Double-peaked Ly α emission during reionization requires nearby voids and a favourable local ionizing background
Abstract
Several Lyman-alpha (Ly α) emitters deep into the reionization era exhibit double-peaked Ly α emission profiles, raising the question of how the intergalactic medium can transmit photons blueward of the Ly α resonance at such high redshifts. To investigate this, we compute Ly α transmission along sightlines originating from galaxies in the Cosmic Dawn III simulation and identify cases that closely reproduce the observed double-peaked emission. In these cases, the sightlines intersect highly underdense voids located a few comoving megaparsecs from the source galaxy. These voids allow photons emitted blueward of Ly α to redshift through resonance without scattering while traversing them. The low opacity arises because the neutral hydrogen density scales with the square of the underlying gas density under ionization equilibrium, making sufficiently underdense regions with 30 per cent of cosmic mean density highly transmissive. Such voids naturally occur in the fluctuating cosmic density field, even in the vicinity of galaxies, and can also be associated with transmissive spikes in the Ly α forest. We find that the global probability of observing double-peaked emission is ∼0 . 5 per cent during reionization at an 80 per cent global ionization fraction, while no cases are found at 60 per cent ionization. We also find that this probability depends sensitively on the local ionizing background intensity, increasing by a factor of ∼10³ for a tenfold increase in intensity. Recent accretion episodes of supermassive black holes can further increase this probability. These results suggest that double-peaked Ly α emission in high-z galaxies can serve as a sensitive probe of the ionizing background during the late stages of cosmic reionization
Related
- Resonantly Scattered CIV Emission in Local Star-forming Galaxies: Radiative Transfer Constraints on High-ionization Gas in Reionization Analogs
- The JWST Emission Line Survey (JELS): A narrow-band determination of the Hα Luminosity Function and Cosmic Star Formation into the Epoch of Reionization
- Untangling dust emission and cosmic infrared background anisotropies with the scattering transform statistics
- An ultra-faint, chemically primitive galaxy forming in the reionization era
- EDGE-INFERNO: The chemical signatures of assembly history in reionization fossils
BibTeX
@article{park2026doublepeaked,
title = {{Double-peaked Ly α emission during reionization requires nearby voids and a favourable local ionizing background}},
author = {Hyunbae Park and Aaron Smith and Intae Jung and Hidenobu Yajima and Pierre Ocvirk and Joseph Lewis and Luke Conaboy and Paul R Shapiro and Ilian T Iliev and Kyungjin Ahn and Joohyun Lee and Jenny G Sorce and Yohan Dubois and Dominique Aubert},
journal = {Monthly Notices of the Royal Astronomical Society},
year = {2026},
month = sep,
doi = {10.1093/mnras/stag1454},
url = {https://hal.science/hal-05736191v1},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - Double-peaked Ly α emission during reionization requires nearby voids and a favourable local ionizing background
AU - Hyunbae Park
AU - Aaron Smith
AU - Intae Jung
AU - Hidenobu Yajima
AU - Pierre Ocvirk
AU - Joseph Lewis
AU - Luke Conaboy
AU - Paul R Shapiro
AU - Ilian T Iliev
AU - Kyungjin Ahn
AU - Joohyun Lee
AU - Jenny G Sorce
AU - Yohan Dubois
AU - Dominique Aubert
PY - 2026
DA - 2026/09/02
JO - Monthly Notices of the Royal Astronomical Society
DO - 10.1093/mnras/stag1454
UR - https://hal.science/hal-05736191v1
AB - Several Lyman-alpha (Ly α) emitters deep into the reionization era exhibit double-peaked Ly α emission profiles, raising the question of how the intergalactic medium can transmit photons blueward of the Ly α resonance at such high redshifts. To investigate this, we compute Ly α transmission along sightlines originating from galaxies in the Cosmic Dawn III simulation and identify cases that closely reproduce the observed double-peaked emission. In these cases, the sightlines intersect highly underdense voids located a few comoving megaparsecs from the source galaxy. These voids allow photons emitted blueward of Ly α to redshift through resonance without scattering while traversing them. The low opacity arises because the neutral hydrogen density scales with the square of the underlying gas density under ionization equilibrium, making sufficiently underdense regions with 30 per cent of cosmic mean density highly transmissive. Such voids naturally occur in the fluctuating cosmic density field, even in the vicinity of galaxies, and can also be associated with transmissive spikes in the Ly α forest. We find that the global probability of observing double-peaked emission is ∼0 . 5 per cent during reionization at an 80 per cent global ionization fraction, while no cases are found at 60 per cent ionization. We also find that this probability depends sensitively on the local ionizing background intensity, increasing by a factor of ∼10³ for a tenfold increase in intensity. Recent accretion episodes of supermassive black holes can further increase this probability. These results suggest that double-peaked Ly α emission in high-z galaxies can serve as a sensitive probe of the ionizing background during the late stages of cosmic reionization
SN - 0035-8711
SN - 1365-2966
ER -