One H₂ molecule per ten million H atoms reveals sub-parsec-scale cold overdensities at z ∼ 4^★

¹ Institut d’Astrophysique de Paris, CNRS-SU, UMR 7095 98bis bd Arago 75014 Paris, France
² Ioffe Institute Polyteknicheskaya 26 194021 Saint-Petersburg, Russia
³ NRC Herzberg Astronomy and Astrophysics Research Centre 5071 West Saanich Road Victoria BC V9E 2E7, Canada
⁴ Department of Physics and Astronomy, Camosun College 3100 Foul Bay Road Victoria BC V8P 5J2, Canada
⁵ INAF – Osservatorio Astronomico di Trieste, Via G. B. Tiepolo 11 34143 Trieste, Italy
⁶ IFPU – Institute for Fundamental Physics of the Universe Via Beirut 2 34151 Trieste, Italy
⁷ National Institute for Nuclear Physics Via Valerio 2 34127 Trieste, Italy
⁸ Departamento de Astronomía, Universidad de Chile Casilla 36-D Santiago 7550000, Chile
⁹ LERMA, Observatoire de Paris, Université PSL, Sorbonne Université 75014 Paris, France
¹⁰ Laboratoire de Physique de l’École Normale Supérieure, PSL, CNRS, SU, Université de Paris 75005 Paris, France
¹¹ Centre for Extragalactic Astronomy, Durham University South Road Durham DH1 3LE, UK

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

Received: 18 December 2025
Accepted: 23 January 2026

Abstract

We present the detection and analysis of H₂ absorption at z = 4.24 towards the bright quasar J 0007−5705, which was observed with the Very Large Telescope as part of the ESPRESSO QUasar Absorption Line Survey (EQUALS). The high resolving power of R ≈ 120 000 enables the identification of extremely weak H₂ lines in several rotational levels at a total column density of N(H₂)≈2 × 10¹⁴ cm⁻², which is among the lowest ever measured in quasar absorption systems. Remarkably, this constitutes the highest redshift H₂ detection to date. Two velocity components are resolved that are separated by only 3 km s⁻¹: a narrow (b ∼ 1.7 km s⁻¹) and a broader (b ≃ 6.2 km s⁻¹) component. Modelling the rotational population of H₂ yields a density of log n_H/cm⁻³ ∼ 2.8 and temperature of ∼40 K (typical of the cold neutral medium) for the narrow component and log n_H/cm⁻³ ∼ 1.4, T ∼ 600 K for the warmer, more turbulent component under a moderate ultraviolet (UV) field, suggesting at least a several-megaparsec distance from the quasar. This system reveals the existence of tiny (down to ∼0.01 pc), cold overdensities in the neutral medium. Their detection among only seven damped Lyman-α systems in EQUALS suggests that they may be widespread yet usually remain undetected. H₂ provides an exceptionally sensitive probe of these structures: even a minute molecular fraction produces measurable Lyman-Werner absorption lines along the extremely narrow optical beam –the size of the quasar’s accretion disc– when observed at sufficiently high spectral resolution. High-resolution spectroscopy on extremely large telescopes may routinely detect and resolve such structures in the distant Universe, where 21-cm absorption traces the collective contribution of many cold cloudlets towards larger radio background sources.