The Shape of Dark Matter Haloes: Results from Weak Lensing

Abstract

Dark matter haloes are expected to have a triaxial shape, appearing elliptical in projection. Understanding the orientation of these dark matter haloes, and how they are aligned, has implications for theories of structure formation in the universe. Additionally, a better understanding of halo alignment will allow us to account for intrinsic alignments and improve future weak lensing studies. We measure dark matter halo ellipticity using weak galaxy-galaxy lensing. We study the anisotropic shear around luminous red galaxies (LRGs) from the Baryon Oscillation Spectroscopic Survey (BOSS), using galaxies from the Canada-France Imaging Survey (CFIS) as background source galaxies. By aligning with the major axis of the galaxy light before measuring the shear, we can stack many lens galaxies to obtain a detection of halo ellipticity. We model the dark matter mass distribution as a multipole expansion, with an isotropic monopole component and a quadrupole that accounts for the angular dependence and ellipticity. We calculate 6 quadrupole estimators, which are designed to measure anisotropic lensing signal while minimising any isotropic contributions. Taking the results from these estimators into account, we obtain a mean halo ellipticity of e = 0.226 ± 0.102. We also study the lensing signal when aligning with filaments, using pairs of LRGs as a proxy. This also yields a detection of dark matter anisotropy. A meaningful measure of ellipticity along the filaments will require further development of the model to account for anisotropy on larger scales.