Population III and the near-infrared background excess

Abstract We make a critical assessment of models that attribute the recently detected near-infrared background ‘excess’ (NIRBE) to the redshifted light from Population III objects. To supply the required 25 keV per baryon at redshift 9, Population III massive stars must form with an efficiency exceeding 30 per cent in all ‘minihaloes’ with virial temperatures above a few hundred kelvins; to avoid excessive metal pollution, most of the baryons once in Population III stars must end up in intermediate-mass black holes (IMBHs). Gas accretion on to such IMBHs must either be inhibited or lead to early miniquasars with steep ultraviolet/X-ray spectra, in order not to overproduce the present-day unresolved soft X-ray background. In the latter case (NIRBE dominated by ‘X-ray-quiet miniquasars’), the total mass density of IMBHs at z ∼ 9 must be ≳50 times higher than the mass density of supermassive black holes observed today in the nuclei of galaxies. A stellar-dominated NIRBE is less economical energetically: ≳5 per cent of all baryons in the Universe must be processed into Population III stars. We survey various aspects of the Population III hypothesis for the NIRBE, and show that the ionizing photon budget required to account for the NIRBE is much larger than that required to explain the high electron scattering optical depth measured by the WMAP satellite.