The contribution of high-redshift galaxies to the near-infrared background

Several independent measurements have confirmed the existence of fluctuations (delta F-obs similar to 0.1 nW m(-2) sr(-1) at 3.6 mu m) up to degree angular scales in the source-subtracted near-infrared background (NIRB), but their origin is unknown. By combining high-resolution cosmological N-body/hydrodynamical simulations with an analytical model, and by matching galaxy luminosity functions (LFs) and the constraints on reionization simultaneously, we predict the NIRB absolute flux and fluctuation amplitude produced by high-redshift (z > 5) galaxies (some of which harbour Population III stars, shown to provide a negligible contribution). This strategy also allows us to make an empirical determination of the evolution of the ionizing photon escape fraction: we find f(esc) = 1 at z >= 11, decreasing to similar to 0.05 at z = 5. In the wavelength range 1.0-4.5 mu m, the predicted cumulative flux is F = 0.2-0.04 nW m(-2) sr(-1). However, we find that the radiation from high-redshift galaxies (including those undetected by current surveys) is insufficient to explain the amplitude of the observed fluctuations: at l = 2000, the fluctuation level attributable to z > 5 galaxies is delta F = 0.01-0.002 nW m(-2) sr(-1), with a wavelength-independent relative amplitude delta F/F = 4 per cent. The source of the missing power remains unknown. This might indicate that an unknown component/foreground, with a clustering signal very similar to that of high-redshift galaxies, dominates the source-subtracted NIRB fluctuation signal.