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

Abstract: We present JWST NIRSpec IFU spectroscopic measurements of one of the faintest (M$<em>{UV}&gt;-13.6$) known ionizing sources at $z=5.66$, dubbed Small And Lensed Source Arc, SALSA. This source is highly magnified ($μ&gt;100$) 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$α$ emission and use it to predict the H$β$ flux assuming negligible dust attenuation. We also measure the [OIII]$λ$5007 emission and estimate oxygen abundance resulting in a high R3 index $2.82</em>{-0.25}<sup>{+0.34}$ and relatively low metallicity $\mathrm{12+log(O/H)}= 7.43\pm0.09$. SALSA presents a high ionizing production efficiency log$(ξ<em>{ion})=25.49</em>{-0.08}<sup>{+0.09}$ Hz erg $<sup>{-1}$, consistent with theoretical models from very massive stars and constant star formation rate, and a high Ly$α$ escape fraction $f_{\mathrm{esc}}<sup>{\mathrm{Ly}α}=0.39\pm0.14$. These properties place SALSA among the most extreme star-forming sources known at this epoch.