The cosmic optical background excess, dark matter, and line-intensity mapping

TL;DR

This paper explores whether the excess in cosmic optical background measured by New Horizons could be due to decaying axion-like dark matter, predicting detectable signals in future line-intensity mapping experiments.

Contribution

It proposes a novel dark matter decay explanation for the optical background excess and identifies specific axion parameter space consistent with observations.

Findings

Excess flux exceeds deep galaxy count expectations by about a factor of two.

Axion-like particles with 8-20 eV masses and specific couplings can explain the excess.

Future line-intensity mapping can test the dark matter decay hypothesis.

Abstract

Recent studies using New Horizons LORRI images have returned the most precise measurement of the cosmic optical background to date, yielding a flux that exceeds that expected from deep galaxy counts by roughly a factor of two. We investigate whether this excess, detected at $\sim 4\sigma$ significance, is due to axion-like dark matter that decays to monoenergetic photons. We compute the spectral energy distribution from such decays and the contribution to the flux measured by LORRI. Assuming that axion-like particles make up all of the dark matter, the parameter space unconstrained to date that explains the measured excess spans masses and effective axion-photon couplings of 8 - 20 eV masses and 3 - 6 $\times 10^{-11}$ GeV$^{-1}$, respectively. If the excess arises from dark-matter decay to a photon line, there will be a significant signal in forthcoming line-intensity mapping measurements that will allow the discrimination of this hypothesis from other candidates.