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

TL;DR

This study reports the first detection of molecular hydrogen in a DLA at $z_{\rm abs} \approx 4.24$ toward J0007-5705 using ESPRESSO at $R \sim 120000$, uncovering two closely separated H$_2$-bearing components with markedly different densities and temperatures. Through Voigt-profile fitting and Cloudy photoionization modelling, the authors derive $\log n_H \approx 2.8$ and $1.4$, $T \approx 40$ K and $\sim 600$ K, and cloudlet sizes from $\sim 0.01$ pc to a few pc, embedded in low-metallicity gas with $Z \approx 0.01 Z_\odot$; a Galactic-like UV field and a possible partial coverage scenario are discussed. The results imply that tiny cold overdensities exist in the high-redshift neutral medium and may be more common than previously thought, detectable only with very high spectral resolution along bright background sources. This work demonstrates the power of next-generation telescopes to routinely resolve sub-parsec CNM structure in the early Universe and motivates re-observation of other H$_2$-bearing DLAs to build a census of multiphase gas at high redshift.

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.

Figures (8)

• Figure 1: Hi Lyman series at $z=4.24$ towards J 0007-5705. ESPRESSO data is shown in black, with the synthetic Hi profile overplotted in red. For Ly-$\alpha$, the reconstructed quasar emission profile is indicated by the dashed blue line.
• Figure 2: Voigt-profile fit to H$_2$ and metal lines towards J 0007-5705. The origin is set at $z_{{\rm H}_2}=4.242745$. The observed normalised spectrum is shown in black, with the total synthetic absorption in red. H$_2$ components are shown in blue (narrow) and orange (wide) while metal components are shown in green. The transmission from the DLA Hi lines, as well as telluric lines (visible in e.g., Sii$\lambda$1253 and Feii$\lambda$1125 panels), is shown in purple. Additional components in grey, fitted jointly with H$_2$ and metal lines, represent intervening Lyman-$\alpha$ forest lines (the $v\sim20$${\rm km\,s^{-1}}$ component in the H$_2$W3-0R0 panel is actually Ly-$\beta$ from a $z=3.838$ system, also visible in Ly-$\alpha$ at the blue edge of the Feii$\lambda1121$ panel).
• Figure 3: Rotational J=0-3 population of H$_2$ for both components. Points are artificially shifted by $\pm$5 K along the x-axis for clarity. Dashed lines show single-temperature fits (with shaded uncertainties), and solid lines our best fiducial Cloudy models.
• Figure 4: Posterior distributions of the UV field and number density in the broad (orange) and narrow (blue) components. The likelihood was derived by comparing the observed H$_2$ J=0 to J=3 column densities with those from a grid of Cloudy models and converted into 2D posterior distribution assuming independent flat priors in logarithmic space. The 1D marginalised posteriors are shown along the top and right axis.
• Figure 5: Comparison of the best-fit model including partial coverage for the narrow component (red) and assuming full coverage (violet). Only a subset of H$_2$ lines is shown, focusing on transitions where profile differences are most pronounced. Graphical elements follow Fig. \ref{['fig:J0007']}. The green dashed horizontal line depicts $(1-C_f)$, the fraction of the background source uncovered by the narrow H$_2$ component.