The Effects of Population III X-ray and Radio Backgrounds on the Cosmological 21-cm Signal

TL;DR

This study models how Population III stars and their black hole remnants influence the 21-cm cosmological signal, aligning with EDGES observations and exploring the effects of X-ray and radio emissions.

Contribution

It combines semi-analytic Pop III star formation models with 21-cm simulations to explain the EDGES signal without exotic physics, highlighting the role of black hole emissions.

Findings

Pop III star formation models match EDGES timing.

Strong radio and weak X-ray emissions can reproduce the signal depth.

Narrow IMF around 140 M_sun produces sharp 21-cm features.

Abstract

We investigate the effects of Population III (Pop III) stars and their remnants on the cosmological 21-cm global signal. By combining a semi-analytic model of Pop III star formation with a global 21-cm simulation code, we investigate how X-ray and radio emission from accreting Pop III black holes may affect both the timing and depth of the 21-cm absorption feature that follows the initial onset of star formation during the Cosmic Dawn. We compare our results to the findings of the EDGES experiment, which has reported the first detection of a cosmic 21-cm signal. In general, we find that our fiducial Pop III models, which have peak star formation rate densities of $\sim 10^{-4}$ M$_\odot$ yr$^{-1}$ Mpc$^{-3}$ between $z \sim 10$ and $z \sim 15$, are able to match the timing of the EDGES signal quite well, in contrast to models that ignore Pop III stars. To match the unexpectedly large depth of the EDGES signal without recourse to exotic physics, we vary the parameters of emission from accreting black holes (formed as Pop III remnants) including the intrinsic strength of X-ray and radio emission as well as the local column density of neutral gas. We find that models with strong radio emission and relatively weak X-ray emission can self-consistently match the EDGES signal, though this solution requires fine-tuning. We are only able to produce signals with sharp features similar to the EDGES signal if the Pop~III IMF is peaked narrowly around $140 \, M_\odot$.