Chasing Cosmic Reionization: An Extremely Faint Highly Magnified Source at $z=5.66$ with high ${\xi }_{\mathrm{ion}}$

Abstract

We present JWST NIRSpec IFU spectroscopic measurements of one of the faintest (M${}_{UV}>-13.6$) known ionizing sources at $z=5.66$, dubbed Small And Lensed Source Arc, SALSA. This source is highly magnified ($\mu >100$) by the lensing galaxy cluster Abell 2744, providing a unique opportunity to investigate the physical properties of faint sources in the Epoch of Reionization. We characterize SALSA's nebular emission using rest-frame UV and optical emission lines and investigate the relationship between its ionizing efficiency, nebular excitation, and chemical enrichment. We robustly detect H$\alpha$ emission and use it to predict the H$\beta$ flux assuming negligible dust attenuation. We also measure the [OIII]$\lambda$5007 emission and estimate oxygen abundance resulting in a high R3 index $2.82_{-0.25}^{+0.34}$ and relatively low metallicity $12+\log (O/H)=7.43\pm 0.09$. SALSA presents a high ionizing production efficiency log$({\xi }_{ion})=25.49_{-0.08}^{+0.09}$ Hz erg ${}^{-1}$, consistent with theoretical models from very massive stars and constant star formation rate, and a high Ly$\alpha$ escape fraction $f_{\mathrm{esc}}^{\mathrm{Ly}\alpha }=0.39\pm 0.14$. These properties place SALSA among the most extreme star-forming sources known at this epoch.

Submitted to ApJL. 8 pages, 4 figures, 1 table