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We present a detailed spectroscopic analysis of a galaxy at $z \simeq 4.88$that is, by chance, magnified $\sim 30 \times$ by gravitational lensing. Onlythree sources at $z \gtrsim 5$ are known with such high magnification. Thisparticular source has been shown to exhibit widespread, high equivalent widthCIV $\lambda$ 1549 {\AA} emission, implying it is a unique example of ametal-poor galaxy with a hard radiation field, likely representing the galaxypopulation responsible for cosmic reionisation. Using UV nebular line ratiodiagnostics, VLT/X-shooter observations rule out strong AGN activity,indicating a stellar origin of the hard radiation field instead. We present anew detection of [NeIII] $\lambda$ 3870 {\AA} and use the [NeIII]/[OII] lineratio to constrain the ionisation parameter and gas-phase metallicity. Closelyrelated to the commonly used [OIII]/[OII] ratio, our [NeIII]/[OII] measurementshows this source is similar to local "Green Pea" galaxies and Lyman-continuumleakers. It furthermore suggests this galaxy is more metal poor than expectedfrom the Fundamental Metallicity Relation, possibly as a consequence of excessgas accretion diluting the metallicity. Finally, we present the highestredshift detection of MgII $\lambda$ 2796 {\AA}, observed at high equivalentwidth in emission, in contrast to more evolved systems predominantly exhibitingMgII absorption. Strong MgII emission has been observed in most $z \sim 0$Lyman-continuum leakers known and has recently been proposed as an indirecttracer of escaping ionising radiation. In conclusion, this strongly lensedgalaxy, observed just 300 Myr after reionisation ends, enables testing ofobservational diagnostics proposed to constrain the physical properties ofdistant galaxies in the $\mathit{JWST}$/ELT era.

monthly notices of the royal astronomical society

Assessing the sources of reionisation: a spectroscopic case study of a  30$\times$ lensed galaxy at $z \sim 5$ with Ly{\alpha}, CIV, MgII, and  [NeIII]