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Dissertation/Thesis Abstract

Lights All Askew: Systematics in Galaxy Images from Megaparsecs to Microns
by Bradshaw, Andrew Kenneth, Ph.D., University of California, Davis, 2017, 80; 10640482
Abstract (Summary)

The stars and galaxies are not where they seem. In the process of imaging and measurement, the light from distant objects is distorted, blurred, and skewed by several physical effects on scales from megaparsecs to microns. Charge-coupled devices (CCDs) provide sensitive detection of this light, but introduce their own problems in the form of systematic biases. Images of these stars and galaxies are formed in CCDs when incoming light generates photoelectrons which are then collected in a pixel’s potential well and measured as signal. However, these signal electrons can be diverted from purely parallel paths toward the pixel wells by transverse fields sourced by structural elements of the CCD, accidental imperfections in fabrication, or dynamic electric fields induced by other collected charges. These charge transport anomalies lead to measurable systematic errors in the images which bias cosmological inferences based on them. The physics of imaging therefore deserves thorough investigation, which is performed in the laboratory using a unique optical beam simulator and in computer simulations of charge transport.

On top of detector systematics, there are often biases in the mathematical analysis of pixelized images; in particular, the location, shape, and orientation of stars and galaxies. Using elliptical Gaussians as a toy model for galaxies, it is demonstrated how small biases in the computed image moments lead to observable orientation patterns in modern survey data. Also presented are examples of the reduction of data and fitting of optical aberrations of images in the lab and on the sky which are modeled by physically or mathematically-motivated methods.

Finally, end-to-end analysis of the weak gravitational lensing signal is presented using deep sky data as well as in N-body simulations. It is demonstrated how measured weak lens shear can be transformed by signal matched filters which aid in the detection of mass overdensities and separate signal from noise. A commonly-used decomposition of shear into two components, E- and B-modes, is thoroughly tested and both modes are shown to be useful in the detection of large scale structure. We find several astrophysical sources of B-mode and explain their apparent origin. The methods presented therefore offer an optimal way to filter weak gravitational shear into maps of large scale structure through the process of cosmic mass cartography.

Indexing (document details)
Advisor: Tyson, J. Anthony
Commitee: Fassnacht, Christopher, Knox, Lloyd
School: University of California, Davis
Department: Physics
School Location: United States -- California
Source: DAI-B 79/08(E), Dissertation Abstracts International
Subjects: Astrophysics, Astronomy
Keywords: Astrometry, Charge-coupled devices, Mass mapping, Pixelization, Weak gravitational lensing
Publication Number: 10640482
ISBN: 978-0-355-76373-7
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