Marta Silva, Mario G. Santos, Yan Gong, Asantha Cooray
We calculate the absolute intensity and anisotropies of the Lyman-$\alpha$ radiation field present during the epoch of reionization. We consider emission from both galaxies and the intergalactic medium (IGM) and take into account all of the contributions to the production of Lyman-$\alpha$ photons: recombinations, collisions, continuum emission from the stars and scattering of Lyman-n photons in the IGM. We find that the emission from individual galaxies dominates over the IGM with a total Lyman-$\alpha$ intensity of about $1.3\times 10^{-2}$ and $3.7\times10^{-3}$ nW m$^{-2}$ sr$^{-1}$ at a redshift of 7 and 10, respectively. These intensity levels are well below the extragalactic background intensity from starlight emission from galaxies and it is unlikely that the Lyman-$\alpha$ background during reionization can be established by an experiment aiming at an absolute background light measurement. Instead we consider Lyman-$\alpha$ intensity mapping with the aim of measuring the anisotropy power spectrum. The anisotropy power spectrum has rms fluctuations at the level of $10^{-2}$ nW m$^{-2}$ sr$^{-1}$ at a few Mpc scales. These anisotropies could be measured with a spectrometer at near-IR wavelengths from 0.9 to 1.4 $\mu$m with fields in the order of 0.5 to 1 sq. degrees. We recommend that existing ground-based programs using narrow band filters also pursue intensity fluctuations to study statistics on the spatial distribution of fainter Lyman-$\alpha$ emitters that remain below the individual detection threshold. We also discuss the cross-correlation signal with 21 cm experiments that probe HI in the IGM during reionization. A dedicated sub-orbital or space-based Lyman-$\alpha$ intensity mapping experiment could provide a viable complementary approach to probe reionization, when compared to 21 cm experiments.
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http://arxiv.org/abs/1205.1493
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