The MUSE-Faint survey

II. The dark-matter density profile of the ultra-faint dwarf galaxy Eridanus 2^⋆

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: zoutendijk@strw.leidenuniv.nl
² Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
³ Univ. Lyon, Univ. Lyon1, ENS de Lyon, CNRS, Centre de Recherche Astrophysique de Lyon UMR5574, 69230 Saint-Genis-Laval, France
⁴ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany

Received: 24 December 2020
Accepted: 24 April 2021

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 × 10⁻²⁹ cm³ s⁻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.