One H2 molecule per ten million H-atoms reveals sub-pc scale cold overdensities at z~4

P. Noterdaeme, S. Balashev, T. Berg, S. Cristiani, R. Cuellar, G. Cupani, S. Di Stefano, V. D'Odorico, C. Fian, B. Godard, S. López, D. Milaković, A. Trost, L. Welsh

First listed 2026-01-23 | Last updated 2026-01-23

Abstract

We present the detection and analysis of H2 absorption at z = 4.24 towards the bright quasar J0007-5705, observed with the Very Large Telescope as part of the ESPRESSO QUasar Absorption Line Survey (EQUALS). The high resolving power, R~120000, enables the identification of extremely weak H2 lines in several rotational levels at a total column density of N(H2)~2x10^14 cm^-2, among the lowest ever measured in quasar absorption systems. Remarkably, this constitutes the highest-redshift H2 detection to date. Two velocity components are resolved, separated by only 3 km/s: a narrow (b~1.7 km/s) and a broader (b~6.2 km/s) component. Modelling the rotational population of H2 yields density of log nH/cm^-3 ~ 2.8 with temperature of ~40K (typical of the cold neutral medium) for the narrow component and log nH/cm^-3 ~ 1.4 , T~600K for the warmer, more turbulent component under a moderate ultraviolet (UV) field, suggesting at least several Mpc 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 7 damped Lyman-alpha 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, when 21-cm absorption will trace the collective contribution of many cold cloudlets toward larger radio background sources.

Short digest

Ultra-high-resolution VLT/ESPRESSO (R~120,000) spectroscopy reveals exceptionally weak H2 Lyman–Werner absorption at z=4.24 toward J0007-5705, the highest-redshift H2 detection to date with N(H2)≈2×10^14 cm^-2 (~10^-7 fraction). Two components separated by ~3 km/s are resolved: a CNM-like narrow cloud (b≈1.7 km/s, nH≈10^2.8 cm^-3, T≈40 K) and a warmer, more turbulent phase (b≈6.2 km/s, nH≈10^1.4 cm^-3, T≈600 K) under a moderate UV field. Column densities and excitation imply path lengths down to ~0.01 pc, pointing to tiny cold overdensities in otherwise neutral gas. Finding one such case among just seven EQUALS DLAs argues these sub-pc cloudlets may be common yet usually invisible, showcasing the leverage of ultra-high spectral resolution.

Key figures to inspect

• Figure 1: Inspect the high-order H I Lyman series edges that pin down N(H I) and z; note how the reconstructed Lyα emission anchors the continuum across the DLA trough.
• Figure 2: Check the Voigt-profile decomposition that isolates two H2 components only ~3 km/s apart and their alignment (or lack thereof) with metal components, while verifying which features are Lyα-forest or telluric blends.
• Figure 3: Read the J=0–3 rotational population diagrams for each component—contrast the ~40 K narrow-phase excitation with the ~600 K broader phase and how Cloudy models outperform single-T fits.
• Figure 4: Examine the UV-field vs density posteriors that separate the high-nH narrow cloud from the lower-nH broad phase; note the degeneracies and the moderate UV field consistent with several-Mpc distance from the quasar.