Discovery of a $z\simeq 4.9$ Lyman-$α$ Emitter Protocluster: Wavelength-Dependent Environmental Effects on Galaxy Structure

Abstract

We report the discovery of a Lyman-alpha emitter (LAE) protocluster at z = 4.90 in the COSMOS field, comprising four distinct overdensity peaks spanning ~65 x 36 cMpc^2, with the primary concentration exhibiting a 4-fold 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 a first systematic rest-frame optical and UV morphological comparison of protocluster versus field LAEs at this redshift using JWST NIRCam rest-frame UV (F150W, ~2540 Angstrom) and optical (F277W, ~4700 Angstrom) imaging. Sersic profile fitting for 16 protocluster members and 23 field LAEs reveals a significant size difference: protocluster LAEs are ~40% larger in rest-optical (median R_e = 0.81+0.26/-0.04 kpc vs. 0.58+0.11/-0.04 kpc, p = 0.041) with no significant difference in rest-UV (p = 0.51) or Sersic 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% 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 or an earlier onset of star formation in the dense environment, with no significant environmental difference detected in rest-UV sizes, providing observational evidence for environmental influences on the structure of LAEs during the early build-up phase of cosmic star formation.

Figures (4)

• Figure 1: Overdensity map of NB718 LAEs at $z = 4.90$ revealing a large-scale protocluster structure (the Loktak protocluster) within the 0.54 deg$^2$ COSMOS-Web survey area. The color-coded background represents the smoothed density field derived from Gaussian kernel density estimation (KDE), with warmer colors indicating higher overdensity as shown by the colorbar ($\delta$). Contour lines trace overdensity levels at $\delta = 2$ (magenta), $\delta = 5$ (green), $\delta = 8$ (blue), and $\delta = 10$ (black). Individual LAE positions are shown as white open circles. Spectroscopically confirmed galaxies at $z \simeq 4.9$ are marked as green stars, while those cross-matched with our LAE sample (within 0.5$^{\prime\prime}$) are further highlighted with red open circles overlaid on the stars. The structure exhibits a prominent primary peak (4-fold surface density enhancement within 1.5 pMpc radius), along with three additional significant peaks at distances of 7.6, 6.0, and 11.1 pMpc from the center, forming an interconnected protocluster complex spanning $\sim$65 $\times$ 36 cMpc$^2$ characteristic of large-scale structure at this epoch.
• Figure 2: Size-mass relations for LAEs in rest-UV (F150W, left panel) and rest-optical (F277W, right panel). Blue circles represent field LAEs, and red squares represent protocluster members. Large filled symbols show binned medians with bootstrap uncertainties. In the left panel (rest-UV), no systematic offset is detected between protocluster and field LAEs (field fit: $\beta = 0.05 \pm 0.08$, scatter $= 0.2$ dex), although the large intrinsic scatter limits the sensitivity to environmental differences. In the right panel (rest-optical), protocluster LAEs systematically lie above the field relation, indicating larger sizes at fixed stellar mass. The median rest-optical size enhancement is $\simeq$40% (cluster median $R_e = 0.81$ kpc vs. field $0.58$ kpc). For reference, the right panel includes the rest-optical size-mass relations from Song_et_al_2026 for LAEs at $z \simeq 4\hbox{--}7$ (blue dashed line) and from Allen_et_al_2025 for star-forming galaxies (SFGs) at $z \simeq 4\hbox{--}6$ (green dashed line). Field LAEs are broadly consistent with the Song_et_al_2026 LAE relation (Section \ref{['sec:results_sizemass']}).
• Figure 3: Cumulative distribution functions (CDFs) of size residuals $\Delta\log_{10}(R_e)$ from the field size-mass relation for rest-frame UV (F150W, left) and rest-frame optical (F277W, right). Residuals are calculated as the observed size minus the size predicted from the linear relation fit to our field LAE sample (Section \ref{['sec:results_sizemass']}) at each galaxy's stellar mass. Blue lines show field LAEs (centered near zero by construction), and red lines show protocluster LAEs. In rest-UV (left panel), the distributions are statistically indistinguishable (KS $p = 0.79$; Mann-Whitney $p = 0.42$). In rest-optical (right panel), protocluster LAEs show positive residuals (median $\Delta\log_{10}(R_e) = +0.12_{-0.04}^{+0.07}$ dex; Mann-Whitney $p = 0.033$, $2.1\sigma$; KS $p = 0.029$), indicating they are $\simeq$31% larger than field galaxies of comparable stellar mass.
• Figure 4: Comparison of rest-UV (F150W) and rest-optical (F277W) effective radii for protocluster (red squares) and field (blue circles) LAEs with valid size measurements in both bands. The dashed line indicates the 1:1 relation; galaxies below the line have more compact UV emission relative to their optical extent ($R_{e,{\rm UV}} < R_{e,{\rm opt}}$). Field LAEs are broadly consistent with the 1:1 relation within their scatter. Protocluster LAEs tend to lie below the 1:1 line, but this difference is not statistically significant. The trend is consistent with the significant rest-optical size enhancement identified in the independent single-band comparisons (Figures \ref{['fig:size_mass']}--\ref{['fig:size_cdf']}), which shifts protocluster LAEs to larger optical sizes while no corresponding difference is detected in rest-UV.