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Abstract Long troughs observed in thez> 5.5 Lyαand Lyβforests 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 Lyα. In a companion paper, we quantified the intensity of I-front Lyαemissions 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 ≈ 1 - 5× 10-21erg s-1cm-2arcsec-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.
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This content will become publicly available on January 1, 2026
Quantifying Lyman-α emissions from reionization fronts
Abstract During reionization, intergalactic ionization fronts (I-fronts) are sources of Lyαline radiation produced by collisional excitation of hydrogen atoms within the fronts. In principle, detecting this emission could provide direct evidence for a reionizing intergalactic medium (IGM). In this paper, we use a suite of high-resolution one-dimensional radiative transfer simulations run on cosmological density fields to quantify the parameter space of I-front Lyαemission. We find that the Lyαproduction efficiency — the ratio of emitted Lyαflux to incident ionizing flux driving the front — depends mainly on the I-front speed and the spectral index of the ionizing radiation. IGM density fluctuations on scales smaller than the typical I-front width produce scatter in the efficiency, but they do not significantly boost its mean value. The Lyαflux emitted by an I-front is largest if 3 conditions are met simultaneously: (1) the incident ionizing flux is large; (2) the incident spectrum is hard, consisting of more energetic photons; (3) the I-front is traveling through a cosmological over-density, which causes it to propagate more slowly. We present a convenient parameterization of the efficiency in terms of I-front speed and incident spectral index. We make these results publicly available as an interpolation table and we provide a simple fitting function for a representative ionizing background spectrum. Our results can be applied as a sub-grid model for I-front Lyα emissions in reionization simulations with spatial and/or temporal resolutions too coarse to resolve I-front structure. In a companion paper, we use our results to explore the possibility of directly imaging Lyαemission around neutral islands during the last phases of reionization.
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- PAR ID:
- 10616019
- Publisher / Repository:
- Journal of Cosmology and Astroparticle Physics
- Date Published:
- Journal Name:
- Journal of Cosmology and Astroparticle Physics
- Volume:
- 2025
- Issue:
- 01
- ISSN:
- 1475-7516
- Page Range / eLocation ID:
- 065
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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