Minimum star-forming halo mass in axion cosmology

TL;DR

This paper investigates the minimum halo mass for star formation in axion dark matter models, using astrophysical observations to constrain axion masses and highlight the potential of future probes.

Contribution

It applies the Rees-Ostriker-Silk cooling criterion to fuzzy dark matter models, providing new astrophysical constraints on axion masses and emphasizing the need for detailed simulations.

Findings

Constraints are compatible with ΛCDM but disfavor axion masses below ~10^{-21} eV/c^2.

Existing data can be explained without ruling out standard dark matter models.

Future observations could further constrain or detect ultralight axion dark matter.

Abstract

Elucidating the particle physics nature of dark matter (DM) is one of the great challenges in modern science. The current lack of any direct DM detections in the laboratory heightens the need for astrophysical constraints, extending the search to DM models beyond the popular weakly interacting massive particle (WIMP) scenario. We here apply the classical Rees-Ostriker-Silk cooling criterion for galaxy formation to models with ultralight axion DM, also known as fuzzy dark matter (FDM). The resulting constraints provide a heuristic framework for upcoming observations, and our approximate analysis motivates the need for future, self-consistent simulations of FDM structure formation. We use observational constraints for the DM hosts of ultra faint dwarf (UFD) galaxies in the Local Group, together with the redshift constraints for the onset of primordial star formation from the recent EDGES 21-cm cosmology measurement, to illustrate this approach. We find that the existing constraints are straightforward to reconcile with standard $\Lambda$CDM, but disfavour FDM axion masses below $\sim 10^{-21}\,{\rm eV}/c^2$. The future potential for harnessing astrophysical probes of DM particle physics is compelling.