The end of an era – the Population III to Population II transition and the near-infrared background

There are only a few ways to constrain the era of reionization and the properties of high-redshift (z ≳ 6) stars through observations. Here, we discuss one of these observables – the spectrum of the near-infrared background – and how it is potentially affected by the transition from Population III to Population II stars. The stronger Lyman α emission expected from massive Population III stars could result in a ‘bump’ in the spectrum of the near-infrared background (referred to in this work as the Lyman α bump). The strength and shape of this bump can reveal properties of Population III stars. The Lyman α bump is predicted to be higher if Population III stars are more massive and present at lower redshifts. The shape of the bump is governed by the star formation rate and the time it takes Population III stars to transition to Population II stars. If Population III stars are indeed massive, a bump is predicted as long as Population III stars exist at z ≲ 15, even if their star formation rate is as low as |10^-7 \; M \, yr^{-1\, Mpc^-3}|⁠. This means that there may be some observational signature in the near-infrared background of small pockets of metal-free gas forming Population III stars at z ∼ 6, even if they are quite rare.