Intensity Mapping of Lyman-alpha Emission During the Epoch of Reionization

Abstract

We calculate the absolute intensity and anisotropies of the Lyα radiation field present during the epoch of reionization. We consider emission from both galaxies and the intergalactic medium (IGM) and take into account the main contributions to the production of Lyα photons: recombinations, collisions, continuum emission from galaxies, and scattering of Lyn photons in the IGM. We find that the emission from individual galaxies dominates over the IGM with a total Lyα intensity (times frequency) of about (1.43-3.57) × 10^(–8) erg s^(–1) cm^(–2) sr^(–1) at a redshift of 7. This intensity level is low, so it is unlikely that the Lyα background during reionization can be established by an experiment aiming at an absolute background light measurement. Instead, we consider Lyα intensity mapping with the aim of measuring the anisotropy power spectrum that has rms fluctuations at the level of 1 × 10^(–1)6 [erg s^(–1) cm^(–2) sr^(–1)]^2 at a few Mpc scales. These anisotropies could be measured with a spectrometer at near-IR wavelengths from 0.9 to 1.4 μm with fields in the order of 0.5 to 1 deg^2. We recommend that existing ground-based programs using narrowband filters also pursue intensity fluctuations to study statistics on the spatial distribution of faint Lyα emitters. We also discuss the cross-correlation signal with 21 cm experiments that probe H I in the IGM during reionization. A dedicated sub-orbital or space-based Lyα intensity mapping experiment could provide a viable complimentary approach to probe reionization, when compared to 21 cm experiments, and is likely within experimental reach.