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

Takanobu Kirihara, Yohei Miki, Masao Mori

Published 2014-08-21Version 1

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. Large-scale stellar structures detected by the recent observations in the halo of the Andromeda galaxy (M31) provide an attractive window to explore the structure of the dark matter halo. Using an N-body simulation of the interaction between an accreting satellite galaxy and M31, we investigate the mass-density profile of the dark matter halo in 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.