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

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<inf>UV</inf> = - 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+OIII) ≳ 1000 Å, and elevated ionizing photon production efficiencies, log(ξ<inf>ion</inf>/erg^{- 1}Hz) = 25.2-25.7. NIRCam deep imaging reveals the presence of compact rest-UV-bright regions along with individual star clusters of R<inf>eff</inf> = 3 - 8 pc in size and M ~ 2 · 10⁵ - 5 · 10⁶ M<inf>⊙</inf> in mass. These clusters are characterized by steep UV slopes, β<inf>UV</inf> ≲ - 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<inf>⊙</inf>, the NIRSpec-IFU map also reveals the presence of a low-luminosity, metal-poor region with Z ≲ 2% Z<inf>⊙</inf>, 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.