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

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-* approximate to 9 x 10(4) M-circle dot. 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 similar to 10(-0.6)) for cold dark matter (a cuspy halo) over self-interacting dark matter (a cored halo) and weak evidence (Bayes factor similar to 10(-0.4)) for fuzzy dark matter over cold dark matter. We find a virial mass M-200 similar to 10(8) M-circle dot and astrophysical factors J(alpha(j)(c)) similar to 10(11 )M(circle dot)(2) kpc(-5) and D(alpha(D)(c)) similar to 10(2)-10(2.5) M-circle dot 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 f Gamma < 2.2 x 10(-2)(9) cm(3) s(-1) eV(-1) c(2) and a fuzzy-dark-matter particle mass m(a) > 4.0 x 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.

Automated summary

Using stellar line-of-sight velocities, the study constrained the dark matter density profile of the ultra-faint dwarf galaxy Eridanus 2, deriving constraints on self-interacting and fuzzy dark matter scenarios. The results favored cold dark matter over self-interacting dark matter, and provided evidence for fuzzy dark matter. The study also found consistency with the literature regarding the mass-to-light ratio and set limits on core and soliton sizes. The constraints on self-interaction and fuzzy-dark matter particle mass offered complementary information and challenged previous results obtained for larger dwarf galaxies.