Static structure of chameleon dark matter as an explanation of dwarf spheroidal galaxy cores

Abstract

We propose a novel mechanism that explains the cored dark matter density profile in recently observed dark matter rich dwarf spheroidal galaxies. In our scenario, dark matter particle mass decreases gradually as a function of distance towards the center of a dwarf galaxy due to its interaction with a chameleon scalar. At closer distance towards the Galactic center the strength of attractive scalar fifth force becomes much stronger than gravity and is balanced by the Fermi pressure of the dark matter cloud; thus, an equilibrium static configuration of the dark matter halo is obtained. Like the case of soliton star or fermion Q-star, the stability of the dark matter halo is obtained as the scalar achieves a static profile and reaches an asymptotic value away from the Galactic center. For simple scalar-dark matter interaction and quadratic scalar self-interaction potential, we show that dark matter behaves exactly like cold dark matter (CDM) beyond a few kpc away from the Galactic center but at closer distance it becomes lighter and Fermi pressure cannot be ignored anymore. Using Thomas-Fermi approximation, we numerically solve the radial static profile of the scalar field, fermion mass and dark matter energy density as a function of distance. We find that for fifth force mediated by an ultralight scalar, it is possible to obtain a flattened dark matter density profile towards the Galactic center. In our scenario, the fifth force can be neglected at distance r ≥ 1 kpc from the Galactic center and dark matter can be simply treated as heavy nonrelativistic particles beyond this distance, thus reproducing the success of CDM at large scales.

Formation and Abundance of Late-forming Primordial Black Holes as Dark Matter

“…We solve numerically for the static profile of the f field. We refer to Brouzakis & Tetradis (2006), Lee & Pang (1987), and Chanda & Das (2017) for detailed derivations of these equations. We consider the DM to be a fermion field, ψ, self-interacting through a scalar field, f. We discuss a more general interaction as compared to the original mass-varying neutrino model (Fardon et al 2004), which considers the Majorana mass term to be a linear function of f. In this analysis, we introduce a function f (f) so that the DM mass is obtained as…”