The cosmic optical background excess, dark matter, and line-intensity mapping (2203.11236v2)

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.

• The paper finds that the COB excess may be explained by axion-like dark matter decays, with LORRI data showing a 4σ excess over galaxy count predictions.
• The authors define a specific ALP parameter space (8–20 eV mass and 3–6×10⁻¹¹ GeV⁻¹ coupling) that reconciles the observed spectral intensities.
• The study proposes that upcoming LIM experiments like SPHEREx and HETDEX could decisively test the dark matter decay hypothesis.

The Cosmic Optical Background Excess: A Link to Dark Matter and Observational Prospects

This paper examines the intriguing cosmic optical background (COB) excess detected using New Horizons' Long Range Reconnaissance Imager (LORRI) data. The observed COB flux surpasses predictions from galaxy counts by approximately a factor of two, a discrepancy that the authors attribute, with a 4σ significance level, potentially to axion-like dark matter decays into monoenergetic photons. The paper explores how such decays could generate the spectral energy distribution contributing to the excess observed by LORRI, proposing a specific parameter space for axion-like particles (ALPs) that includes masses of 8 - 20 eV and effective axion-photon couplings between 3 - 6 × 10^{-11} GeV^{-1}.

The paper's contribution lies in postulating that if the COB excess derives from dark-matter decay into photon lines, forthcoming line-intensity mapping (LIM) measurements will provide significant signals to either substantiate or refute this hypothesis. LIM experiments, such as those planned with SPHEREx and HETDEX, will use spectral lines' three-dimensional mapping to detect potential unidentified emission lines from ALP decays. The authors specifically anticipate ALP parameter space not previously constrained, suggesting that these upcoming observations could yield pivotal insights.

Key results showcased in the paper include the spectral intensity calculations for various ALP masses and the associated decay rates necessary to account for the LORRI-detected excess. In the mass range examined (3 - 20 eV), the predicted intensities align with an unexplored area in the axion parameter space, allowing the COB excess to be reconciled with an elusive, straightforward ALP decay scenario. Meanwhile, other astrophysical explanations—for instance, faint galaxies or star streams not accounted for in current models—are acknowledged but deemed unlikely given the simplicity and predictive power of the dark matter interpretation.

The paper's implications extend beyond merely explaining the COB excess. It ventures into constraints on dark-matter models, revealing that LIM methodologies might soon test hypotheses about ALP behaviors. Future experiments could drastically refine or even resolve questions about the microphysical nature of dark matter via observed photon emissions. Additionally, given that current EBL reconstructions agree with predicted emissions from galaxy counts, any detection of ALP-induced measurements may reshape understanding in cosmic distribution metrics, potentially altering theoretical perspectives on dark energy distributions.

Such comprehensive analyses have further observational opportunities: any contributions in the ultraviolet range, as indicated by the New Horizons UV spectrograph, might independently confirm photonic decay connections and crucially inform limits on photon extragalactic background light (EBL) from γ-ray attenuation studies in blazar analyses. These spectral data points create a cross-validation potential across observational domains, lending robustness to any future alignment of these excess observations with ALP decay predictions.

In summary, this paper details a robust framework connecting presumed axion-like dark matter decay mechanisms with observed cosmic optical signals. Future developments in LIM experiments, bolstered by existing γ-ray observations, might either substantiate the dark matter hypothesis for describing cosmic light excesses or pivot scientific inquiry toward alternative phenomena. Through methodological foresight and comprehensive parameter modeling, the paper propels forward the convergence of observational cosmology and particle theory, potentially unveiling pivotal insights into the dark universe.