Structure Formation Limits on Axion-Like Dark Matter

TL;DR

This paper investigates the constraints on keV-scale axion-like particles as dark matter candidates using structure formation data, revealing mass limits and prospects for future observational improvements.

Contribution

It provides the first detailed analysis of structure formation limits on freeze-in produced ALP dark matter, considering quantum statistics and complex production rate divergences.

Findings

Excludes photophilic ALP DM below ~19 keV mass.

Weak limits for photophobic ALP DM due to lower momentum distribution.

Future observations could raise the mass limit to ~80 keV.

Abstract

We derive structure formation limits on dark matter (DM) composed of keV-scale axion-like particles (ALPs), produced via freeze-in through the interactions with photons and Standard Model (SM) fermions. We employ Lyman-alpha (Ly-{\alpha}) forest data sets as well as the observed number of Milky Way (MW) subhalos. We compare results obtained using Maxwell-Boltzmann and quantum statistics for describing the SM bath. It should be emphasized that the presence of logarithmic divergences complicates the calculation of the production rate, which can not be parameterized with a simple power law behaviour. The obtained results, in combination with X-ray bounds, exclude the possibility for a photophilic "frozen-in" ALP DM with mass below $\sim 19\,\mathrm{keV}$. For the photophobic ALP scenario, in which DM couples primarily to SM fermions, the ALP DM distribution function is peaked at somewhat lower momentum and hence for such realization we find weaker limits on DM mass. Future facilities, such as the upcoming Vera C. Rubin observatory, will provide measurements with which the current bounds can be significantly improved to $\sim 80\,\mathrm{keV}$.