The clustering of C IV and Si IV at the end of reionization

A perspective from the E-XQR-30 survey

¹ INAF – Osservatorio Astronomico di Trieste, Via G. B. Tiepolo 11, I-34143 Trieste, Italy
² IFPU – Institute for Fundamental Physics of the Universe, Via Beirut 2, I-34151 Trieste, Italy
³ Centre for Extragalactic Astronomy, Durham University, South Road, Durham DH1 3LE, UK
⁴ Scuola Normale Superiore, Piazza dei Cavalieri 7, I-56126 Pisa, Italy
⁵ Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Hawthorn, Victoria 3122, Australia
⁶ Institute for Theoretical Physics, Heidelberg University, Philosophenweg 12, D-69120 Heidelberg, Germany
⁷ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁸ Department of Physics & Astronomy, University of California, Riverside, CA 92521, USA
⁹ Institute for Astronomy, University of Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
¹⁰ ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO3D), Canberra, ACT 2611, Australia
¹¹ Dipartimento di Fisica, Universitá di Trieste, Sezione di Astronomia, Via G.B. Tiepolo 11, I-34131 Trieste, Italy
¹² Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
¹³ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
¹⁴ Augustana Campus, University of Alberta, Camrose, AB T4V2R3, Canada
¹⁵ Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA
¹⁶ Commonwealth Scientific and Industrial Research Organisation (CSIRO), Space & Astronomy, PO Box 1130 Bentley, WA 6102, Australia
¹⁷ Tianjin Normal University, Binshuixidao 393, 300387 Tianjin, China
¹⁸ Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia

^⋆ Corresponding author: louise.welsh@inaf.it

Received: 10 March 2025
Accepted: 25 August 2025

Abstract

Aims. We studied the clustering of metal absorption lines and the structures that they arise in as a function of cosmic time. We focused on the behaviour of C IV and Si IV ionic species. These C IV and Si IV absorption features are identified along a given quasar sightline.

Methods. We exploited the two-point correlation function (2PCF) to investigate the clustering of these structures as a function of their separation. We utilised the E-XQR-30 data to perform a novel analysis at z > 5. We also drew on literature surveys (including XQ-100) of lower redshift quasars to investigate the possible evolution of this clustering towards cosmic noon (i.e. z ∼ 2 − 3).

Results. We find no significant evolution with redshift when considering the separation of absorbers in velocity space. Since we were comparing data across a large interval of cosmic time, we also considered the separation between absorbers in the reference frame of physical distances. In this reference frame, we find that the amplitude of the clustering increases with cosmic time for both C IV and Si IV on scales of < 1500 physical kpc.

Conclusions. For the first time, we assessed the 2PCF of C IV and Si IV close to the epoch of reionization utilising the absorber catalogue from the E-XQR-30 survey. We compared this with lower redshift data and find that, on small scales, the clustering of these structures grows with cosmic time. We compared these results to the clustering of galaxies in the GAEA simulations. It appears that the structures traced by C IV are broadly comparable to those of the galaxies from the considered simulations. The clustering is most similar to that of the galaxies with virial masses (M) of ∼ 10^10.5 M_⊙. We do not draw direct comparisons at the smallest separations, to avoid the clustering traced by C IV at z ∼ 5 being dominated by contributions from absorbers within a single halo. We require tailor-made simulations to investigate the full range of factors contributing to the observed clustering of the detected metal absorbers. Future ground-based spectrographs will further facilitate surveys of absorbers at this epoch with increased sensitivity.