Cosmic Optical Background Excess, Dark Matter, and Line-Intensity Mapping

Recent studies using New Horizons’s Long Range Reconnaisance Imager (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 2. We investigate whether this excess, detected at $\sim 4\sigma$ significance, is due to axionlike 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 axionlike 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 $3–6\times {10}^{-11}{\mathrm{GeV}}^{-1}$, 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.

Figure 1

Specific intensity per observed wavelength from ALP radiative decays as ALP mass varies (color coded following the color bar). We show predictions for a fixed decay rate of $8\times {10}^{-24}{\mathrm{s}}^{-1}$ (top left panel), and for a fixed total contribution to the measured intensity equal to the COB excess (bottom left panel), with the corresponding contributions to measured intensities along with the COB excess (top right panel) and required decay rates to match the COB excess (bottom right panel). We also indicate in the color bar the redshift at which the decay occurs at the LORRI pivot wavelength ${\lambda }_{\mathrm{obs}}=0.608\mathrm{\mu }\mathrm{m}$ for each mass. The gray shaded region corresponds to the unnormalized shape of the LORRI responsivity as function of wavelength.

Figure 2

Comparison between the required decay rates (top panel) and the corresponding effective ALP-photon coupling (bottom panel) to explain the cosmic optical background and existing bounds from spectroscopic searches and the cooling of horizontal branch stars in globular clusters, as well as forecasted sensitivities for optical LIM experiments (solid and dotted lines show the forecasts using the LIM power spectrum and voxel intensity distribution, respectively). All values correspond to 95% confidence level, except for the 68% confidence level favored regions from the analysis of $\gamma$-ray attenuation and claimed HST limits.