Population III and the Near-Infrared Background Excess

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/baryon at redshift 9, Pop III massive stars must form with an efficiency exceeding 30% in all minihalos’’ with virial temperatures above a few hundred kelvins: to avoid excessive metal pollution, most of the baryons once in Pop III stars must end up in intermediate-mass black holes (IMBHs). Gas accretion onto such IMBHs must either be inhibited or lead to early miniquasars with steep UV/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 redshift 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: more than 5% 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.