# EPOCHS IX. When cosmic dawn breaks: Evidence for evolved stellar populations in $7 < z < 12$ galaxies from PEARLS GTO and public NIRCam imaging

**Authors:** James A. A. Trussler, Christopher J. Conselice, Nathan Adams, Duncan Austin, Leonardo Ferreira, Tom Harvey, Qiong Li, Aswin P. Vijayan, Stephen M. Wilkins, Rogier A. Windhorst, Rachana Bhatawdekar, Cheng Cheng, Dan Coe, Seth H. Cohen, Simon P. Driver, Brenda Frye, Norman A. Grogin, Nimish Hathi, Rolf A. Jansen, Anton Koekemoer  
**arXiv:** [2308.09665](https://arxiv.org/abs/2308.09665)  
**Date:** 2023-08-18  
**Summary page:** https://www.summarizepaper.com/en/paper/2308.09665v1/

## Key points

- Study focuses on evolved stars in high-redshift galaxies and their implications for understanding star formation in the Universe
- Combination of PEARLS GTO and public NIRCam data used to search for Balmer break candidate galaxies at redshifts between 7 and 12
- Balmer break candidates at redshift around 10.5 tend to be older, with an age of approximately 115 million years
- These galaxies exhibit lower [O III] + Hβ emission line equivalent widths, indicating less intense star formation activity
- They also have lower specific star formation rates and redder UV slopes compared to a control sample of galaxies
- Trends become less pronounced at redshift around 8, where F444W filter allows probing strong rest-frame optical emission lines
- Weaker trends likely reflect bursty nature of Epoch of Reionisation galaxies, leading to disconnect between current star formation activity and spectral energy distribution profiles
- Factors contributing to photometric excess seen in rest-frame optical region discussed: strong emission lines, cumulative effects of weak emission lines, dusty continua, active galactic nuclei (AGN)
- Additional medium-band imaging deemed essential for more accurate identification of Balmer break galaxies
- Balmer break informative about galaxy properties but not definitive proxy for stellar age due to complex dependence on star formation history
- Deep NIRSpec continuum spectroscopy and MIRI imaging expected to provide stronger indirect constraints on formation era of first galaxies in the Universe

## Summary

The study focuses on the presence of evolved stars in high-redshift galaxies and their implications for understanding the onset of star formation in the Universe. The authors utilize a combination of PEARLS GTO (Guaranteed Time Observations) and public NIRCam (Near-Infrared Camera) photometric data to search for Balmer break candidate galaxies at redshifts between 7 and 12. The researchers find that their Balmer break candidates at a redshift of around 10.5 tend to be older, with an age of approximately 115 million years. These galaxies also exhibit lower inferred [O III] + Hβ emission line equivalent widths, indicating less intense star formation activity. Additionally, they have lower specific star formation rates and redder UV slopes compared to a control sample of galaxies. However, these trends become less pronounced at a redshift of around 8, where the F444W filter allows for probing strong rest-frame optical emission lines. This provides additional insights into the current star formation activity of these galaxies. The weaker trends observed at this redshift likely reflect the bursty nature of Epoch of Reionisation galaxies, which can lead to a disconnect between their current star formation activity and spectral energy distribution profiles. The study discusses various factors that can contribute to the photometric excess seen in the rest-frame optical region, mimicking the signature of a Balmer break. These factors include strong emission lines, cumulative effects of weak emission lines, dusty continua, and active galactic nuclei (AGN). To more accurately identify Balmer break galaxies, additional medium-band imaging is deemed essential. While the Balmer break is informative about galaxy properties, it cannot serve as a definitive proxy for stellar age due to its complex dependence on star formation history. Ultimately, deep NIRSpec continuum spectroscopy and MIRI (Mid-Infrared Instrument) imaging are expected to provide stronger indirect constraints on the formation era of the first galaxies in the Universe, shedding light on when cosmic dawn breaks.