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UBC Theses and Dissertations

A new approach to photometric redshift contamination, providing critical insight for weak lensing cosmology Benjamin, Jonathan Remby Embro


Light travelling through the Universe is deflected by the presence of mass, this effect will distort the size and shape of observed galaxies. Weak gravitational lensing measures the amount of mass in the Universe by observing these subtle changes in the shapes of distant galaxies. In order to properly interpret the observed shapes of galaxies their distances must be accurately known, this information is encoded in the redshift distribution. A detailed spectroscopic observation is the most reliable way to measure the redshift of a galaxy. Unfortunately this is a time-intensive process and weak lensing surveys are composed of millions of galaxies many of which are too faint for spectroscopic observation. For this reason photometric redshifts are used. Photometric redshifts are less accurate than spectroscopic redshifts but are easier to obtain since they rely on only a few measurements over large ranges of wavelength. Thorough knowledge of uncertainties in the photometric redshifts is vital to weak lensing because photometric redshifts provide the distances necessary to understand the weak lensing signal. In this thesis we present a new technique to measure the reliability of photometric redshifts with the goal of improving the estimated redshift distribution for use in weak lensing studies. Mock observational surveys are used to test the technique before applying it to two surveys: the Deep component of the Canada-France-Hawaii Telescope Legacy Survey (CFHTLS-Deep) and the CFHT Lensing Survey (CFHTLenS). We demonstrate our ability to construct both the true redshift distribution and the true average redshift of galaxies in a given photometric redshift range. Furthermore, we show that the photometric redshift probability distribution function can be used as an accurate measure of the true redshift distribution when summed for an ensemble of galaxies. Using our tested redshift distribution we present cosmological constraints for CFHTLenS from a weak lensing analysis. We present constraints on cosmological parameters for a model of the Universe with dark energy and cold (non-interacting) dark matter (ΛCDM). We find that our weak lensing analy- sis, combined with other cosmological probes, improves the precision of these measurements by a factor of 1.5 to 2.

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