Strong Balmer break objects at z ~ 7-10 uncovered with JWST

TL;DR

This study uses JWST data to identify and spectroscopically confirm a significant number of high-redshift galaxies with Balmer breaks, revealing diverse stellar populations and star formation histories in the early universe.

Contribution

It provides the first large spectroscopic sample of Balmer break galaxies at z ~ 7-10, demonstrating photometry's effectiveness in identifying evolved stellar populations at high redshift.

Findings

Confirmed 10-11 new Balmer break galaxies at z >7

Approximately 10-20% of high-z galaxies show evolved stellar populations

Detected diverse star formation histories, including bursty activity

Abstract

We report the discovery of robust spectroscopically confirmed Balmer break (BB) galaxies and candidates, with secure spectroscopic redshifts $7.1 \le z \le 9.6$ from publicly available JWST extra-galactic photometric and spectroscopic surveys. To do so, we used dedicated filters probing the Balmer break and inspected the objects with NIRSpec spectroscopy. We recover the previously known objects with strong Balmer breaks and reveal 10-11 new objects with clear BBs, thus tripling the number of spectroscopically confirmed galaxies with a BB at z >7. Approximately half of them show a pure BB and no signs of recent star formation, whereas the other half shows BB and emission lines, indicating most likely galaxies whose star formation ceased earlier and has restarted recently. Overall we find that ~10-20% of all galaxies from our sample show signatures of an evolved stellar population. Furthermore, we find that the strength of the BB does not significantly depend on the rest-UV and rest-optical brightness of these sources. In short, our work confirms that photometry alone has the potential to measure BB strengths and to identify evolved stellar populations at high redshift and that such objects may be more frequent than previously thought. The presence of galaxies with a range of break strengths and the joint presence of BB and emission lines indicate a bursty nature of the star formation in the early Universe.