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

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.

5 pages, 4 figures, accepted for publication in PASJ Letter

References in corpus (4)

• The M31 Velocity Vector. II. Radial Orbit Towards the Milky Way and Implied Local Group Mass
• The Shape of the Gravitational Potential in Cold Dark Matter Halos
• Investigating the Andromeda Stream: III. A Young Shell System in M31
• The Once and Future Andromeda Stream

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• Elemental Abundances in M31: Gradients in the Giant Stellar Stream
• Kinematics and Metallicity of Red Giant Branch Stars in the Northeast Shelf of M31
• The survey of planetary nebulae in Andromeda (M31) VII. Predictions of a major merger simulation model compared with chemodynamical data of the disc and inner halo substructures
• Prospects for annihilating dark matter from M31 and M33 observations with the Cherenkov Telescope Array
• The origin of the black hole offset in M31