Enhanced Detectability of Pre-reionization 21-cm Structure

TL;DR

This paper suggests that large 21-cm fluctuations caused by early X-ray heating sources could be more detectable than previously thought, enabling probing of the universe's state before reionization with current and upcoming radio telescopes.

Contribution

It demonstrates that early X-ray heating sources produce large, detectable 21-cm signals, improving prospects for observing the universe before reionization with existing facilities.

Findings

Large 21-cm fluctuations (~250 mK) are caused by early X-ray heating.

Current telescopes like Arecibo and FAST can detect these signals at high significance.

Existing arrays like MWA and LOFAR can statistically detect fluctuations from smaller black holes.

Abstract

Before the universe was reionized, it was likely that the spin temperature of intergalactic hydrogen was decoupled from the CMB by UV radiation from the first stars through the Wouthuysen-Field effect. If the IGM had not yet been heated above the CMB temperature by that time, then the gas would appear in absorption relative to the CMB. Large, rare sources of X-rays could inject sufficient heat into the neutral IGM, so that the differential brightness temperature was greater than zero at comoving distances of tens to hundreds of Mpc, resulting in large 21-cm fluctuations with amplitudes of about 250 mK on arcminute to degree angular scales, an order of magnitude larger in amplitude than that caused by ionized bubbles during reionization, about 25 mK. This signal could therefore be easier to detect and probe higher redshifts than that due to patchy reionization. For the case in which the first objects to heat the IGM are QSOs hosting 10^7-solar mass black holes with an abundance exceeding about 1 per Gpc^3 at z~15, observations with either the Arecibo Observatory or the Five Hundred Meter Aperture Spherical Telescope (FAST) could detect and image their fluctuations at greater than 5-sigma significance in about a month of dedicated survey time. Additionally, existing facilities such as MWA and LOFAR could detect the statistical fluctuations arising from a population of 10^5-solar mass black holes with an abundance of about 10^4 per Gpc^3 at z~10-12.