Discovery of Super-enriched Gas ∼1 Gyr after the Big Bang

Abstract

Abundances of chemical elements in the interstellar and circumgalactic media of high-redshift galaxies offer important constraints on the nucleosynthesis by early generations of stars. Damped Lyα absorbers (DLAs) in spectra of high-redshift background quasars are excellent sites for obtaining robust measurements of element abundances in distant galaxies. Past studies of DLAs at redshifts z > 4 have measured abundances of ≲0.01 solar. Here we report the discovery of a DLA at z = 4.7372 with an exceptionally high degree of chemical enrichment. We estimate the H i column density in this absorber to be log (N H I/cm−2) = 20.48 ± 0.15. Our analysis shows unusually high abundances of carbon and oxygen ([C/H] = 0.88 ± 0.17, [O/H] = 0.71 ± 0.16). Such a high level of enrichment a mere 1.2 Gyr after the Big Bang is surprising because of insufficient time for the required amount of star formation. To our knowledge, this is the first supersolar absorber found at z > 4.5. We find the abundances of Si and Mg to be [Si/H] = −0.56−0.35+0.40 and [Mg/H] = 0.59−0.50+0.27 , confirming the metal-rich nature of this absorber. By contrast, Fe shows a much lower abundance ([Fe/H] = −1.53−0.15+0.15 ). We discuss implications of our results for galactic chemical evolution models. The metallicity of this absorber is higher than that of any other known DLA and is >2 orders of magnitude above predictions of chemical evolution models and the N H I-weighted mean metallicity from previous studies at z > 4.5. The relative abundances (e.g., [O/Fe] = 2.29 ± 0.05, [C/Fe] = 2.46 ± 0.08) are also highly unusual compared to predictions for enrichment by early stars.