Semi-analytical modelling of Pop. III star formation and metallicity evolution – II. Impact on 21 cm power spectrum

Simulating Population (Pop.) III star formation in mini-haloes in a large cosmological simulation is an extremely challenging task but it is crucial to estimate its impact on the 21 cm power spectrum. In this work, we develop a framework within the semi-analytical code MERAXES to estimate the radiative backgrounds from Pop. III stars needed for the computation of the 21 cm signal. We computed the 21 cm global signal and power spectrum for different Pop. III models varying star formation efficiency, initial mass function and specific X-ray luminosity per unit of star formation (L_X/SFR). In all the models considered, we find Pop. III stars have little to no impact on the reionization history but significantly affect the thermal state of the intergalactic medium (IGM) due to the strong injection of X-ray photons from their remnants that heat the neutral IGM at z ≥ 15. This is reflected not only on the 21 cm sky-averaged global signal during the Cosmic Dawn but also on the 21 cm power spectrum at z ≤ 10 where models with strong Pop. III X-ray emission have larger power than models with no or mild Pop. III X-ray emission. We estimate observational uncertainties on the power spectrum using 21CMSENSE and find that models where Pop. III stars have a stronger X-ray emission than Pop. II are distinguishable from models with no or mild Pop. III X-ray emission with 1000 h observations of the upcoming SKA1-low.