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

Journal Article · Tue Jul 13 00:00:00 EDT 2021 · Monthly Notices of the Royal Astronomical Society

DOI:https://doi.org/10.1093/mnras/stab2591· OSTI ID:1982535

Witstok, Joris  ^[1]; Smit, Renske  ^[2]; Maiolino, Roberto  ^[1]; Curti, Mirko  ^[1]; Laporte, Nicolas  ^[1]; Massey, Richard  ^[3]; Richard, Johan  ^[4]; Swinbank, Mark  ^[3]

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1. Univ. of Cambridge (United Kingdom). Kavli Institute for Cosmology and Cavendish Laboratory
2. Univ. of Cambridge (United Kingdom). Kavli Institute for Cosmology and Cavendish Laboratory; Liverpool John Moores Univ. (United Kingdom)
3. Durham Univ. (United Kingdom)
4. Centre National de la Recherche Scientifique (CNRS) (France); Univ. of Lyon (France)

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.

• Research Organization: US Department of Energy (USDOE), Washington, DC (United States). Office of Science, Sloan Digital Sky Survey (SDSS)
• Sponsoring Organization: USDOE; European Research Council (ERC); Science and Technology Facilities Council (STFC)
• Grant/Contract Number: 695671; ST/T000244/1; ST/T002565/1; NAS 5-26555
• OSTI ID: 1982535
• Journal Information: Monthly Notices of the Royal Astronomical Society, Vol. 508, Issue 2; ISSN 0035-8711
• Publisher: Oxford University Press
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
Astronomy
Astrophysics
Gravitational lensing
Methods
Strong
Observational
Techniques
Spectroscopic
Galaxies
High-redshift
Dark ages
Reionization
First stars