Resolving Ambiguities in the Inferred Star Formation Histories of Intense [O III] Emitters in the Reionisation Era

Understand

Early JWST spectroscopic campaigns have confirmed the presence of strong [O III] line-emitting galaxies in the redshift interval $7<z<9$.

• Although deduced earlier from Spitzer photometry as indicative of young stellar populations, some studies suggested the relevant photometric excesses attributed to [O III] emission could, in part, be due to Balmer breaks arising from older stars.
• We demonstrate that this is likely the case by exploiting medium-band near-infrared JWST photometry in the Hubble Ultra Deep Field.
• We locate a sample of 6 galaxies with redshifts 8.2$<z<$8.6 for which the relevant medium-band filters enables us to separate the contributions of [O III] emission and a Balmer break, thereby breaking earlier degeneracies of interpretation.