Cores in Dwarf Galaxies from Fermi Repulsion

TL;DR

This paper demonstrates that Fermi repulsion can produce cored density profiles in dwarf galaxies using sub-keV fermionic dark matter modeled as a self-gravitating Fermi gas, addressing core formation and mass constraints.

Contribution

It introduces a model of dwarf galaxy cores based on Fermi repulsion with sub-keV fermionic dark matter, including new density profile calculations and mass bounds.

Findings

Suitable dwarf galaxy cores (>130 pc) are achievable with 70 eV - 400 eV fermion mass.

Lower bounds on fermion mass are established at 70 eV and 470 eV from different constraints.

The tension between core sizes and mass bounds can be relaxed in non-thermal dark matter scenarios.

Abstract

We show that Fermi repulsion can lead to cored density profiles in dwarf galaxies for sub-keV fermionic dark matter. We treat the dark matter as a quasi-degenerate self-gravitating Fermi gas and calculate its density profile assuming hydrostatic equilibrium. We find that suitable dwarf galaxy cores of larger than 130 pc can be achieved for fermion dark matter with mass in the range 70 eV - 400 eV. While in conventional dark matter scenarios, such sub-keV thermal dark matter would be excluded by free streaming bounds, the constraints are ameliorated in models with dark matter at lower temperature than conventional thermal scenarios, such as the Flooded Dark Matter model that we have previously considered. Modifying the arguments of Tremaine and Gunn we derive a conservative lower bound on the mass of fermionic dark matter of 70 eV and a stronger lower bound from Lyman-$\alpha$ clouds of about 470 eV, leading to slightly smaller cores than have been observed. We comment on this result and how the tension is relaxed in dark matter scenarios with non-thermal momentum distributions.