Anatomy of a z = 6 Lyman-α emitter down to parsec scales

Extreme UV slopes, metal-poor regions, and possibly leaking star clusters

¹ INAF – OAS, Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, via Gobetti 93/3, I-40129 Bologna, Italy
² Dipartimento di Fisica, Università degli Studi di Milano, Via Celoria 16, I-20133 Milano, Italy
³ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monteporzio Catone, Rome, Italy
⁴ Department of Physics and Astronomy, University of Missouri, Columbia, MO 65211, USA
⁵ NRC Herzberg, 5071 West Saanich Rd, Victoria, BC V9E 2E7, Canada
⁶ School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-1404, USA
⁷ , INAF – Osservatorio Astronomico di Capodimonte, Via Moiariello 16, I-80131 Napoli, Italy
⁸ Dipartimento di Fisica e Scienze della Terra, Università degli Studi di Ferrara, Via Saragat 1, I-44122 Ferrara, Italy
⁹ Department of Astronomy, Oskar Klein Centre, Stockholm University, AlbaNova University Centre, SE-106 91 Stockholm, Sweden
¹⁰ Dipartimento di Fisica e Astronomia, Università degli Studi di Padova, Vicolo dell’Osservatorio 3, I-35122 Padova, Italy
¹¹ INAF Osservatorio Astronomico di Padova, vicolo dell’Osservatorio 5, 35122 Padova, Italy
¹² University of Ljubljana, Department of Mathematics and Physics, Jadranska ulica 19, SI-1000 Ljubljana, Slovenia
¹³ Department of Physics and Astronomy, University of California Davis, 1 Shields Avenue, Davis, CA 95616, USA
¹⁴ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹⁵ Jodrell Bank Centre for Astrophysics, Alan Turing Building, University of Manchester, Oxford Road, Manchester M13 9PL, UK
¹⁶ International Centre for Radio Astronomy Research (ICRAR) and the International Space Centre (ISC), The University of Western Australia, M468, 35 Stirling Highway, Crawley, WA 6009, Australia
¹⁷ NSF’s National Optical-Infrared Astronomy Research Laboratory, 950 N. Cherry Ave., Tucson, AZ 85719, USA
¹⁸ Department of Astronomy/Steward Observatory, University of Arizona, 933 N. Cherry Avenue, Tucson, AZ 85721, USA
¹⁹ INAF – IASF Milano, via A. Corti 12, I-20133 Milano, Italy
²⁰ Astrophysics Science Division, NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA
²¹ Center for Research and Exploration in Space Science and Technology II, Department of Physics, Catholic University of America, 620 Michigan Ave N.E., Washington, DC 20064, USA
²² Department of Astronomy, University of Maryland, College Park 20742, USA

^⋆ Corresponding author; matteo.messa@inaf.it

Received: 29 July 2024
Accepted: 13 November 2024

Abstract

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 (M_UV = −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(ξ_ion/erg⁻¹Hz) = 25.2–25.7. NIRCam deep imaging reveals the presence of compact rest-UV-bright regions along with individual star clusters of R_eff = 3 − 8 pc in size and M ∼ 2 ⋅ 10⁵ − 5 ⋅ 10⁶ M_⊙ in mass. These clusters are characterized by steep UV slopes, β_UV ≲ −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.