Ancient Galaxies Formed Stars at Unexpectedly High Rates Just after Cosmic Dawn

Researchers are currently investigating an unexpectedly large number of UV-bright galaxies observed at high redshifts. Clara L. Pollock, Kasper E. Heintz, and Joris Witstok, working with colleagues from the Cosmic Dawn Center (DAWN), Denmark and Niels Bohr Institute, University of Copenhagen, Denmark, DTU Space, Technical University of Denmark, Institute for Computational Cosmology, Department of Physics, Durham University, UK, Canadian Institute for Theoretical Astrophysics, University of Toronto, Canada, David A. Dunlap Department of Astronomy and Astrophysics, University of Toronto, Canada, Department of Physics, University of Toronto, Canada, and California Institute of Technology, USA, present an analysis of galaxies spectroscopically observed by the JWST/NIRSpec Prism at redshifts greater than nine. By modelling the damped Lyman-alpha absorption features, the team places observational constraints on the inflow of hydrogen gas during the epoch of reionisation. Their findings demonstrate that these galaxies exhibit remarkably efficient star formation on very short timescales, exceeding expectations based on established local universe relationships and current galaxy formation simulations, suggesting that dense neutral gas plays a crucial role in driving their intense UV luminosity.

Scientists have uncovered evidence of unexpectedly rapid star formation in a population of distant galaxies, challenging current models of galactic assembly. These galaxies, observed at redshifts greater than nine, are forming stars on timescales of only 10-100 million years, a rate significantly faster than predicted by existing simulations.
This accelerated star formation appears to be fuelled by abundant, pristine neutral atomic hydrogen gas, offering a crucial insight into the conditions of the early universe. The research provides the first robust observational constraints on the impact of this pristine gas on early galaxy assembly, potentially explaining the unexpectedly high luminosity of galaxies at cosmic dawn.

Observations made with the James Webb Space Telescope’s NIRSpec Prism have enabled detailed spectroscopic analysis of these galaxies, allowing researchers to model the absorption features of neutral hydrogen. By carefully examining the damped Lyman-α absorption, a signature of neutral hydrogen, they have been able to constrain the amount of gas surrounding these early galaxies.

The derived hydrogen column densities, combined with measurements of star-formation rates, reveal an extraordinary efficiency in star formation. This efficiency greatly exceeds that observed in the local universe and deviates from predictions made by state-of-the-art galaxy formation simulations. The abundance of dense neutral gas also appears to influence the galaxies’ position relative to the fundamental-metallicity relation, a key indicator of galactic chemical evolution.

While the dust-to-gas ratio seems consistent with local galaxies, the lowest metallicity sight-lines suggest a unique composition in these early systems. This combination of efficient star formation and low dust obscuration provides a compelling explanation for the observed UV-brightness of galaxies at the very beginning of cosmic history. The findings represent a significant step forward in understanding how the earliest galaxies assembled and evolved, offering a glimpse into the conditions that prevailed during the epoch of reionisation.

High-redshift galaxy spectra from JWST enable detailed emission line analysis

JWST/NIRSpec Prism spectroscopy underpins this work, providing detailed observations of galaxies at high redshift. Crucially, the study prioritised sources exhibiting a median signal-to-noise ratio exceeding 3 in the rest-frame UV region, around 1500 Å, ensuring robust spectral modelling. A convolution factor of 1.3 was applied to account for improvements in resolution relative to pre-launch predictions.

These lines were superimposed on a power-law continuum, enabling accurate modelling of the underlying stellar emission. For galaxies with detectable rest-frame optical emission, additional lines such as Hβ, [O iii]λλ4959, 5007, Hγ, [O iii]λ4363, [Ne iii]λ3869, and [O ii]λλ3727, 3729 were also modelled.

The [O iii] doublet was constrained with a fixed ratio of 1:2.97, based on theoretical calculations, and the unresolved [O ii] doublet was treated as a single Gaussian. A first-order polynomial was used to model the underlying continuum in the optical regime. The selection of these specific emission lines and the detailed modelling approach were chosen to maximise the precision of redshift measurements and to accurately characterise the physical conditions within these distant galaxies.

Rapid Star Formation and Gas-Rich Discs in Early Galaxies

Galaxies at high redshift exhibit star formation on remarkably rapid timescales of approximately 10, 100 million years. This depletion timescale is substantially faster than predicted by current galaxy formation simulations and deviates significantly from the canonical Kennicutt-Schmidt relation observed in the local universe.

These observations reveal an unexpectedly efficient conversion of gas into stars in these early galactic systems. Detailed spectroscopic analysis reveals high neutral hydrogen (H i) column densities, exceeding 10 22cm -2 , detected through damped Lyman-α absorption features. This abundance of neutral gas is not easily explained by models focusing solely on the intergalactic medium, strongly suggesting that the observed H i is directly associated with the galaxies themselves.

The presence of such dense gas appears to drive an offset from the fundamental-metallicity relation, indicating a unique chemical enrichment pathway in these early galaxies. Furthermore, the dust-to-gas ratio within these galaxies remains consistent with values observed in local galaxies, except for the lowest metallicity sight-lines.

This suggests that despite the intense star formation, dust production has not yet reached levels comparable to more mature galaxies. The combination of efficient star formation and limited dust obscuration likely contributes to the exceptional UV-brightness observed in these galaxies at cosmic dawn. The research provides the first robust observational constraints on the impact of pristine H i gas on early galaxy assembly.

These findings imply that the rapid star formation is fuelled by abundant, pristine hydrogen gas, challenging existing models and offering new insights into the assembly of the earliest galaxies in the universe. The observed efficiency in converting gas into stars, coupled with the relatively low dust content, presents a compelling scenario for understanding the luminous nature of these distant objects.

The Bigger Picture

Scientists peering into the early universe have long struggled to reconcile theory with observation when it comes to the first galaxies. These nascent cosmic structures appear to be forming stars at a rate that defies existing models, a puzzle that has vexed astronomers for years. Now, observations from the James Webb Space Telescope are beginning to offer a compelling explanation: an abundance of pristine hydrogen gas is fueling this unexpectedly rapid star formation.

What makes this work notable is not just the detection of this gas, but the sheer efficiency with which these early galaxies are converting it into stars. The timescales involved, just tens of millions of years, are dramatically shorter than predicted, suggesting that the conditions in the early universe were far more conducive to star birth than previously thought.

This challenges our understanding of how galaxies assembled themselves from the primordial soup of the cosmos. The implications extend beyond simply refining galaxy formation models. Understanding the processes at play in these distant galaxies provides a window into the universe’s formative years, shedding light on the origins of the elements and the eventual emergence of structures like our own Milky Way.

While this research offers a significant step forward, questions remain about the precise mechanisms driving this efficiency and whether these galaxies represent typical examples or a rare, extreme population. Future observations, particularly those focusing on a wider range of galaxies and utilising different wavelengths, will be crucial to building a more complete picture. The search for the universe’s first galaxies is far from over, but this work suggests we are finally beginning to grasp the conditions that allowed them to ignite.