Assessing the sources of reionization: a spectroscopic case study of a 30× lensed galaxy at z ∼ 5 with Lyα, C IV, Mg II, and [Ne III]

Witstok, Joris; Smit, Renske; Maiolino, Roberto; Curti, Mirko; Laporte, Nicolas; Massey, Richard; Richard, Johan; Swinbank, Mark

Abstract

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 CIV λ1549A˚ 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 [NeIII] λ3870A˚ and use the [NeIII]/[OII] line ratio to constrain the ionization 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 λ2796A˚⁠, 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 ∼ 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 300Myr 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.

Citation

Witstok, J., Smit, R., Maiolino, R., Curti, M., Laporte, N., Massey, R., Richard, J., & Swinbank, M. (2021). Assessing the sources of reionization: a spectroscopic case study of a 30× lensed galaxy at z ∼ 5 with Lyα, C IV, Mg II, and [Ne III]. Monthly Notices of the Royal Astronomical Society, 508(2), 1686-1700. https://doi.org/10.1093/mnras/stab2591

• Journal Article Type: Article
• Acceptance Date: Sep 7, 2021
• Online Publication Date: Jul 13, 2021
• Publication Date: 2021-12
• Deposit Date: Sep 29, 2021
• Publicly Available Date: Nov 11, 2021
• Journal: Monthly Notices of the Royal Astronomical Society
• Print ISSN: 0035-8711
• Electronic ISSN: 1365-2966
• Publisher: Royal Astronomical Society
• Peer Reviewed: Peer Reviewed
• Volume: 508
• Issue: 2
• Pages: 1686-1700
• DOI: https://doi.org/10.1093/mnras/stab2591
• Public URL: https://durham-repository.worktribe.com/output/1239648