Double-peaked Ly α emission during reionization requires nearby voids and a favourable local ionizing background
Year: 2026
Type: article
Abstract: ABSTRACT Several Lyman-alpha (Ly $\alpha$) emitters deep into the reionization era exhibit double-peaked Ly $\alpha$ emission profiles, raising the question of how the intergalactic medium can transmit photons blueward of the Ly $\alpha$ resonance at such high redshifts. To investigate this, we compute Ly $\alpha$ 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 $\alpha$ 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 ${\lesssim} 30~{{\ \rm per\ cent}}$ 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 $\alpha$ forest. We find that the global probability of observing double-peaked emission is ${\sim} 0.5~{{\ \rm per\ cent}}$ 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 ${\sim} 10^3$ 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 $\alpha$ emission in high-z galaxies can serve as a sensitive probe of the ionizing background during the late stages of cosmic reionization.
Institutions: University of Tsukuba, Institute for Basic Science, Center for Computational Sciences, University of Tsukuba, The University of Texas at Dallas, Chungbuk National University, Centre National de la Recherche Scientifique, Observatoire astronomique de Strasbourg, Université de Strasbourg, Commissariat à l'Énergie Atomique et aux Énergies Alternatives, Institut d'Astrophysique de Paris, Astrophysique, Instrumentation et Modélisation, CEA Paris-Saclay, Institut de Recherche sur les Lois Fondamentales de l'Univers, Université Paris Diderot, Université Pierre-et-Marie-Curie, University of Nottingham, The University of Texas at Austin, University of Sussex, Chosun University, Université de Lille, Université Paris-Saclay, Institut d'Astrophysique Spatiale, Centre de Recherche en Informatique, Signal et Automatique de Lille, Centrale Lille Institut
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $3,046
Funders: National Science Foundation, Université de Lille, Ministry of Education, Culture, Sports, Science and Technology, Korea Astronomy and Space Science Institute, National Research Foundation of Korea, Science and Technology Facilities Council, Japan Society for the Promotion of Science, Japan Science and Technology Agency, Kavli Institute for Theoretical Physics, University of California, Santa Barbara
Awards: G114141472, G2949333978, G3398237449, G4689434848, 宇宙物理輻射輸送計算で拓く新しい生体医用光学, G6454862874, Kavli Institute for Theoretical Physics, 新時代銀河シミュレーションと多波長観測の融合で明かす初期宇宙の銀河形成メカニズム, G7283855938, 高精度原始銀河団シミュレーションによる銀河形成と宇宙再電離研究の新展開, G7913129754, Astronomy and Astrophysics at the University of Nottingham - 2023 to 2026
OpenAlex ID: W7159099411