Evidence of Pop III stars’ chemical signature in neutral gas at z ∼ 6

A study based on the E-XQR-30 spectroscopic sample

¹ INAF – Osservatorio Astronomico di Trieste, Via G. Tiepolo 11, 34143 Trieste, Italy
e-mail: valentina.dodorico@inaf.it
² Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy
³ IFPU – Institute for Fundamental Physics of the Universe, Via Beirut 2, 34151 Trieste, Italy
⁴ Dipartimento di Fisica e Astronomia, Università degli Studi di Firenze, Via G. Sansone 1, Sesto Fiorentino FI, Italy
⁵ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁶ Dipartimento di Fisica, Sezione di Astronomia, Università di Trieste, Via Tiepolo 11, 34143 Trieste, Italy
⁷ Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Hawthorn, Victoria 3122, Australia
⁸ ARC Centre of Excellence for All-Sky Astrophysics in 3 Dimensions (ASTRO 3D), Canberra, Australia
⁹ Department of Physics & Astronomy, University of California, Riverside, CA 92521, USA
¹⁰ Max Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹¹ Institute for Theoretical Physics, University of Heidelberg, Philosophenweg 16, 69120 Heidelberg, Germany
¹² Gemini Observatory, NSF’s NOIRLab, 670 N A’ohoku Place, Hilo, HI 96720, USA
¹³ Institute for Astronomy, University of Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
¹⁴ Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai, 400005, India
¹⁵ Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia

Received: 21 December 2023
Accepted: 10 May 2024

Abstract

Aims. This study explores the metal enrichment signatures attributed to the first generation of stars (Pop III) in the Universe, focusing on the E-XQR-30 sample – a collection of 42 high signal-to-noise ratio spectra of quasi-stellar objects (QSOs) with emission redshifts ranging from 5.8 to 6.6. We aim to identify traces of Pop III metal enrichment by analyzing neutral gas in the interstellar medium of primordial galaxies and their satellite clumps, detected in absorption.

Methods. To chase the chemical signature of Pop III stars, we studied metal absorption systems in the E-XQR-30 sample, selected through the detection of the neutral oxygen absorption line at 1302 Å. The O I line is a reliable tracer of neutral hydrogen and allowed us to overcome the challenges posed by the Lyman-α forest’s increasing saturation at redshifts above ∼5 to identify damped Lyman-α systems (DLAs). We detected and analyzed 29 O I systems at z ≥ 5.4, differentiating between proximate DLAs (PDLAs) and intervening DLAs. Voigt function fits were applied to obtain ionic column densities, and relative chemical abundances were determined for 28 systems. These were then compared with the predictions of theoretical models.

Results. Our findings expand the study of O I systems at z ≥ 5.4 fourfold. No systematic differences were observed in the average chemical abundances between PDLAs and intervening DLAs. The chemical abundances in our sample align with literature systems at z >  4.5, suggesting a similar enrichment pattern for this class of absorption systems. A comparison between these DLA-analogs at 4.5 <  z <  6.5 with a sample of very metal-poor DLAs at 2 <  z <  4.5 shows in general similar average values for the relative abundances, with the exception of [C/O], [Si/Fe] and [Si/O] which are significantly larger for the high-z sample. Furthermore, the dispersion of the measurements significantly increases in the high-redshift bin. This increase is predicted by the theoretical models and indicates a potential retention of Pop III signatures in the probed gas.

Conclusions. This work represents a significant advancement in the study of the chemical properties of highly neutral gas at z ≥ 5.4, shedding light on its potential association with the metal enrichment from Pop III stars. Future advancements in observational capabilities, specifically high-resolution spectrographs, are crucial for refining measurements and addressing current limitations in the study of these distant absorption systems.