The JWST EXCELS survey: tracing the chemical enrichment pathways of high-redshift star-forming galaxies with O, Ar and Ne abundances

We present an analysis of nine star-forming galaxies with ⟨z⟩=3.95 from the JWST EXCELS survey for which we obtain robust chemical abundance estimates for the α-elements O, Ne and Ar. The α-elements are primarily produced via core-collapse supernovae (CCSNe) which should result in α-element abundance ratios that do not vary significantly across cosmic time. However, Type Ia supernovae (SNe Ia) models predict an excess production of Ar relative to O and Ne. The Ar/O abundance ratio can therefore be used as a tracer of the relative enrichment of CCSNe and SNe Ia in galaxies. Our sample approximately doubles the number of sources with measurements of Ar/O at z>2, and we find that our sample exhibits sub-solar Ar/O ratios on average, with Ar/O=0.62±0.10(Ar/O)⊙. In contrast, the average Ne/O abundance is fully consistent with the solar ratio, with Ne/O=1.07±0.12(Ne/O)⊙. Our results support a scenario in which Ar has not had time to build up in the interstellar medium of young high-redshift galaxies, which are dominated by CCSNe enrichment. We show that these abundance estimates are in good agreement with recent Milky Way chemical evolution models, and with Ar/O trends observed for planetary nebulae in the Andromeda galaxy. These results highlight the potential for using multiple element abundance ratios to constrain the chemical enrichment pathways of early galaxies with JWST