Metal enrichment and evolution in four z > 6.5 quasar sightlines observed with JWST/NIRSpec

¹ Cosmic Dawn Center (DAWN), University of Copenhagen, 2200 Copenhagen N, Denmark
e-mail: lichrist@nbi.ku.dk
² Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200 Copenhagen N, Denmark
³ National Research Council of Canada, Herzberg Astronomy & Astrophysics Research Centre, 5071 West Saanich Road, Victoria BC V9E 2E7, Canada
⁴ Centro de Astrobiología (CAB), CSIC-INTA, Ctra. de Ajalvir km 4, Torrejón de Ardoz, 28850 Madrid, Spain
⁵ Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
⁶ Sorbonne Université, CNRS, UMR 7095, Institut d’Astrophysique de Paris, 98 bis bd Arago, 75014 Paris, France
⁷ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
⁸ Cavendish Laboratory, University of Cambridge, 19 JJ Thomson Avenue, Cambridge CB3 0HE, UK
⁹ Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK

Received: 11 September 2023
Accepted: 11 October 2023

Abstract

We present JWST/NIRSpec R ≃ 2700 spectra of four high-redshift quasars: VDES J0020–3653 (z = 6.860), DELS J0411–0907 (z = 6.825), UHS J0439+1634 (z = 6.519), and ULAS J1342+0928 (z = 7.535). The exquisite data quality, signal-to-noise ratio of 50–200, and large 0.86 μm ≤ λ ≤ 5.5 μm spectral coverage allowed us to identify between 13 and 17 intervening and proximate metal absorption line systems in each quasar spectrum, with a total number of 61 absorption-line systems detected at 2.42 < z < 7.48 including the highest redshift intervening O Iλ1302 and Mg II systems at z = 7.37 and z = 7.44. We investigated the evolution of the metal enrichment in the epoch of re-ionisation (EoR) at z > 6 and found the following: i) a continued increase in the low-ionisation O I, C II, and Si II incidence, ii) decreasing high-ionisation C IV and Si IV incidence with a transition from predominantly high- to low-ionisation at z ≈ 6.0, and iii) a constant Mg II incidence across all redshifts. The observations support a change in the ionisation state of the intergalactic medium in the EoR rather than a change in metallicity. The abundance ratio of [Si/O] in five z > 6 absorption systems show enrichment signatures produced by low-mass Pop III pair instability supernovae, and possibly Pop III hypernovae. In the Gunn-Peterson troughs, we detected transmission spikes where Lyα photons can escape. From 22 intervening absorption line systems at z > 5.7, only a single low-ionisation system out of 13 lies within 2000 km s⁻¹ from a spike, while four high-ionisation systems out of nine lie within ∼2000 km s⁻¹ from a spike. Most spikes do not have associated metal absorbers close by. This confirms that star-forming galaxies responsible for producing the heavy elements that are transported to the circumgalactic medium via galaxy winds do so in predominantly high-density, neutral environments, while lower density environments are ionised without being polluted by metals at z ≈ 6 − 7.