Halo Shapes From Weak Lensing: The Impact of Galaxy--Halo Misalignment

TL;DR

This study investigates how galaxy--halo misalignment affects the ability of weak lensing to measure dark matter halo shapes, revealing that misalignments significantly obscure the ellipticity signal and complicate interpretation.

Contribution

It provides a comprehensive analysis of galaxy--halo misalignment effects on weak lensing measurements using simulations and models, highlighting the challenges in detecting halo ellipticity.

Findings

Significant misalignments wash out the halo ellipticity signal.

Only aligned galaxy-halo axes produce a measurable ellipticity below 0.95.

Theoretical uncertainties exceed statistical errors, complicating interpretation.

Abstract

We analyse the impact of galaxy--halo misalignment on the ability of weak lensing studies to constrain the shape of dark matter haloes, using a combination of the Millennium dark matter N-body simulation and different semi-analytic galaxy formation models, as well as simpler Monte Carlo tests. Since the distribution of galaxy--halo alignments is not known in detail, we test various alignment models, together with different methods of determining the halo shape. In addition to alignment, we examine the interplay of halo mass and shape, and galaxy colour and morphology with the resulting stacked projected halo shape. We find that only in the case where significant numbers of galaxy and halo minor axes are parallel does the stacked, projected halo axis ratio fall below 0.95. When using broader misalignment distributions, such as those found in recent simulations of galaxy formation, the halo ellipticity signal is washed out and would be extremely difficult to measure observationally. It is important to note that the spread in stacked halo axis ratio due to theoretical unknowns (differences between semi-analytic models, and between alignment models) are much bigger than any statistical uncertainty: It is naive to assume that, simply because LCDM predicts aspherical haloes, the stacked projected shape will be elliptical. In fact, there is no robust LCDM prediction yet for this procedure, and the interpretation of any such elliptical halo signal from lensing in terms of physical halo properties will be extremely difficult.