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 ($\mu =17-21$) 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 (${\mathrm{M}}_{\mathrm{UV}}=-17.8$) Lyman-$\alpha$ emitter, yet the large magnification allows the detailed characterization of structures on sub-galactic scales (down to a few parsecs). Prominent optical $\mathrm{H}\alpha$, $\mathrm{H}\beta$ and [OIII]$\lambda \lambda 4959,5007$ lines are spatially resolved with the high spectral resolution grating (G395H, R~2700), with large equivalent widths, EW($\mathrm{H}\beta$+[OIII])$\gtrsim 1000$ Å, and elevated ionising photon production efficiencies $\log ({\xi }_{\mathrm{ion}}/er{\mathrm{g}}^{-1}\mathrm{Hz})=25.2-25.7$. NIRCam deep imaging reveals the presence of compact rest-UV bright regions along with individual star clusters of ${\mathrm{R}}_{\mathrm{eff}}=3-8\mathrm{pc}$ in size and $\mathrm{M}\sim 2\cdot 10^5-5\cdot 10^6{\mathrm{M}}_{\odot }$ in mass. These clusters are characterised by steep UV slopes, ${\beta }_{\mathrm{UV}}\lesssim -2.5$, 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-$\alpha$ emission peaking at $\sim 100\mathrm{km}{\mathrm{s}}^{-1}$ from the systemic redshift. While the entire system is characterised by low-metallicity, $\sim 0.1Z_{\odot }$, the NIRSpec-IFU map also reveals the presence of a low-luminosity, metal-poor region with $\mathrm{Z}\lesssim 2\%{\mathrm{Z}}_{\odot }$, barely detected in NIRCam imaging; this region is displaced by $>200\mathrm{pc}$ 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.

18 pages (11 figures) + appendix (6 pages, 6 figures, 2 tables). Published in A&A

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