Strong Constraints on Fuzzy Dark Matter from Ultrafaint Dwarf Galaxy Eridanus II

TL;DR

This paper uses the survival of a star cluster in Eridanus II to place new constraints on fuzzy dark matter particle mass, considering core oscillations and density fluctuations.

Contribution

It introduces the effect of core oscillations on star cluster heating in FDM, providing the first constraints on FDM mass from star cluster survival.

Findings

FDM particle mass must be greater than approximately 0.6-1×10^{-19} eV for FDM to be all dark matter.

Constraints from the Milky Way subhalo mass function suggest m_a > 0.8×10^{-21} eV.

A narrow mass window between 10^{-21} eV and 10^{-20} eV remains uncertain and requires further study.

Abstract

The fuzzy dark matter (FDM) model treats DM as a bosonic field with astrophysically large de Broglie wavelength. A striking feature of this model is $\mathcal{O}(1)$ fluctuations in the dark matter density on time scales which are shorter than the gravitational timescale. Including for the first time the effect of core oscillations, we demonstrate how such fluctuations lead to heating of star clusters, and thus an increase in their size over time. From the survival of the old star cluster in Eridanus II we infer $m_a\gtrsim 0.6\rightarrow 1\times 10^{-19}\text{ eV}$ within modelling uncertainty if FDM is to compose all of the DM, and derive constraints on the FDM fraction at lower masses. The subhalo mass function in the Milky Way implies $m_a\gtrsim 0.8\times 10^{-21}\text{ eV}$ to successfully form Eridanus II. The window between $10^{-21}\text{ eV}\lesssim m_a\lesssim 10^{-20}\text{ eV}$ is affected by narrow band resonances, and the limited applicability of the diffusion approximation. Some of this window may be consistent with observations of Eridanus II and more detailed investigations are required.