Anisotropic cosmic optical background bound for decaying dark matter in light of the LORRI anomaly

TL;DR

This paper investigates whether decaying dark matter could explain the anomalous cosmic optical background flux reported by New Horizon, but finds anisotropy measurements strongly constrain such models, disfavoring them.

Contribution

It provides the first anisotropy-based constraints on decaying dark matter as an explanation for the COB excess, challenging previous hypotheses.

Findings

Decaying dark matter models are constrained by HST anisotropy data.

Decay rates larger than 10^{-24}-10^{-23} s^{-1} are excluded for 5-20 eV mass range.

Decaying cold dark matter is disfavored as the source of the COB excess.

Abstract

Recently anomalous flux in the cosmic optical background (COB) has been reported by the New Horizon observations. The COB flux is $16.37\pm 1.47\, \rm nW m^{-2} sr^{-1}$, at the LORRI pivot wavelength of $0.608\,\rm \mu m$, which is $\sim 4\sigma$ level above the expected flux from the Hubble Space Telescope (HST) galaxy count. It would be great if this were a hint for the eV scale dark matter decaying into photons. In this paper, we point out that such a decaying dark matter model predicts a substantial amount of anisotropy in the COB flux, which is accurately measured by the HST. The data of the HST excludes the decay rate of the dominant cold dark matter larger than $10^{-24}$-$10^{-23}\,{\rm s}^{-1}$ in the mass range of $5$-$20$eV. As a result, the decaying cold dark matter explaining the COB excess is strongly disfavored by the anisotropy bound. We discuss some loopholes: e.g. warm/hot dark matter or two-step decay of the dark matter, to explain the COB excess.