Probing Spatial Variations in IGM Transmission: Higher Lyα Visibility in an Hα-Selected Galaxy Overdensity at z≃6.2
Abstract
The visibility of Lyα emission during the epoch of reionization depends on both galaxy properties and the transmission of Lyα photons through the intergalactic medium (IGM). Using JWST/NIRCam F470N imaging, we identify a prominent projected overdensity of Hα emitters (HAEs) at z≃6.2 in CEERS field. We use Subaru/HSC NB872 imaging to measure their Lyα fluxes and compare the Lyα visibility of galaxies within 2' of the density center with that of galaxies outside this region. The Lyα-emitter (LAE) fraction is 0.41+0.15−0.14 in the inner region and 0.27+0.19−0.15 in the outer region. Stacking the Hα and Lyα images gives median Lyα escape fractions of 0.100+0.091−0.053 and 0.063+0.130−0.063 in the inner and outer regions, respectively. Both measures indicate higher Lyα visibility in the inner region, although only at modest significance. HAEs closer to the density center are systematically more massive and have redder UV slopes, which instead favor lower Lyα escape. The environmental difference in Lyα visibility is therefore difficult to explain solely by variations in underlying galaxy population. Our results are consistent with reduced Lyα attenuation through a more highly ionized IGM in the overdense region, as expected if large H\,{\sc ii} bubbles preferentially form around such galaxy overdensities. The wide-field LAE distribution provides complementary support for this scenario, as the HAE-defined density center coincides with the strongest LAE overdensity across the 1.4 deg2 HSC field. These findings highlight the importance of spatial variations in IGM attenuation in shaping Lyα visibility near the end of reionization.
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BibTeX
@misc{shimizu2026probing,
title = {{Probing Spatial Variations in IGM Transmission: Higher Ly$α$ Visibility in an H$α$-Selected Galaxy Overdensity at $z\simeq6.2$}},
author = {Shunta Shimizu and Nobunari Kashikawa and Junya Arita and Ryo Emori and Kohei Inayoshi and Akio K. Inoue and Kei Ito and Satoshi Kikuta and Kentaro Koretomo and Mariko Kubo and Yongming Liang and Rieko Momose and Kentaro Nagamine and Masafusa Onoue and Rhythm Shimakawa and Hisakazu Uchiyama},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.35221},
eprint = {2609.35221},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.35221},
}
RIS
TY - UNPB
TI - Probing Spatial Variations in IGM Transmission: Higher Lyα Visibility in an Hα-Selected Galaxy Overdensity at z≃6.2
AU - Shunta Shimizu
AU - Nobunari Kashikawa
AU - Junya Arita
AU - Ryo Emori
AU - Kohei Inayoshi
AU - Akio K. Inoue
AU - Kei Ito
AU - Satoshi Kikuta
AU - Kentaro Koretomo
AU - Mariko Kubo
AU - Yongming Liang
AU - Rieko Momose
AU - Kentaro Nagamine
AU - Masafusa Onoue
AU - Rhythm Shimakawa
AU - Hisakazu Uchiyama
PY - 2026
DA - 2026/09/28
PB - arXiv
DO - 10.48550/arxiv.2609.35221
UR - https://arxiv.org/abs/2609.35221
AB - The visibility of Lyα emission during the epoch of reionization depends on both galaxy properties and the transmission of Lyα photons through the intergalactic medium (IGM). Using JWST/NIRCam F470N imaging, we identify a prominent projected overdensity of Hα emitters (HAEs) at z≃6.2 in CEERS field. We use Subaru/HSC NB872 imaging to measure their Lyα fluxes and compare the Lyα visibility of galaxies within 2' of the density center with that of galaxies outside this region. The Lyα-emitter (LAE) fraction is 0.41^+0.15_−0.14 in the inner region and 0.27^+0.19_−0.15 in the outer region. Stacking the Hα and Lyα images gives median Lyα escape fractions of 0.100^+0.091_−0.053 and 0.063^+0.130_−0.063 in the inner and outer regions, respectively. Both measures indicate higher Lyα visibility in the inner region, although only at modest significance. HAEs closer to the density center are systematically more massive and have redder UV slopes, which instead favor lower Lyα escape. The environmental difference in Lyα visibility is therefore difficult to explain solely by variations in underlying galaxy population. Our results are consistent with reduced Lyα attenuation through a more highly ionized IGM in the overdense region, as expected if large H\,{\sc ii} bubbles preferentially form around such galaxy overdensities. The wide-field LAE distribution provides complementary support for this scenario, as the HAE-defined density center coincides with the strongest LAE overdensity across the 1.4 deg^2 HSC field. These findings highlight the importance of spatial variations in IGM attenuation in shaping Lyα visibility near the end of reionization.
ER -