To be, or not to be: Balmer breaks in high-z galaxies with JWST

TL;DR

This study measures the Balmer break in high-redshift galaxies using JWST data to evaluate the accuracy of current galaxy formation models, finding general agreement but some potential discrepancies in outlier frequencies.

Contribution

First measurement of Balmer break strengths in galaxies at $z=6$ to $12$ using JWST, comparing observations with simulations to test star formation models in the early universe.

Findings

Observed Balmer break strengths align with simulations within uncertainties.

No extremely strong Balmer breaks detected, consistent with models.

Potential excess of outliers suggests areas for model refinement.

Abstract

Standard models of structure formation allow us to predict the cosmic timescales relevant for the onset of star formation and the assembly history of galaxies at high redshifts ($z > 10$). The strength of the Balmer break represents a well-known diagnostic of the age and star formation history of galaxies, which enables us to compare observations with contemporary simulations - thus shedding light on the predictive power of our current models of star formation in the early universe. Here, we measure the Balmer break strength for 23 spectroscopically confirmed galaxies at redshifts $6 \lesssim z \lesssim 12$ using public JWST NIRSpec data from the cycle 1 GO 1433 and GO 2282 programs (PI Coe), as well as public spectroscopic data from the JWST Deep Extragalactic Survey (JADES). We find that the range of observed Balmer break strengths agree well with that of current simulations given our measurement uncertainties. No cases of anomalously strong Balmer breaks are detected, and therefore no severe departures from the predictions of contemporary models of star formation. However, there are indications that the number of outliers in the observed distribution, both in direction of strong and weak Balmer breaks, is higher than that predicted by simulations.