Puzzling Outer-Density Profile of the Dark Matter Halo in the Andromeda Galaxy

TL;DR

This study investigates the dark matter halo of the Andromeda galaxy, revealing a steeper outer density profile than standard CDM predictions through N-body simulations of satellite interactions.

Contribution

It provides the first detailed simulation-based analysis of M31's dark matter halo density profile, challenging the standard CDM model predictions.

Findings

Outer density profile index is approximately -3.7, steeper than CDM prediction.

N-body simulations successfully reproduce observed stellar structures in M31.

Results suggest possible deviations from standard dark matter halo models.

Abstract

The cold dark matter (CDM) cosmology, which is the standard theory of the structure formation in the universe, predicts that the outer density profile of dark matter halos decreases with the cube of distance from the center. However, so far not much effort has examined this hypothesis. In the halo of the Andromeda galaxy (M31), large-scale stellar structures detected by the recent observations provide a potentially suitable window to investigate the mass--density distribution of the dark matter halo. We explore the density structure of the dark matter halo in M31 using an N-body simulation of the interaction between an accreting satellite galaxy and M31. To reproduce the Andromeda Giant Southern Stream and the stellar shells at the east and west sides of M31, we find the sufficient condition for the power-law index $\alpha$ of the outer density distribution of the dark matter halo. The best-fit parameter is $\alpha=-3.7$, which is steeper than the CDM prediction.