Observing Neutral Hydrogen Above Redshift 6: The "Global" Perspective

TL;DR

This paper discusses the potential of using the redshifted 21 cm line to study the evolution of neutral hydrogen and the early universe during the epoch of reionization, highlighting ongoing experiments like EDGES.

Contribution

It provides an overview of the EDGES project and discusses future prospects for measuring the global 21 cm signal to understand early galaxy formation.

Findings

Experiments aim to measure the mean brightness temperature of the 21 cm line.

The 21 cm line probes the neutral hydrogen spin temperature during reionization.

Future results could reveal the heating and ionization history of the IGM.

Abstract

Above redshift 6, the dominant source of neutral hydrogen in the Universe shifts from localized clumps in and around galaxies and filaments to a pervasive, diffuse component of the intergalactic medium (IGM). This transition tracks the global neutral fraction of hydrogen in the IGM and can be studied, in principle, through the redshifted 21 cm hyperfine transition line. During the last half of the reionization epoch, the mean (global) brightness temperature of the redshifted 21 cm emission is proportional to the neutral fraction, but at earlier times (10 < z < 25), the mean brightness temperature should probe the spin temperature of neutral hydrogen in the IGM. Measuring the (of order 10 mK) mean brightness temperature of the redshifted 21 cm line as a function of frequency (and hence redshift) would chart the early evolution of galaxies through the heating and ionizing of the IGM by their stellar populations. Experiments are already underway to accomplish this task or, at least, provide basic constraints on the evolution of the mean brightness temperature. We provide a brief overview of one of these projects, the Experiment to the Detect the Global EOR Signature (EDGES), and discuss prospects for future results.