The prolate dark matter halo of the Andromeda galaxy

TL;DR

This paper constrains the shape of Andromeda's dark matter halo, finding it to be prolate, which aligns with cold dark matter models and enhances understanding of galaxy formation and dynamics.

Contribution

It introduces generalized non-spherical mass models applied to Andromeda, revealing a prolate dark matter halo consistent with CDM predictions.

Findings

Andromeda's dark halo is prolate in shape.

Prolate halo aligns with anisotropic satellite distribution.

Results support CDM models and cosmic web connection.

Abstract

We present new limits on the global shape of the dark matter halo in the Andromeda galaxy using and generalizing non-spherical mass models developed by Hayashi & Chiba and compare our results with theoretical predictions of cold dark matter (CDM) models. This is motivated by the fact that CDM models predict non-spherical virialized dark halos, which reflect the process of mass assembly in the galactic scale. Applying our models to the latest kinematic data of globular clusters and dwarf spheroidal galaxies in the Andromeda halo, we find that the most plausible cases for Andromeda yield a prolate shape for its dark halo, irrespective of assumed density profiles. We also find that this prolate dark halo in Andromeda is consistent with theoretical predictions in which the satellites are distributed anisotropically and preferentially located along major axes of their host halos. It is a reflection of the intimate connection between galactic dark matter halos and the cosmic web. Therefore, our result is profound in understanding internal dynamics of halo tracers in Andromeda, such as orbital evolutions of tidal stellar streams, which play important roles in extracting the abundance of CDM subhalos through their dynamical effects on stream structures.