Don't cross the streams: caustics from Fuzzy Dark Matter

TL;DR

This paper investigates how wave interference in fuzzy dark matter causes density fluctuations that disturb stellar streams, proposing a new simulation method to constrain the dark matter particle mass.

Contribution

It introduces a simple approach to simulate FDM wavefunctions in static potentials and analyzes their impact on tidal streams to infer dark matter properties.

Findings

FDM causes density fluctuations with a sharp cutoff at the de Broglie wavelength.

Nonlinear stream perturbations lead to universal fold caustics.

Stream density signatures of FDM can be inferred from angular momentum or action fluctuations.

Abstract

We study how tidal streams from globular clusters may be used to constrain the mass of ultra-light dark matter particles, called `fuzzy' dark matter (FDM). A general feature of FDM models is the presence of ubiquitous density fluctuations in bound, virialized dark matter structures, on the scale of the de Broglie wavelength, arising from wave interference in the evolving dark matter distribution. These time-varying fluctuations can disturb the motions of stars, leading to potentially observable signatures in cold thin tidal streams in our own Galaxy. The study of this effect has been hindered by the difficulty in simulating the FDM wavefunction in Milky Way-sized systems. We present a simple method to evolve realistic wavefunctions in nearly static potentials, that should provide an accurate estimate of this granulation effect. We quantify the impact of FDM perturbations on tidal streams, and show that initially, while stream perturbations are small in amplitude, their power spectra exhibit a sharp cutoff corresponding to the de Broglie wavelength of the FDM potential fluctuations. Eventually, when stream perturbations become nonlinear, fold caustics generically arise that lead to density fluctuations with universal behavior. This erases the signature of the de Broglie wavelength in the stream density power spectrum, but we show that it will still be possible to determine the FDM mass in this regime, by considering the fluctuations in quantities like angular momenta or actions.