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

TL;DR

This study uses stellar velocities from the ultra-faint dwarf galaxy Eridanus 2 to analyze its dark matter profile, providing constraints on different dark matter models and finding evidence favoring cold dark matter over alternatives.

Contribution

It presents new velocity data and dynamical analysis of Eridanus 2, offering novel constraints on self-interacting and fuzzy dark matter scenarios in ultra-faint dwarf galaxies.

Findings

Evidence favors cold dark matter over self-interacting dark matter.

Weak evidence supports fuzzy dark matter over cold dark matter.

Limits on core and soliton sizes suggest no resolved core or soliton in Eridanus 2.

Abstract

Aims. We use stellar line-of-sight velocities to constrain the dark matter-density profile of Eridanus 2, an ultra-faint dwarf galaxy ($M_\mathrm{V} = -7.1$, $M_* \approx 9 \times 10^4\,M_\odot$). 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 MUSE 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. We constrain models of cold dark matter, self-interacting dark matter, and fuzzy dark matter with these tracers, using CJAM and pyGravSphere for the dynamical analysis. Results. We find substantial evidence for cold dark matter over self-interacting dark matter and weak evidence for fuzzy dark matter over cold dark matter. We find a virial mass $M_{200} \sim 10^8\,M_\odot$ and astrophysical factors $J(\alpha_\mathrm{c}^J) \sim 10^{11}\,M_\odot^2\,\mathrm{kpc}^{-5}$ and $D(\alpha_\mathrm{c}^D) \sim 10^2$-$10^{2.5}\,M_\odot\,\mathrm{kpc}^{-2}$. We do not resolve a core ($r_\mathrm{c} < 47\,\mathrm{pc}$, 68-% level) or soliton ($r_\mathrm{sol} < 7.2\,\mathrm{pc}$, 68-% level). These limits are equivalent to an effective self-interaction coefficient $f\Gamma < 2.2 \times 10^{-29}\,\mathrm{cm}^3\,\mathrm{s}^{-1}\,\mathrm{eV}^{-1}\,c^2$ and a fuzzy-dark-matter particle mass $m_\mathrm{a} > 4.0 \times 10^{-20}\,\mathrm{eV}\,c^{-2}$. 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. (Abridged)