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

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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>R</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>e</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>0.8</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>1</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.04</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.26</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> kpc versus <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>0.5</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>8</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.04</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.11</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> 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.