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The MUSE-Faint survey II. The dark-matter density profile of the ultra-faint dwarf galaxy Eridanus 2

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

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 MV = −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(αcJ) ~ 1011 M2 kpc−5 and D(αcD) ~ 102 − 102.5 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 (rc < 47 pc, 68% confidence level) or a soliton (rsol < 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 ma > 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.

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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Faculdade de Ciências da Universidade de Lisboa Universidade do Porto Faculdade de Ciências e Tecnologia da Universidade de Coimbra
Fundação para a Ciência e a Tecnologia COMPETE 2020 PORTUGAL 2020 União Europeia