2021A&A...651A..80Z

Aims. We use stellar line-of-sight velocities to constrain the dark-matter density profile of Eridanus 2, an ultra-faint dwarf galaxy with an absolute V-band magnitude M_V=-7.1 that corresponds to a stellar mass M_*≃9x10⁴M_☉. We furthermore derive constraints on fundamental properties of self-interacting and fuzzy dark matter scenarios.
Methods. We present new observations of Eridanus 2 from MUSE-Faint, a survey of ultra-faint dwarf galaxies with the Multi Unit Spectroscopic Explorer on the Very Large Telescope, and determine line-of-sight velocities for stars inside the half-light radius. Combined with literature data, we have 92 stellar tracers out to twice the half-light radius. With these tracers we constrain models of cold dark matter, self-interacting dark matter, and fuzzy dark matter, using CJAM and pyGravSphere for the dynamical analysis. The models of self-interacting and fuzzy dark matter relate the density profile to the self-interaction coefficient and the dark-matter particle mass, respectively.
Results. We find substantial evidence (Bayes factor ∼10^–0.6) for cold dark matter (a cuspy halo) over self-interacting dark matter (a cored halo) and weak evidence (Bayes factor ∼10^–0.4) for fuzzy dark matter over cold dark matter. We find a virial mass M₂₀₀ ∼ 10⁸ M_☉ and astrophysical factors J(α_c^J)∼10¹¹M_☉²/kpc⁵ and D(α_c^D)∼10²-10^2.5M_☉/kpc² (proportional to dark-matter annihilation and decay signals, respectively), the exact values of which depend on the density profile model. The mass-to-light ratio within the half-light radius is consistent with the literature. We do not resolve a core (r_c<47pc, 68% confidence level) or a soliton (r_sol<7.2pc, 68% confidence level). These limits are equivalent to an effective self-interaction coefficient fΓ<2.2x10^–29cm³s^–1eV^–1c² and a fuzzy-dark-matter particle mass m_a>4.0x10^–20eVc^–2. The constraint on self-interaction is complementary to those from gamma-ray searches. The constraint on fuzzy-dark-matter particle mass is inconsistent with those obtained for larger dwarf galaxies, suggesting that the flattened density profiles of those galaxies are not caused by fuzzy dark matter.