Abstract
Recently anomalous flux in the cosmic optical background (COB) is reported by the New Horizon observations. The COB flux is $16.37\pm1.47\, \rm nW m^{-2} sr^{-1}$, 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 excluded 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.
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