The phase space density of fermionic dark matter haloes

Abstract

We have performed a series of numerical experiments to investigate how the primordial thermal velocities of fermionic dark matter particles affect the physical and phase space density profiles of the dark matter haloes into which they collect. The initial particle velocities induce central cores in both profiles, which can be understood in the framework of phase space density theory. We find that the maximum coarse-grained phase space density of the simulated haloes (computed in 6 dimensional phase space using the EnBid code) is very close to the theoretical fine-grained upper bound, while the pseudo phase space density, Q ~ ρ/σ^3, overestimates the maximum phase space density by up to an order of magnitude. The density in the inner regions of the simulated haloes is well described by a 'pseudo-isothermal' profile with a core. We have developed a simple model based on this profile which, given the observed surface brightness profile of a galaxy and its central velocity dispersion, accurately predicts its central phase space density. Applying this model to the dwarf spheroidal satellites of the Milky Way yields values close to 0.5 keV for the mass of a hypothetical thermal warm dark matter particle, assuming the satellite haloes have cores produced by warm dark matter free streaming. Such a small value is in conflict with the lower limit of 1.2 keV set by observations of the Lyman-α forest. Thus, if the Milky Way dwarf spheroidal satellites have cores, these are likely due to baryonic processes associated with the forming galaxy, perhaps of the kind proposed by Navarro, Eke and Frenk and seen in recent simulations of galaxy formation in the cold dark matter model.

12 Pages, 5 Figures, 2 Tables. Minor changes. MNRAS in press

References in corpus (15)

• Dark Matter Candidates from Particle Physics and Methods of Detection
• Too big to fail? The puzzling darkness of massive Milky Way subhaloes
• The Kinematics of the Ultra-Faint Milky Way Satellites: Solving the Missing Satellite Problem
• Cosmological parameters from combining the Lyman-alpha forest with CMB, galaxy clustering and SN constraints
• The Observed properties of Dark Matter on small spatial scales
• Dark matter and cosmic structure
• Stellar Feedback in Dwarf Galaxy Formation
• A lower bound on the mass of Dark Matter particles
• Phase-space structure in the local dark matter distribution and its signature in direct detection experiments
• How cold is cold dark matter? Small scales constraints from the flux power spectrum of the high-redshift Lyman-alpha forest
• The fine-grained phase-space structure of Cold Dark Matter halos
• On the Structure of Dark Matter Halos at the Damping Scale of the Power Spectrum with and without Relict Velocities
• Lighting the Universe with filaments
• Constraints on dark matter particles from theory, galaxy observations and N-body simulations
• Evolution of Phase-Space Density in Dark Matter Halos