The multiple coherence scales of C IV at cosmic noon

¹ Departamento de Astronomía, Universidad de Chile Casilla 36-D Santiago, Chile
² Instituto de Física, Pontificia Universidad Católica de Valparaíso Casilla 4059 Valparaíso, Chile
³ French-Chilean Laboratory for Astronomy, IRL 3386, CNRS and U. de Chile Casilla 36-D Santiago, Chile
⁴ Centre de Recherche Astrophysique de Lyon, Université de Lyon 1, ENS-Lyon, UMR5574 9 Av Charles André 69230 Saint-Genis-Laval, France
⁵ Kapteyn Astronomical Institute, University of Groningen Landleven 12 9747 AD Groningen, The Netherlands
⁶ Dipartimento di Fisica e Astronomia, Università di Firenze Via G. Sansone 1 50019 Sesto Fiorentino Firenze, Italy
⁷ INAF – Osservatorio Astrofisico di Arcetri Largo E. Fermi 5 Firenze I-50125, Italy

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 15 November 2025
Accepted: 21 January 2026

Abstract

The spatial and kinematic structure of the circumgalactic medium remains poorly constrained observationally. We computed the clustering of C IV absorption systems at cosmic noon using quasar pairs. We analyzed VLT/UVES and Keck/HIRES high-resolution spectra (R ≈ 45 000) of a sample of eight projected and four lensed quasar pairs that probe transverse separations, Δr, from subkiloparsec to a few megaparsec over the redshift range 1.6 ≲ z ≲ 3.3. We detected and fit Voigt profiles to a total of 141 C IV systems, corresponding to 620 velocity components at all quasar lines of sight. We computed the two-point correlation function of C IV, ξ(Δv, Δr), where Δv is the velocity difference between components at all available scales. We found a strong dependence of ξ(Δr) on Δr at all velocities. ξ(Δr) reaches a sharp peak at the smallest scales we analyzed, Δr ≈ 0.1 kpc, decreases steadily up to Δr ≈ 5 kpc, and remains flat up to Δr ≈ 500 kpc, where it again begins to decrease. By fitting power laws to the projected transverse correlation function Ξ(Δr), we inferred two coherence lengths. The first is r₁ = 654⁺¹⁰⁰₋₈₇ kpc, which we interpret as a representative size for the C IV enriched regions at z ≈ 2, and the second is r₂ = 4.70^+1.60_−1.19 kpc for the individual C IV-bearing clouds. When we instead projected this in Δr, we found amplitudes of ξ(Δv) that were consistent with those in previous works that used quasars and extended background sources. Our results suggest that C IV might be a good tracer of not only the small internal structure of the circumgalactic medium, but also of the way in which galaxies cluster at cosmic noon.