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

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.

Accepted for publication in Astronomy and Astrophysics

References in corpus (18)

• Molecfit: A general tool for telluric absorption correction. I. Method and application to ESO instruments
• H3+ in Diffuse Interstellar Clouds: a Tracer for the Cosmic-Ray Ionization Rate
• Molecular hydrogen in high-redshift Damped Lyman-alpha systems: The VLT/UVES database
• Models of turbulent dissipation regions in the diffuse interstellar medium
• Discovery of a Perseus-like cloud in the early Universe: HI-to-H2 transition, carbon monoxide and small dust grains at zabs=2.53 towards the quasar J0000+0048
• Excitation mechanisms in newly discovered H2-bearing Damped Lyman-alpha clouds: systems with low molecular fractions
• Physical conditions in the neutral interstellar medium at z=2.43 toward Q2348-011
• Molecular Hydrogen in a Damped Lyman-alpha System at z_abs=4.224
• X-shooter observations of strong H${}_2$-bearing DLAs at high redshift
• An HST/COS survey of molecular hydrogen in DLAs & sub-DLAs at z < 1: Molecular fraction and excitation temperature
• The spectroscopic follow-up of the QUBRICS bright quasar survey
• Ultra-luminous quasars at redshift $z>4.5$ from SkyMapper
• Physical conditions in diffuse interstellar medium of local and high redshift galaxies: measurements based on excitation of H${}_2$ rotational and CI fine-structure levels
• A detection of H${}_2$ in a high velocity cloud toward the Large Magellanic Cloud
• Radiative and mechanical energies in galaxies I. Contributions of molecular shocks and PDRs in 3C 326 N
• CII*/CII ratio in high-redshift DLAs: ISM phase separation drives the observed bimodality of [CII] cooling rates
• Neutral carbon in diffuse interstellar medium: abundance matching with H2 for DLAs at high redshifts
• Exploring quasar evolution with proximate molecular absorbers: Insights from the kinematics of highly ionized nitrogen