Study of Intrinsic Alignment of Galaxies in Recent Galaxy Surveys




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Weak lensing is one of the most promising new probes of cosmological parameters. However, it also comes with its own series of systematics and challenges to overcome. In this dissertation we will focus on our work done to isolate the astrophysical systematic effect known as intrinsic alignment of galaxies. We first introduce the methodology and underlying theory that gave rise to the field of weak gravitational lensing, explaining how minuscule changes in the shapes of distant galaxies can be used to obtain better constraints on the matter distribution and the makeup of our entire universe. Then we introduce the idea that galaxies are not isolated objects, but rather are located inside clusters of galaxies or along gas-rich filaments of the large scale structure of the universe. This leads to a systematic effect in the form of the galaxy shapes being distorted prior to lensing, because they intrinsically align with the large scale structure around them. This effect causes either false negatives or false positives when interpreted as weak lensing distortions in observations. Intrinsic alignment of galaxies was first detected about a decade and a half ago, and since then a few different approaches have been suggested to mitigate this. Our work is focused on the idea of using the extra information like unused correlations available within modern cosmological surveys to isolate these intrinsic alignment signals. This provides us with the dual advantages of getting rid of a systematic effect but also gives the possibility of studying the intrinsic alignment itself in more detail. Our work has focused on the development of analysis tools for this separation and the subsequent application of these tools to detect of intrinsic alignment of galaxies. This is done in anticipation of the upcoming Vera Rubin Observatory’s Legacy Survey of Space and Time (LSST). The tools described in this dissertation were made specifically to be ready for this survey and were developed as extensions to the analysis tools being developed for the LSST by its Dark Energy Science Collaboration (DESC). Since LSST has not yet begun, we have applied these tools to two earlier surveys: The Kilo Degree Survey (KiDS)’s data release of approximately 450 square degrees and the Dark Energy Survey (DES)’s year one data release. In both cases we have found strong signs of intrinsic alignment using our self-calibration analysis. We also showed how we can use this method to handle both intrinsic alignment in galaxy-galaxy lensing and in cosmic shear correlations. This has strong implications for constraining the physics of galaxy evolution with LSST.



Physics, Astronomy and Astrophysics