Anatomy of a z = 6 Lyman-α emitter down to parsec scales : extreme UV slopes, metal-poor regions, and possibly leaking star clusters

Messa, Matteo (author), Vanzella, Eros (author), Loiacono, Federica (author), Bergamini, Pietro (author), Castellano, Marco (author), Bradač, Maruša (author)

We present a detailed JWST/NIRSpec and NIRCam analysis of a gravitationally lensed galaxy (μ = 17 ‑ 21) at a redshift of 6.14 magnified by the Hubble Frontier Field galaxy cluster MACS J0416. The target galaxy is a typical compact and UV-faint (MUV = ‑17.8) Lyman-α emitter, yet the large magnification allows the detailed characterization of structures on sub-galactic scales (down to a few parsecs). Prominent optical Hα, Hβ, and [OIII]λλ4959, 5007 lines are spatially resolved with the high-spectral-resolution grating (G395H, R 2700), with large equivalent widths, EW(Hβ+O III) ≳ 1000 Å, and elevated ionizing photon production efficiencies, log(ξion/erg‑1Hz) = 25.2–25.7. NIRCam deep imaging reveals the presence of compact rest-UV-bright regions along with individual star clusters of Reff = 3 ‑ 8 pc in size and M ∼ 2 ṡ 105 ‑ 5 ṡ 106 M⊙ in mass. These clusters are characterized by steep UV slopes, βUV ≲ ‑2.5, which in some cases are associated with a dearth of line emission, indicating possible leaking of the ionizing radiation, as also supported by a Lyman-α emission peaking at ∼100 km s‑1 from the systemic redshift. While the entire system is characterized by low metallicity, ∼0.1 Z⊙, the NIRSpec-IFU map also reveals the presence of a low-luminosity, metal-poor region with Z ≲ 2% Z⊙, which is barely detected in NIRCam imaging; this region is displaced by > 200 pc from one of the brightest structures of the system in UV, and would have been too faint to detect if not for the large magnification of the system.

• Language: English
• Keywords: gravitational lensing, HII regions, high-redshift galaxies, star formation
• Typology: 1.01 - Original Scientific Article
• Organization: FMF - Faculty of Mathematics and Physics
• Publication status: Published
• Publication version: Version of Record
• Year: 2025
• Number of pages: 23 str.
• Numbering: Vol. 694, art. A59
• PID: 20.500.12556/RUL-167506
• UDC: 524.8
• ISSN on article: 1432-0746
• DOI: 10.1051/0004-6361/202451695
• COBISS.SI-ID: 227187715
• Publication date in RUL: 25.02.2025

Record is a part of a journal

• Title: Astronomy & astrophysics
• Shortened title: Astron. astrophys.
• Publisher: EDP Sciences
• ISSN: 1432-0746
• COBISS.SI-ID: 392577

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• License: CC BY 4.0, Creative Commons Attribution 4.0 International
• Link: http://creativecommons.org/licenses/by/4.0/
• Description: This is the standard Creative Commons license that gives others maximum freedom to do what they want with the work as long as they credit the author.