"Dark Matter Halo Alignments and Probes of Large-Scale Structure"

ABSTRACT:

 

The alignments of triaxial halo shapes are both a key test of the Cold Dark Matter (CDM) model and a source of systematic uncertainty in cosmological probes. Some recent tests of CDM and galaxy formation models have attempted to measure the alignments of Brightest Cluster Galaxies with the surrounding dark matter distribution or the radial or spatial alignments of satellite galaxies in groups and clusters.

The data and theory predictions largely do not agree, but it is tricky to determine whether the problem lies with convergence in simulations, observational systematics, missing galaxy formation physics, or a failure of CDM. I will discuss the issue of convergence in simulations
based on a joint analysis of the Millennium and Millennium-II simulations yielding measurements of dark matter halo shapes and orientations over an order of magnitude in halo radius and five orders of magnitude in halo mass. I will then use recent measurements of
galaxy alignments in the Sloan Digital Sky Survey to illustrate the clash between observational systematics and models for satellite galaxy accretion. Finally, I will describe how our measurements make new predictions for the intrinsic galaxy alignment contamination in
cosmic shear surveys.