Characterising Ly$α$ damping wings at the onset of reionisation: Evidence for highly efficient star formation driven by dense, neutral gas in UV-bright galaxies at $z>9$

TL;DR

This study uses JWST/NIRSpec Prism spectra of 48 UV-bright galaxies at $z>9$ to model damped Lyα absorption (DLAs) and extract HI column densities $N_{ m HI}$. By combining Lyα damping wings with IGM models, the authors find predominantly dense neutral gas, with $N_{ m HI}$ ranging from $10^{21.7}$ to $>10^{22}$ cm$^{-2}$, and demonstrate that these systems exhibit rapid, highly efficient star formation with depletion times $t_{ m dep}$ around $10$–$100$ Myr and $t_{ m ff}\uparrow$ to $ oughly 25$ Myr, implying star-formation efficiencies $ ilde{ u} ightarrow 0.9$. The HI-rich gas correlates with offsets from the Fundamental Metallicity Relation and the canonical Kennicutt-Schmidt law, suggesting dilution by pristine inflows and elevated star-formation activity in compact, dust-poor environments. The results align broadly with some high-resolution simulations but reveal more extreme DLAs at $z>10$, indicating that early galaxy assembly is governed by dense, central HI reservoirs and rapid gas conversion into stars, with important implications for reionisation and the interpretation of UV-bright galaxies in the dawn of the universe. Future high-resolution spectroscopy will be crucial to disentangle IGM vs DLA contributions, measure the metallicity of absorbing gas, and refine constraints on the early baryon cycle and reionisation timeline.

Abstract

One of the major conundrums in contemporary extragalactic astrophysics is the apparent overabundance of a remarkable population of UV-bright galaxies at redshifts $z\gtrsim 9$. We analyse galaxies spectroscopically observed by JWST/NIRSpec Prism and confirmed to lie at $z>9$, with sufficient signal-to-noise to carefully model their rest-frame UV to optical continua and line emission. In particular, we model the damped Lyman-$α$ (Ly$α$) absorption (DLA) features of each galaxy to place observational constraints on the gas assembly of neutral atomic hydrogen (HI) onto the galaxy halos at the onset of cosmic reionisation. Based on the derived HI column densities and star-formation rate (SFR) surface densities, we show that all galaxies are highly efficient at forming stars on rapid $\sim 10-100\,$Myr depletion timescales, greatly in excess compared to the canonical local universe Kennicutt-Schmidt relation and predictions from state-of-the-art galaxy formation simulations. The dense HI gas appears to also drive the offset from the fundamental-metallicity relation of these galaxies though its dust-to-gas ratio is seemingly consistent with values derived for local galaxies except for the lowest metallicity sight-lines. Our results provide the first robust observational constraints on the impact of pristine HI gas on early galaxy assembly, and imply that a combination of highly efficient star formation and low dust obscuration can likely explain the UV-brightness of galaxies at cosmic dawn.

Figures (12)

• Figure 1: Left: Example of UV emission line and DLA fitting for JADES-GS-z13-0 (DJA ID 3215_20128771) at $z=12.85$. The NIRSpec/Prism spectrum and associated error are shown in green. The marked UV emission lines were modelled and then superimposed on the intrinsic spectrum before modelling the DLA (solid black line). In the inset, we show a zoom on the Ly$\alpha$ region, with the DLA+IGM model as a solid line, and with 100% neutral IGM only ($\mathrm{x_{\rm HI}}=1.0$) as a dashed line. Right: Corner plot of the posterior distributions for the DLA+IGM model, with median, 16th and 84th percentiles marked.
• Figure 2: Calculated column densities across the redshift range of the sample, dark green squares are well-constrained column densities with values $N_{\rm HI}>10^{21}\, \rm cm^{-2}$. Upper limits are plotted as triangles, derived from the 95th percentile of the posterior distribution. The different shaded regions represent low column density ($N_{\rm HI}<10^{21}\, \rm cm^{-2}$) where objects are likely IGM-dominated, high column densities ($N_{\rm HI}>10^{21-22}\,\rm cm^{-2}$) where neutral hydrogen exceeds abundance from a fully neutral IGM, and very strong DLAs ($N_{\rm HI}>10^{22}\, \rm cm^{-2}$) with an extreme over-abundance of neutral gas. Regardless of column density, all objects are optically thick with optical depths at the Lyman limit $\tau > 10^3$, with the highest column densities $N_{\rm HI}=10^{22}\, \mathrm{cm^{-2}}$ corresponding to optical depths $\tau \approx 10^5$. Medians of well-constrained column density binned in redshift ($z<10$, $10<z<12$, $z>12$) are shown as black triangles (with values $N_{\rm HI} = 10^{21.71}, 10^{22.24}$, and $10^{22.37}\, \mathrm{cm^{-2}}$ respectively). These are essentially upper limits, as the fraction of IGM-dominated galaxies/upper limits (which were not included in the median) in each bin is 0.42, 0.50, and 0.29 respectively.
• Figure 3: Measured $\beta_{UV}$ slope and column density $\mathrm{log}(N_{\rm HI}/ \rm cm^{-2})$ for the galaxy sample. The green squares and triangles again represent the well-constrained and upper limits for column density respectively, with the grey shaded region representing possible high nebular continuum where two-photon emission may be masquerading as a DLA Katz25.
• Figure 4: Top: Cumulative histogram of derived column densities for our sample in shades of green; separated for non-DLAs (column densities $<10^{21}\,\mathrm{cm^{-2}}$), and the DLA sample (column densities $>10^{21}\,\mathrm{cm^{-2}}$) for $z=9-10$ and $z>10$. The full sample is shown as grey shaded region, comparing directly to the averages of SERRA-simulated galaxies $z=6-9.5$ in blue Gelli25, showing that while the spread of observed DLAs is broadly consistent, there is a peak of extreme-DLAs ($>10^{22}\,\mathrm{cm^{-2}}$) that are not represented in the averages of simulations, particularly for those $z>10$. Bottom: Similar to top panel, but for a normalised PDF, along with the fraction of galaxies with unconstrained DLAs shown as an IGM limit.
• Figure 5: Measured UV magnitude and column density. Halo masses for our objects are derived from UV magnitudes Mason23. The well-constrained column densities of our sample are consistent with results from SERRA simulations Gelli25. The grey contours represent the average column density of 100 galaxies over random sight-lines, with the 1$\sigma$ error region also shown, representing the scatter across sightlines in simulations.