Quantifying the heart of darkness with GHALO - a multibillion particle simulation of a galactic halo

We perform a series of simulations of a Galactic mass dark matter halo at different resolutions: our largest uses over 3 billion particles and has a mass resolution of 1000 M⊙ . We quantify the structural properties of the inner dark matter distribution and study how they depend on numerical resolution. We can measure the density profile to a distance of 120 pc (0.05 per cent of Rvir ), where the logarithmic slope is −0.8 and −1.4 at (0.5 per cent of Rvir ). We propose a new two-parameter fitting function that has a linearly varying logarithmic density gradient over the resolved radii which fits the GHALO and VL2 density profiles extremely well. Convergence in the halo shape is achieved at roughly three times the convergence radius for the density profile at which point the halo becomes more spherical due to numerical resolution. The six-dimensional phase-space profile is dominated by the presence of the substructures and does not follow a power law, except in the central few kpc which is devoid of substructure even at this resolution. The quantity, ρ/σ3 , which is often used as a proxy for the six-dimensional phase-space density should be used with caution.