Characterising Ly$\alpha$ 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 observations to analyze the neutral hydrogen gas and star formation in galaxies at redshifts greater than 9, revealing highly efficient star formation driven by dense gas during the early universe's reionization.

Contribution

It provides the first observational constraints on how pristine HI gas influences early galaxy assembly and star formation efficiency at cosmic dawn.

Findings

Galaxies exhibit star formation on rapid depletion timescales of 10-100 Myr.

Dense HI gas drives the offset from the fundamental-metallicity relation.

Galaxies are highly efficient at forming stars, exceeding local universe relations.

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-$\alpha$ (Ly$\alpha$) 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.