On the dusty proximate damped Lyman-α system toward Q 2310−3358 at z = 2.40

¹ Cosmic Dawn Center (DAWN) Copenhagen 2200, Denmark
² Niels Bohr Institute, University of Copenhagen Jagtvej 155 2200 Copenhagen N, Denmark
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, UMR5574 69230 Saint-Genis-Laval, France
⁵ European Southern Observatory, Alonso de Córdova 3107, Vitacura Casilla 19001 Santiago, Chile
⁶ Instituto de Astrofísica de Canarias, Vía Láctea s/n 38205 La Laguna Tenerife, Spain
⁷ Gran Telescopio Canaias (GRANTECAN) 38205 San Cristóbal de La Laguna Tenerife, Spain

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

Received: 28 October 2025
Accepted: 27 November 2025

Abstract

Quasar absorption systems not only affect the way quasars are selected, but also serve as key probes of galaxies, providing insight into their chemical evolution and interstellar medium (ISM). Recently, a method based on Gaia astrometric measurements has aided the selection of quasars reddened by dust hitherto overlooked. We conducted a spectroscopic study using VLT/X-Shooter on one such dust-reddened quasar, Q 2310–3358. This quasar, at z = 2.3909 ± 0.0022, is associated with a damped Lyman-α absorber (DLA) at nearly the same redshift 2.4007 ± 0.0003, with a neutral hydrogen column density of log N(H I) = 21.214 ± 0.003. The DLA is very metal-rich (close to solar metallicity after correction for depletion on dust grains). Its properties align with the metal-to-dust ratio and the mass-metallicity relation established in previous large samples of DLAs. Surprisingly, given its proximity to the quasar in redshift, the absorber has strong cold gas characteristics, including C I and H₂. Based on the derived kinetic temperature of 71₋₁₅⁺²⁸ K, we infer the presence of a strong UV radiation field, which in turn suggests that the quasar and the DLA are in close proximity, i.e., part of the same galaxy and not just different objects in the same overdensity of galaxies. We used the line ratios of the C I fine-structure lines to constrain the density of the cold gas, yielding n_H ∼ 10³ cm⁻³. Our analysis extends the understanding of z_abs ≈ z_em absorption line systems and provides valuable constraints on the interplay between dust, metals, and neutral gas in the ISM of early galaxies.