JWST spectra measure oxygen, carbon and nitrogen in early galaxies

A set of nine distant star-forming galaxies has yielded unusually direct measurements of its chemical composition, after astronomers detected a faint temperature-sensitive emission line in every object. The results come from ultra-deep observations made with the James Webb Space Telescope’s Near-Infrared Spectrograph, or NIRSpec, in the GOODS-North field.

The study, reported by astrobiology.com, is based on galaxies at redshifts between 6.0 and 8.3. They were selected from the Origins of Metal Enrichment in Galaxies at cosmic dAwn, known as the OMEGA survey. The researchers combined observations from two NIRSpec instrument configurations to cover wavelengths corresponding to both ultraviolet and optical light in the galaxies’ own rest frames.

Their analysis is presented as a pilot study and has been submitted to the Monthly Notices of the Royal Astronomical Society. The paper was posted to arXiv on 28 September 2026 by Raunaq Singh Rai and colleagues.

A direct route to oxygen abundance

The key observation was the detection of the auroral [O III] λ4363 line in all nine galaxies. This weak feature can be used to determine the temperature of the ionised gas. Once those temperatures are established, the researchers can calculate oxygen abundances directly rather than relying mainly on relationships between stronger emission lines.

For the sample, the resulting values of 12 + log(O/H) range from 7.14 to 8.18. Oxygen abundance is used here as a measure of the amount of heavy-element enrichment in the galaxies. The direct method matters because strong-line calibrations can introduce the largest systematic uncertainties in studies at these redshifts, according to the researchers.

The observations were especially valuable because they provided continuous coverage from the rest-frame ultraviolet through the optical. That range contains the emission features needed not only for oxygen measurements but also for examining the physical conditions and relative abundances of other elements.

Carbon and nitrogen offer a more detailed picture

With the oxygen measurements in hand, the team compared them with carbon-to-oxygen and nitrogen-to-oxygen ratios. These comparisons are intended to show how the chemical mix in very early galaxies relates to their overall oxygen enrichment.

The galaxies follow the trend seen in nearby systems when carbon-to-oxygen ratio is compared with oxygen abundance. The researchers describe this result as robust across their sample, indicating that the relationship between those two quantities is already evident in galaxies at redshifts from 6.0 to 8.3.

The nitrogen results are less straightforward. The inferred nitrogen-to-oxygen ratio changes depending on which nitrogen indicator is used. The relevant measurements come from ultraviolet N IV] or N III] lines, or from the optical [N II] line. The study therefore highlights the importance of the diagnostic chosen when interpreting nitrogen in galaxies from this era.

This distinction is significant for comparisons between observations: different wavelength ranges may not produce interchangeable estimates of the same abundance ratio. The result does not remove the value of ultraviolet measurements, but shows that conclusions about nitrogen need to account for the particular emission line being used.

From a pilot sample to a wider survey

The observations demonstrate what ultra-deep NIRSpec spectroscopy can achieve when ultraviolet and optical diagnostics are combined for individual high-redshift galaxies. Detecting [O III] λ4363 in every member of this small sample allowed the researchers to determine electron temperatures and obtain oxygen abundances without making strong-line calibrations the foundation of the analysis.

The authors present the work as a demonstration rather than a final description of the wider population. Their next requirement is a statistical sample of galaxies at redshifts of 6 and above. A larger set would be needed to establish how representative these nine systems are and to map the range of nebular properties across early star-forming galaxies.

For now, the study provides a detailed chemical view of a small group of galaxies during cosmic dawn. It also shows how combining rest-frame ultraviolet and optical spectroscopy can reveal both the broad enrichment level, through oxygen, and the more specific behaviour of carbon and nitrogen relative to it.