First Light and Reionization Epoch Simulations (FLARES) – XIV. The Balmer/4000 Å breaks of distant galaxies

Stephen M. Wilkins, Christopher C. Lovell, Dimitrios Irodotou, Aswin P. Vijayan, Anton Vikaeus, Erik Zackrisson, Joseph Caruana, E. R. Stanway, Christopher J. Conselice, Louise T. C. Seeyave, William J. Roper, Katherine Chworowsky, Steven L. Finkelstein

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With the successful launch and commissioning of JWST we are now able to routinely spectroscopically probe the rest-frame optical emission of galaxies at z > 6 for the first time. Among the most useful spectral diagnostics used in the optical is the Balmer/4000 Å break; this is, in principle, a diagnostic of the mean ages of composite stellar populations. However, the Balmer break is also sensitive to the shape of the star formation history, the stellar (and gas) metallicity, the presence of nebular continuum emission, and dust attenuation. In this work, we explore the origin of the Balmer/4000 Å break using the synthesizer synthetic observations package. We then make predictions of the Balmer/4000 Å break using the First Light and Reionization Epoch Simulations at 5 < z < 10. We find that the average break strength weakly correlates with stellar mass and rest-frame far-ultraviolet luminosity, but that this is predominantly driven by dust attenuation. We also find that break strength provides a weak diagnostic of the age but performs better as a means to constrain star formation and stellar mass, alongside the ultraviolet and optical luminosity, respectively.

Original languageEnglish
Pages (from-to)7965-7973
Number of pages9
JournalMonthly Notices of the Royal Astronomical Society
Issue number3
Early online date6 Dec 2023
Publication statusPublished - 1 Jan 2024


  • galaxies: evolution
  • galaxies: formation
  • galaxies: high-redshift
  • infrared: galaxies
  • methods: numerical
  • UKRI
  • STFC
  • ST/K00042X/1
  • ST/P002293/1
  • ST/R002371/1
  • ST/S002502/1
  • ST/R000832/1
  • ST/X001040/1

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