The inprint of cosmic dark ages on the near-infrared background

The redshifted light of the first (Population III) stars might contribute substantially to the near-infrared background (NIRB). By fitting recent data with models including up-to-date Population III stellar spectra, we find that such stars can indeed account for the whole NIRB residual (i.e. after ‘normal’ galaxy contribution subtraction) if the high-redshift star formation efficiency is f★ = 10–50 per cent, depending on the initial mass function (the top-heaviest requiring lowest efficiency) and on the unknown galaxy contribution in the L band (our models, however, suggest it to be negligible). Such an epoch of Population III star formation ends in all models by zend≈ 8.8, with a hard limit zend < 9 set by J-band observations. To prevent an associated intergalactic medium (IGM) overenrichment with heavy elements compared with observed levels in the IGM, pair-instability supernovae must be the dominant heavy element source. Alternative explanations must break the light—metal production link by advocating very massive stars M > 260 M⊙, locking their nucleosynthetic products in the compact remnant or by postulating an extremely inhomogeneous metal enrichment of the Lyα forest. We discuss these possibilities in detail along with the uncertainties related to the adopted zodiacal light model.