The Near-IR Background Intensity and Anisotropies During The Epoch of Reionization

TL;DR

This paper models the near-infrared background during reionization, estimating its intensity and anisotropies based on galaxy emission and comparing predictions with observations, revealing unexplained fluctuations.

Contribution

It provides a new estimate of the near-IR background intensity and anisotropy power spectrum during reionization, incorporating galaxy emission models and observational constraints.

Findings

Estimated near-IR background intensity of 0.1-0.3 nW/m²/sr at 1.1 μm

Predicted anisotropy fluctuations are much smaller than observed

Existing measurements are incompatible with reionization-era origins

Abstract

A fraction of the extragalactic near-infrared (near-IR) background light involves redshifted photons from the ultraviolet (UV) emission from galaxies present during reionization at redshifts above 6. The absolute intensity and the anisotropies of the near-IR background provide an observational probe of the first-light galaxies and their spatial distribution. We estimate the extragalactic background light intensity during reionization by accounting for the stellar and nebular emission from first-light galaxies. We require the UV photon density from these galaxies to generate a reionization history that is consistent with the optical depth to electron scattering from cosmic microwave background measurements. We also require the bright-end luminosity function of galaxies in our models to reproduce the measured Lyman drop-out luminosity functions at redshifts of 6 to 8. The absolute intensity is about 0.1 to 0.3 nW m$^{-2}$ sr$^{-1}$ at the peak of its spectrum at $\sim$ 1.1 $\mu$m. We also discuss the anisotropy power spectrum of the near-IR background using a halo model to describe the galaxy distribution. We compare our predictions for the anisotropy power spectrum to existing measurements from deep near-IR imaging data from {\it Spitzer}/IRAC, {\it Hubble}/NICMOS, and {\it AKARI}. The predicted rms fluctuations at tens of arcminute angular scales are roughly an order of magnitude smaller than the existing measurements. While strong arguments have been made that the measured fluctuations do not have an origin involving faint low-redshift galaxies, we find that the existing measurements are also incompatible with an origin during the era of reionization. The measured near-IR background anisotropies remain unexplained and could be associated with an unidentified non-astrophysical origin.