Halo Cores and Phase Space Densities: Observational Constraints on Dark Matter Physics and Structure Formation

TL;DR

This work tests dark matter models by analyzing halo core sizes and the coarse-grained phase-space density $Q$ across ~8 orders of magnitude in mass, arguing that neither generic warm DM nor self-interacting DM can explain cores on all scales and that cores must have dynamical origins in some systems. A simple hierarchical merging argument yields a robust scaling $Q \\propto M^{-1} \\propto \\sigma^{-3} \\propto R^{-3}$, which is supported by observational data from dwarf spheroidals, rotating dwarfs/LSBs, and clusters, and is complemented by links to elliptical galaxies and non-homology. The analysis identifies three factors setting the normalization of the $Q$–$\\sigma$ relation (primordial $Q_0$, violent relaxation, and the collapse epoch) and derives strong lower bounds on warm dark matter particle masses, $m_X > 669$ eV (thermal fermions) and $m_X > 322$ eV (degenerate fermions). Together, these results constrain DM particle properties, support a bottom-up formation scenario, and highlight the role of collisionless dynamics and phase-space mixing in shaping halo cores.

Abstract

We explore observed dynamical trends in a wide range of dark matter dominated systems (about seven orders of magnitude in mass) to constrain hypothetical dark matter candidates and scenarios of structure formation. First, we argue that neither generic warm dark matter (collisionless or collisional) nor self-interacting dark matter can be responsible for the observed cores on all scales. Both scenarios predict smaller cores for higher mass systems, in conflict with observations; some cores must instead have a dynamical origin. Second, we show that the core phase space densities of dwarf spheroidals, rotating dwarf and low surface brightness galaxies, and clusters of galaxies decrease with increasing velocity dispersion like Q ~ sigma^-3 ~ M^-1, as predicted by a simple scaling argument based on merging equilibrium systems, over a range of about eight orders of magnitude in Q. We discuss the processes which set the overall normalization of the observed phase density hierarchy. As an aside, we note that the observed phase-space scaling behavior and density profiles of dark matter halos both resemble stellar components in elliptical galaxies, likely reflecting a similar collisionless, hierarchical origin. Thus, dark matter halos may suffer from the same systematic departures from homology as seen in ellipticals, possibly explaining the shallower density profiles observed in low mass halos. Finally, we use the maximum observed phase space density in dwarf spheroidal galaxies to fix a minimum mass for relativistically decoupled warm dark matter candidates of roughly 700 eV for thermal fermions, and 300 eV for degenerate fermions.

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