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The cold dark matter (CDM) cosmology, which is the standard theory of thestructure formation in the universe, predicts that the outer density profile ofdark 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 theAndromeda galaxy (M31), large-scale stellar structures detected by the recentobservations provide a potentially suitable window to investigate themass--density distribution of the dark matter halo. We explore the densitystructure of the dark matter halo in M31 using an N-body simulation of theinteraction between an accreting satellite galaxy and M31. To reproduce theAndromeda Giant Southern Stream and the stellar shells at the east and westsides 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-fitparameter is $\alpha=-3.7$, which is steeper than the CDM prediction.

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