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

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. Only
three sources at $z gtrsim 5$ are known with such high magnification. This
particular source has been shown to exhibit widespread, high equivalent width
CIV $lambda$ 1549 {AA} emission, implying it is a unique example of a
metal-poor galaxy with a hard radiation field, likely representing the galaxy
population responsible for cosmic reionisation. Using UV nebular line ratio
diagnostics, VLT/X-shooter observations rule out strong AGN activity,
indicating a stellar origin of the hard radiation field instead. We present a
new detection of [NeIII] $lambda$ 3870 {AA} and use the [NeIII]/[OII] line
ratio to constrain the ionisation parameter and gas-phase metallicity. Closely
related to the commonly used [OIII]/[OII] ratio, our [NeIII]/[OII] measurement
shows this source is similar to local "Green Pea" galaxies and Lyman-continuum
leakers. It furthermore suggests this galaxy is more metal poor than expected
from the Fundamental Metallicity Relation, possibly as a consequence of excess
gas accretion diluting the metallicity. Finally, we present the highest
redshift detection of MgII $lambda$ 2796 {AA}, observed at high equivalent
width in emission, in contrast to more evolved systems predominantly exhibiting
MgII absorption. Strong MgII emission has been observed in most $z sim 0$
Lyman-continuum leakers known and has recently been proposed as an indirect
tracer of escaping ionising radiation. In conclusion, this strongly lensed
galaxy, observed just 300 Myr after reionisation ends, enables testing of
observational diagnostics proposed to constrain the physical properties of
distant galaxies in the $mathit{JWST}$/ELT era.
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