Intracluster light is a close tracer of the dark matter halo shape

TL;DR

This study shows that intracluster light closely traces the shape and orientation of dark matter halos in galaxy clusters, making it a reliable proxy for understanding halo ellipticity.

Contribution

It demonstrates that ICL aligns well with dark matter halo shapes and is more accurate than satellite galaxy distributions as a tracer.

Findings

ICL and DM are closely aligned with minor axis offsets less than 10 degrees.

The axis ratios of ICL and DM are similar, with only about 0.07 difference.

ICL provides a better estimate of DM shape than satellite galaxy distribution.

Abstract

We investigate whether the intracluster light (ICL) can serve as a reliable tracer of the shape of the underlying dark matter (DM) haloes in galaxy clusters. Using the cosmological Hydrangea cluster simulations, we measure the 3D and projected shapes of both components with a shape tensor computed in concentric ellipsoidal shells, out to the virial radius $R_\mathrm{200c}$ for each cluster. The ICL and DM are closely aligned, with their major axes typically offset from each other by $\lesssim$10 degrees. Their axis ratios also match closely, with a typical difference of only $\approx\! 0.07$ for both the major-to-minor and major-to-intermediate axes, the DM being slightly rounder than the ICL. These trends are consistent across 2D and 3D measurements and agree well with results from isophotal fitting of mock images. In detail, the axis ratio offset is sensitive to the method used to remove satellites, and may also depend on the choice of subgrid physics models. We demonstrate that the ICL traces the DM shape better than the distribution of satellite galaxies, which exhibits larger scatter in the axis ratio and misalignment angle and is overall more elliptical. Together, these results indicate that the ICL can act as a useful proxy for DM halo ellipticity and orientation.