Charting the Parameter Space of the Global 21-cm Signal

TL;DR

This paper explores the wide range of possible high-redshift parameters affecting the global 21-cm signal, providing a comprehensive simulation-based mapping to guide future observations and interpret their results.

Contribution

It systematically models the global 21-cm signal across diverse astrophysical parameters, revealing key correlations and shape characteristics to aid interpretation of upcoming measurements.

Findings

Global 21-cm signal spans a large parameter space

Key features correlate with Lyα intensity, X-ray heating, and ionizing photon production

Shape of the 21-cm curve remains generally consistent across models

Abstract

The early star-forming Universe is still poorly constrained, with the properties of high-redshift stars, the first heating sources, and reionization highly uncertain. This leaves observers planning 21-cm experiments with little theoretical guidance. In this work we explore the possible range of high-redshift parameters including the star formation efficiency and the minimal mass of star-forming halos; the efficiency, spectral energy distribution, and redshift evolution of the first X-ray sources; and the history of reionization. These parameters are only weakly constrained by available observations, mainly the optical depth to the cosmic microwave background. We use realistic semi-numerical simulations to produce the global 21-cm signal over the redshift range $z = 6-40$ for each of 193 different combinations of the astrophysical parameters spanning the allowed range. We show that the expected signal fills a large parameter space, but with a fixed general shape for the global 21-cm curve. Even with our wide selection of models we still find clear correlations between the key features of the global 21-cm signal and underlying astrophysical properties of the high redshift Universe, namely the Ly$\alpha$ intensity, the X-ray heating rate, and the production rate of ionizing photons. These correlations can be used to directly link future measurements of the global 21-cm signal to astrophysical quantities in a mostly model-independent way. We identify additional correlations that can be used as consistency checks.