Maximum amplitude of the high-redshift 21-cm absorption feature

TL;DR

This paper investigates the theoretical maximum strength of the high-redshift 21-cm absorption feature in the cosmic background, considering key physical effects that limit its amplitude, including heating and density fluctuations.

Contribution

It introduces a comprehensive analysis of the maximum possible 21-cm absorption, incorporating the effects of 21-cm heating, Lyα heating, and intergalactic density fluctuations, with the latter being a novel consideration.

Findings

Maximum 21-cm absorption is reduced by about 20% due to combined effects.

Inclusion of density fluctuations decreases the absorption by approximately 10%.

The study provides a refined theoretical upper limit for the 21-cm absorption signal at high redshift.

Abstract

We examine the maximum possible strength of the global 21-cm absorption dip on the Cosmic Background Radiation at high-redshift caused by the atomic intergalactic medium, when the Lyman-$\alpha$ coupling is maximum, assuming no exotic cooling mechanisms from interactions with dark matter. This maximum absorption is limited by three inevitable factors that need to be accounted for: $(a)$ heating by energy transferred from the Cosmic Background Radiation to the hydrogen atoms via 21-cm transitions, dubbed as 21-cm heating; $(b)$ Ly$\alpha$ heating by scatterings of Ly$\alpha$ photons from the first stars; $(c)$ the impact of the expected density fluctuations in the intergalactic gas in standard Cold Dark Matter theory, which reduces the mean 21-cm absorption signal. Inclusion of this third novel effect reduces the maximum global 21-cm absorption by $\sim 10\%$. Overall, the three effects studied here reduce the 21-cm global absorption by $\sim 20\%$ at $z \simeq 17$.