JWST Spectra Reveal Early Chemical Enrichment in Seven-Billion-Year-Old Galaxies

Scientists using the James Webb Space Telescope have examined the chemical composition of eight distant galaxies observed when the universe was less than 500 million years old. The galaxies, selected for their bright ultraviolet emission, were studied with medium-resolution NIRSpec spectra, allowing detection of absorption lines from carbon, oxygen, magnesium, aluminum, silicon and iron.

Five of the galaxies display multiple low-ionization absorption lines, while the remaining three show only one or no significant detections. The weaker lines in the more compact systems (effective radius ≲ 100 pc) that exhibit high Hβ-to-UV luminosity ratios and star-formation-rate surface densities suggest that intense star formation or nuclear activity may reduce the covering fraction of absorbing gas.

By fitting several absorption features simultaneously, the team derived covering fractions and column densities that follow the local relation between these two quantities. The resulting elemental abundance ratios place the five galaxies in the same region of the [Si/O]-[C/O] and [Fe/Si]-[Al/Si] parameter spaces as damped Lyman-α systems, indicating similar gas properties.

Two galaxies at redshift ≈ 7 show iron-to-silicon ratios near the solar value ([Fe/Si] ≈ -0.2). The authors note that such enrichment could arise from early Type Ia supernovae or pair-instability supernovae, but the observed aluminum-to-silicon ratios ([Al/Si] ≈ 0) lack the strong odd-even effect expected from pair-instability events, favoring a Type Ia origin.

"The iron enrichment observed at these early epochs points to a rapid cycle of star formation and supernova explosions," the study states.

The findings demonstrate JWST’s capability to trace the buildup of heavy elements in the first galaxies, providing constraints on the timing and nature of early stellar nucleosynthesis.