First Light And Reionisation Epoch Simulations (FLARES) XIV: The Balmer/4000~\AA\ Breaks of Distant Galaxies

TL;DR

This study uses simulations and synthetic observations to analyze the Balmer/4000 Å break in high-redshift galaxies, assessing its diagnostic power for galaxy properties with implications for JWST observations.

Contribution

It explores the origins of the Balmer/4000 Å break and predicts its behavior in distant galaxies using FLARES simulations, enhancing understanding of galaxy evolution diagnostics.

Findings

Break strength weakly correlates with stellar mass and UV luminosity, mainly due to dust attenuation.

The Balmer/4000 Å break is a weak age indicator but better constrains star formation and stellar mass.

Simulations predict observable trends of the break in galaxies at 5<z<10.

Abstract

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. Amongst the most useful spectral diagnostics used in the optical is the Balmer/4000~\AA\ 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~\AA\ break using the SYNTHESIZER synthetic observations package. We then make predictions of the Balmer/4000~\AA\ break using the First Light and Reionisation Epoch Simulations (FLARES) at $5<z<10$. We find that the average break strength weakly correlates with stellar mass and rest-frame far-UV 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 UV and optical luminosity, respectively.