2025A&A...694A..59M

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.