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Biases in weak lensing measurements of galaxy simulated images
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Resumo(s)
Weak gravitational lensing due to the large-scale distribution of matter in the Universe is responsible for introducing distortions to the images of distant galaxies, in a process known as cosmic shear. This occurs due to the deflection of light by the gravitational field produced by matter inhomogeneities along the line-of-sight, which coherently deforms the observed ellipticity of such galaxies. The dependence of this effect on the dynamics of large-scale structures makes it a powerful tool for probing the nature of dark matter and the evolution of dark energy, allowing constraints to be made to cosmological parameters. Over the years, an increase in precision of cosmological data has been accompanied by an increase in sensitivity to systematic errors. These must be modelled and removed from the measurements, with the accuracy of this calibration becoming a relevant limitation to the performance of future cosmological observations, such as the ones made by the Euclid mission. Following this growing concern, this thesis is focused on the quantification of biases introduced by specific image manipulation procedures, and the evaluation of their impact in the constraint of the cosmological parameters Ωm and Ó8. The first step of this work involved the simulation of parametric galaxy images with realistic properties. To these, a mock variable cosmic shear signal was introduced, after which a point spread function (PSF) was applied, in conjunction with noise as a function of exposure time. This defined the first calibration tests, aiming to quantify the bias from the PSF removal algorithm in the presence of a realistic sample, with noise properties, and as a function of an input cosmic shear signal. The bias analysis from these first tests was done based on galaxy properties, confirming that smaller, fainter galaxies are more susceptible to bias. The results of this analysis also showed that the residual errors from the PSF correction method used in this work strongly depend on both galaxy and PSF ellipticities. The next steps of this work used simulated Euclid-like images, with the objective of quantifying the systematics introduced by the process of dithering and subsequent stacking of dithered images, and the bias introduced by Euclid’s PSF variation along its field-of-view (FoV). Analysing the residual errors remaining after correcting the PSF in different set-ups demonstrated the importance of an accurately modelled PSF, and the calibration of each data processing step regarding systematics. The bias values obtained in the first part of the thesis were then included in the procedure to constrain Ωm and Ó8 parameters. This was done by computing biased correlation functions and the respective maximum likelihood regions, in simulated Euclid-like observing conditions. The results of this analysis allow an overview of the impact that these routine image manipulation procedures and PSF variations might have on weak lensing measurements.
Descrição
Tese de mestrado, Física (Astrofísica e Cosmologia), Universidade de Lisboa, Faculdade de Ciências, 2021
Palavras-chave
lentes gravitacionais simulação de imagens efeitos sistemáticos análise de dados parâmetros cosmológicos Teses de mestrado - 2021