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]

TL;DR

This study analyzes a highly magnified galaxy at z~4.88, revealing its metal-poor, star-forming nature, and its potential role in cosmic reionisation through spectroscopic diagnostics and detection of key emission lines.

Contribution

It provides the first detailed spectroscopic analysis of a highly magnified galaxy at this redshift, including new detections of [NeIII] and MgII lines, and assesses its ionisation and metallicity properties.

Findings

Galaxy is metal-poor with a hard radiation field.

Spectroscopic diagnostics suggest stellar origin of ionising radiation.

Detected MgII emission indicates potential Lyman-continuum leakage.

Abstract

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.