The Balmer break — a drop in a galaxy's light caused by many older stars — is a key clue to how old a galaxy's stars are and how it formed. Using deep images from the James Webb Space Telescope (JWST), researchers measured how the strength of this break changes from redshift about 3.5 to 10. They find that the typical break grows stronger with time, rising from a flux ratio of about 1.1 in the earliest epochs to about 1.5 by the era known as cosmic noon. The change is mostly driven by the increasing average age of stars in galaxies.

The team measured the break from broadband photometry taken with JWST’s NIRCam instrument. They used data from several public programs (CEERS, JADES, FRESCO, and PRIMER) and selected galaxies in redshift windows that avoid strong emission lines. To translate the colors into physical quantities they fitted galaxy models with the SED-fitting code CIGALE and also ran mock galaxy simulations to test the measurements.

The starting photometric catalog contained about 351,000 non-stellar sources. After requiring well-constrained photometric redshifts and good model fits, the final sample used in the study included 16,881 objects with redshifts above about 2.8. The authors report that their median Balmer-break trend agrees with recent spectroscopic estimates in the literature, lending support to the photometric method.

In addition to the gradual increase in the median break, the study finds rare objects with very large Balmer breaks (flux ratio > 3) at redshifts 3.5–4 and again at 7–10. At z≈3.5–4 these large breaks likely reflect strong dust extinction combined with an older stellar population and include examples of so-called Little Red Dots (LRDs) — compact, very red objects now seen up to z≈10. At the highest redshifts the extreme breaks appear mainly in LRDs. When the authors interpret their results under two simple star-formation histories (constant star formation versus an instantaneous burst), the average stellar age shrinks with redshift: from about 350 million years (50 million years for the burst case) at z≈3.5 down to about 20 million years (10 million years for the burst case) at z≈10.