Assessing the sources of reionization: a spectroscopic case study of a 30× lensed galaxy at z ∼ 5 with Lyα, C\u2009iv, Mg\u2009ii, and [Ne\u2009iii]

Abstract

\n We present a detailed spectroscopic analysis of a galaxy at z ≃ 4.88 that is, by chance, magnified ∼30× by gravitational lensing. Only three sources at z ≳ 5 are known with such high magnification. This particular source has been shown to exhibit widespread, high equivalent width ${{\\rm C\\, \\small {IV}}}\\ \\lambda 1549\\, \\mathring{\\rm A}$ 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 reionization. Using ultraviolet (UV) nebular line ratio diagnostics, Very Large Telescope (VLT)/X-shooter observations rule out strong active galactic nuclei (AGN) activity, indicating a stellar origin of the hard radiation field instead. We present a new detection of ${[{\\rm Ne\\, \\small {III}}]}\\ \\lambda 3870\\, \\mathring{\\rm A}$ and use the [${\\rm Ne\\, \\small {III}}$]/[${\\rm O\\, \\small {II}}$] line ratio to constrain the ionization parameter and gas-phase metallicity. Closely related to the commonly used [${\\rm O\\, \\small {III}}$]/[${\\rm O\\, \\small {II}}$] ratio, our [${\\rm Ne\\, \\small {III}}$]/[${\\rm O\\, \\small {II}}$] 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 ${{\\rm Mg\\, \\small {II}}}\\ \\lambda 2796\\, \\mathring{\\rm A}$, observed at high equivalent width in emission, in contrast to more evolved systems predominantly exhibiting ${\\rm Mg\\, \\small {II}}$ absorption. Strong ${\\rm Mg\\, \\small {II}}$ emission has been observed in most z ∼ 0 Lyman-continuum leakers known and has recently been proposed as an indirect tracer of escaping ionizing radiation. In conclusion, this strongly lensed galaxy, observed just $300\\, \\mathrm{Myr}$ after reionization ends, enables testing of observational diagnostics proposed to constrain the physical properties of distant galaxies in the James Webb Space Telescope (JWST)/Extremely Large Telescope (ELT) era.