Imaging reionization's last phases with I-front Lyman-$\alpha$ emissions

TL;DR

This paper investigates the potential to directly image neutral hydrogen islands during reionization using narrowband Ly$ ext{alpha}$ observations, modeling their emissions and assessing detectability with current and future telescopes.

Contribution

It applies reionization simulations to model I-front Ly$ ext{alpha}$ emissions from neutral islands, exploring their observability and dependence on ionizing spectra and I-front speeds.

Findings

Neutral islands appear as diffuse structures tens of Mpc across.

Surface brightness of islands ranges from 1 to 5×10^{-21} erg s^{-1} cm^{-2} arcsec^{-2}.

Detection is challenging but potentially feasible with wide-field narrowband imaging.

Abstract

Long troughs observed in the $z > 5.5$ Ly$\alpha$ and Ly$\beta$ forests are thought to be caused by the last remaining neutral patches during the end phases of reionization -- termed neutral islands. If this is true, then the longest troughs mark locations where we are most likely to observe the reionizing intergalactic medium (IGM). A key feature of the neutral islands is that they are bounded by ionization fronts (I-fronts) which emit Lyman series lines. In this paper, we explore the possibility of directly imaging the outline of neutral islands with a narrowband survey targeting Ly$\alpha$. In a companion paper, we quantified the intensity of I-front Ly$\alpha$ emissions during reionization and its dependence on the spectrum of incident ionizing radiation and I-front speed. Here we apply those results to reionization simulations to model the emissions from neutral islands. We find that neutral islands would appear as diffuse structures that are tens of comoving Mpc across, with surface brightnesses in the range $\approx 1 - 5\times 10^{-21}$ erg s$^{-1}$ cm$^{-2}$ arcsec$^{-2}$. The islands are brighter if the spectrum of ionizing radiation driving the I-fronts is harder, and/or if the I-fronts are moving faster. We develop mock observations for current and futuristic observatories and find that, while extremely challenging, detecting neutral islands is potentially within reach of an ambitious observing program with wide-field narrowband imaging. Our results demonstrate the potentially high impact of low-surface brightness observations for studying reionization.