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Discovery of a z ≃ 4.9 Ly α Emitter Protocluster: Wavelength-dependent Environmental Effects on Galaxy Structure

Year: 2026
Type: article
Abstract: We report the discovery of a Ly α emitter (LAE) protocluster at z = 4.90 in the COSMOS field, comprising four distinct overdensity peaks spanning ∼65 × 36 cMpc 2 , with the primary concentration exhibiting a fourfold surface density enhancement relative to the field within a 1.5 proper Mpc (pMpc) radius. Using SILVERRUSH narrowband survey data combined with JWST COSMOS-Web imaging, we perform one of the first systematic rest-frame optical and UV morphological comparisons of protocluster versus field LAEs at this redshift using JWST NIRCam rest-frame UV (F150W, ∼2540 Å) and optical (F277W, ∼4700 Å) imaging. Sérsic profile fitting for 16 protocluster members and 23 field LAEs reveals a size difference: protocluster LAEs are ∼40% larger in rest-optical (median R e = 0.8 1 − 0.04 + 0.26 kpc versus 0.5 8 − 0.04 + 0.11 kpc, p = 0.041), with no significant difference in rest-UV ( p = 0.51) or Sérsic index. At fixed stellar mass, protocluster LAEs are offset by +0.12 dex (≃31%) in rest-optical size from the field size–mass relation (68% confidence interval (CI): [+0.08, +0.21]; Mann–Whitney p = 0.033), with 75% exhibiting positive size residuals compared to 44% of field LAEs. This wavelength-dependent environmental signature suggests that protocluster environments at z ≃ 5 preferentially affect extended stellar populations, possibly through tidal interactions, with no significant environmental difference detected in rest-UV sizes, providing observational evidence for environmental influences on the structure of LAEs during the early buildup phase of cosmic star formation.
DOI: 10.3847/2041-8213/ae5824
Authors: Ronaldo Laishram, Yusei Koyama, Haruka Kusakabe, Satoshi Kikuta, Shunta Shimizu, Tadayuki Kodama
Language: English
Open Access status: gold
Estimated APC paid: $2,400
OpenAlex ID: W7151298596
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