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

The redshifted light of the first (Pop III) stars might substantially contribute to the near infrared background (NIRB). By fitting recent data with models including up-to-date Pop 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%, depending on the IMF (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 epoch of Pop III star formation ends in all models by z_end ~ 8.8, with a hard limit z_end < 9 set by J band observations. To prevent an associated IGM over-enrichment with heavy elements compared to observed levels in the IGM, pair-instability supernovae must be the dominant heavy element sources. Alternative explanations must break the light-metal production link by advocating very massive stars (M > 260 M_sun) locking their nucleosynthetic products in the compact remnant or by postulating an extremely inhomogeneous metal enrichment of the Ly alpha forest. We discuss these possibilities in detail along with the uncertainties related to the adopted zodiacal light model.