B. Zoutendijk, J. Brinchmann, N. F. Bouché, M. den Brok, D. Krajnović, K. Kuijken, M. V. Maseda, J. Schaye

Abstract
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_* ≈ 9 × 10⁴ M_⊙. 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) ~ 10¹¹ M_⊙² kpc⁻⁵ and D(α_c^D) ~ 10² − 10^2.5 M_⊙ kpc⁻² (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 < 47 pc, 68% confidence level) or a soliton (r_sol < 7.2 pc, 68% confidence level). These limits are equivalent to an effective self-interaction coefficient fΓ < 2.2 × 10⁻²⁹cm³s⁻¹ eV⁻¹ c² and a fuzzy-dark-matter particle mass m_a > 4.0 × 10⁻²⁰ eV c⁻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.

Keywords
dark matter; galaxies: individual: Eridanus 2; stars: kinematics and dynamics; techniques: imaging spectroscopy; Astrophysics - Astrophysics of Galaxies; Astrophysics - Cosmology and Nongalactic Astrophysics

Astronomy & Astrophysics
Volume 651, Article Number A80, Number of pages 23
2021 July

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