Discovery of a $z\simeq 4.9$ Lyman-$α$ Emitter Protocluster: Wavelength-Dependent Environmental Effects on Galaxy Structure
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arXiv
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| Natura: | Preprint |
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2026
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| author | Laishram, Ronaldo Koyama, Yusei Kusakabe, Haruka Kikuta, Satoshi Shimizu, Shunta Kodama, Tadayuki |
| author_facet | Laishram, Ronaldo Koyama, Yusei Kusakabe, Haruka Kikuta, Satoshi Shimizu, Shunta Kodama, Tadayuki |
| contents | 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 size difference: protocluster LAEs are $\sim$40% larger in rest-optical (median $R_e = 0.81_{-0.04}^{+0.26}$ kpc vs. $0.58_{-0.04}^{+0.11}$ 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 ($\simeq$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 $z \simeq 5$ 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 build-up phase of cosmic star formation. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2603_03570 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | Discovery of a $z\simeq 4.9$ Lyman-$α$ Emitter Protocluster: Wavelength-Dependent Environmental Effects on Galaxy Structure Laishram, Ronaldo Koyama, Yusei Kusakabe, Haruka Kikuta, Satoshi Shimizu, Shunta Kodama, Tadayuki Astrophysics of Galaxies 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 size difference: protocluster LAEs are $\sim$40% larger in rest-optical (median $R_e = 0.81_{-0.04}^{+0.26}$ kpc vs. $0.58_{-0.04}^{+0.11}$ 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 ($\simeq$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 $z \simeq 5$ 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 build-up phase of cosmic star formation. |
| title | Discovery of a $z\simeq 4.9$ Lyman-$α$ Emitter Protocluster: Wavelength-Dependent Environmental Effects on Galaxy Structure |
| topic | Astrophysics of Galaxies |
| url | https://arxiv.org/abs/2603.03570 |