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

TL;DR

This study uses JWST observations of a highly magnified galaxy at z=6.14 to analyze its sub-parsec structures, revealing star clusters, ionizing radiation leakage, and low-metallicity regions, advancing understanding of early galaxy formation.

Contribution

First detailed JWST analysis of a z=6.14 galaxy at parsec scales, identifying star clusters, ionizing radiation leakage, and metallicity variations within the galaxy.

Findings

Detection of star clusters with sizes 3-8 pc and masses 2x10^5 to 5x10^6 M_sun.

Evidence of ionizing radiation leakage from steep UV slopes and Ly-alpha emission.

Identification of a low-metallicity region displaced from bright UV structures.

Abstract

We present a detailed JWST/NIRSpec and NIRCam analysis of a gravitationally-lensed galaxy ($\rm \mu=17-21$) at redshift 6.14 magnified by the Hubble Frontier Field galaxy cluster MACS J0416. The target galaxy is overall a typical compact and UV-faint ($\rm M_{UV}=-17.8$) Lyman-$\alpha$ emitter, yet the large magnification allows the detailed characterization of structures on sub-galactic scales (down to a few parsecs). Prominent optical $\rm H\alpha$, $\rm H\beta$ and [OIII]$\lambda\lambda4959,5007$ lines are spatially resolved with the high spectral resolution grating (G395H, R~2700), with large equivalent widths, EW($\rm H\beta$+[OIII])$\gtrsim1000$ \AA, and elevated ionising photon production efficiencies $\rm log(\xi_{ion}/erg^{-1}Hz)=25.2-25.7$. NIRCam deep imaging reveals the presence of compact rest-UV bright regions along with individual star clusters of $\rm R_{eff}=3-8~pc$ in size and $\rm M\sim2\cdot10^5-5\cdot10^{6}~M_\odot$ in mass. These clusters are characterised by steep UV slopes, $\rm\beta_{UV}\lesssim-2.5$, 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-$\rm \alpha$ emission peaking at $\rm \sim100~km~s^{-1}$ from the systemic redshift. While the entire system is characterised by low-metallicity, $\sim0.1~Z_\odot$, the NIRSpec-IFU map also reveals the presence of a low-luminosity, metal-poor region with $\rm Z\lesssim2\%~Z_\odot$, barely detected in NIRCam imaging; this region is displaced by $\rm >200~pc$ 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.