Weak gravitational lensing - the subtle distortion of the images of background galaxies caused by the bending of light by intervening mass - is one of the most potentially powerful probes of dark energy. But measurement and interpretation of the lensing signal is complicated by intrinsic correlations between the alignments of galaxy images. Intrinsic correlations can arise when physically close galaxies are aligned by the gravitational field of the matter surrounding them, or when a matter structure links the orientation of a galaxy close to it to the lensed image of a galaxy far behind. Previous studies have shown that if intrinsic alignments are ignored, the lensing constraints on the dark energy equation of state are biased by up to 50%. This project will study the 3-point intrinsic correlations of alignments and how their effects can be removed from the lensing signal. The Principal Investigator, collaborator, and graduate student supported by the project will carry out analytical calculations and numerical integrations to develop the method of "self-calibration," which uses additional correlations that can all be extracted from the same lensing survey to calculate the intrinsic alignment contamination and remove it. They will derive statistical estimators of the 3-point correlations and study their dependencies on redshift, luminosity, color, and morphology in order to understand their scaling properties. The work will have broad impact in cosmology, in developing an essential correction to lensing surveys. It will also contribute to graduate education and to the development research program at the Principal Investigator's institution. The Principal Investigator will also be involved in outreach to local amateur astronomy groups and through participation in public presentations and professional development programs for teachers.

Project Report

Intellectual merit: This investigation addressed open questions related to the 2- and 3-point intrinsic alignments of galaxy ellipticities as a contaminant of gravitational weak lensing. First, the investigation allowed the extension of the self-calibration method to the 3-point intrinsic alignments and their removal. Understanding these systematic effects and providing techniques to remove them is crucial for precision cosmology using ongoing and future lensing surveys such as LSST, DES, Euclid, and WFIRST, and will allow gravitational lensing to reach its full potential. For example, studies have shown that if intrinsic alignments are ignored then the lensing constraints on the dark energy equation of state are biased by up to 50%. There are 2 types of intrinsic alignments: the first are intrinsic ellipticity correlations, also known as the II signal for the 2-points and III for the 3-points, which arise due to the fact that physically close galaxies are aligned by the tidal force field of the same matter surrounding them. The second type occurs when a matter structure causes the alignment of a nearby galaxy and also contributes to the lensing of a background galaxy, producing an anti-correlation between lensing and intrinsic ellipticities, also known as the GI signal for the 2-points and GGI, GII for the 3-points. While the type II and III alignments can be greatly deduced using cross-spectra of galaxies in different redshift bins this does not work for the GI, GGI, and GII since this happens between galaxies at different redshifts and large separations. There has been some active research work to develop methods to eliminate GI contaminations, and the self-calibration method has been recognized to work the best and without throwing away part of the lensing signal. The method is based on using other correlations that can all be extracted from the same lensing survey and can be used in order to calculate the GI signal and remove it. We performed analytical calculations and numerical integrations in order to extend the self-calibration method to the 3-points GGI and GII, and thus facilitate their removal. We derived self-calibration relations along with the assessment of the residual errors. We also derived some estimators of the 3-point correlations that will allow their measurements in ongoing and future surveys. We found that the contamination can be suppressed by a factor of 10 or greater using the self-calibration techniques. This will allow the GGI (as well as the GII) self-calibration to reduce the correlations to the statistical limit of the lensing survey and near the level of the GI self-calibration. Together these promise to be an efficient technique to isolate both the 2- and 3-point IA signals from the cosmic shear signal. This investigation also allowed the characterization of the cross-correlation between cosmic microwave background lensing and galaxy intrinsic alignment as a contaminant to gravitational lensing cross-correlated probes of the universe and the development of self-calibration method to remove it. The results of the investigation have been published in eight journal papers including a review article on the intrinsic alignment of galaxies and its impact on weak gravitational lensing in an era of precision Cosmology. Broader impact: The investigation involved the training of a graduate student, Michael Troxel who contributed to all aspects of the research and also the process of preparing scientific publications. The research also involved indirectly three other graduate students in the group via discussions during our weekly group meetings. Some of the research has also been subject to a news article published online for the whole university community at UD-Dallas. During the conduction of the project, the PI also sponsored two of the Monthly meetings of the Texas Astronomy Society on the campus of UT-Dallas. These meetings include typically 200 amateur astronomers with great interest in what is happening on the research front in astronomy and cosmology, including people with various ages ranging from high school students to senior retired individuals. Also, during the conduction of this project, PI and student have developed several complex computer codes. These are available on demand to the scientific community working on lensing and cosmology. The project led to 8 talks presented at the American Astronomy Society, the Texas-APS section meetings and invited colloquia. PI has supervised three undergraduate students (Thomas Griffin, Nathan Newton and Jonathan Woodbury) during this project. PI Co-chaired the organization of the 50th anniversary of the Texas Symposium on Relativistic Astrophysics in Dallas in December 2013 with 470 participants and over 300 scientific talks. Two parallel sessions were dedicated to gravitational lensing and intrinsic alignments.

National Science Foundation (NSF)
Division of Astronomical Sciences (AST)
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Patricia Knezek
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University of Texas at Dallas
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