic alignments. Using the DES Y1 data, we show a clear dependence in alignment amplitude on galaxy type, in agreement with previous results. We subject these findings to a series of initial robustness tests. We conclude with a short overview of the work presented, and discuss prospects for the future.
Date of Award  31 Dec 2017 

Original language  English 

Awarding Institution   The University of Manchester


Supervisor  Michael Brown (Supervisor) & Sarah Bridle (Supervisor) 

 Cosmology
 Weak Lensing
 Cosmic Shear
 Galaxy Shape Measurement
Systematic Biases in Weak Lensing Cosmology with the Dark Energy Survey
Samuroff, S. (Author). 31 Dec 2017
Student thesis: Phd