Flattening of dark matter cusps during mergers: model of M31.

We run high resolution fully GPU N-body simulations to show that dark matter distribution in M31 is well-fitted by a core-like profile. Rich observational data especially on giant stellar stream provide stringent constraints on the initial conditions of our simulations. We demonstrate that accretion of a satellite on a highly eccentric orbit heats up the central parts of M31, causes an outward migration of dark matter particles, flattens the central cusp over more than a decade in scale and generates a new model-independent dark matter profile that is well-fit by a core. Our results imply that core-like central profiles could be a common feature of galaxies that have been initially cuspy but have accreted satellites on eccentric orbits.