2023MNRAS.523.3018L

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 an 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 six 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. The technique is particularly valuable since it provides photometric redshifts whose precision, $\Delta \, z\simeq \, \pm 0.08$, approaches that of spectroscopic campaigns now underway with JWST. Although some sources are young, a third of our sample have prominent Balmer breaks consistent with stellar ages of ≥150 Myr. Our results indicate that even intense [O III] emitters experienced episodes of earlier star formation to z ∼10 and beyond, as is now being independently deduced from direct detection of the progenitors of similar systems.