Near-infrared radiation background, gravitational wave background, and star formation rate of Pop III and Pop II during cosmic reionization

Context. The transition from Population III (Pop III) to Population II (Pop II) stars plays an important role in the history of the Universe. The huge amount of ionizing photons generated by Pop III stars begin to ionize the intergalactic medium (IGM) at the early stage of reionization. Meanwhile, the feedback from reionization and metal enrichment changes the evolution of different populations. The near-infrared radiation background (NIRB) and the stochastic background of gravitational waves (SBGWs) from these early stars will provide important information about the transition form Pop III to Pop II stars. Aims. We obtain the NIRB and SBGWs from the early stars, which are constrained by the observation of reionization and star formation rate (SFR). Methods. We studied the transition from Pop III to Pop II stars via the star formation model of different populations, which takes into account the reionization and the metal enrichment evolution. We calculated the two main metal pollution channels arising from the supernova-driven protogalactic outflows and genetic channel. We obtained the SFRs of Pop III and Pop II and their NIRB and SBGWs radiation. Results. We predict that the upper limit of metallicity in metal-enriched IGM (the galaxies that are polluted via the genetic channel) reaches Z(crit) = 10(-3.5) Z(circle dot) at z similar to 13 (z similar to 11), which is consistent with our star formation model. We constrain the SFR of Pop III stars from reionization observations. The peak intensity of NIRB is about 0.03-0.2 nW m(-2) sr(-1) at similar to 1 mu m for z > 6. The predicted NIRB signal is consistent with the metallicity evolution. We also obtain the gravitational wave background from the black holes formed by these early stars. The predicted gravitational wave background has a peak amplitude of Omega(GW) similar or equal to 8 x 10(-9) at nu = 158 Hz for Pop II star remnants. However, the background generated by Pop III.2 stars is much lower than that of Pop II stars, with a peak amplitude of Omega(GW) similar or equal to 1.2 x 10(-11) at nu = 28 Hz. The background of Pop III.1 shifts to lower frequencies, and the amplitude of Omega(GW) for Pop III.1 stars shows a minimum value at nu similar or equal to 10 Hz that is due to the lack of gravitational wave signals from stars with 140 M-circle dot < M-* < 260 M-circle dot.