# COSMOS-Web: Intrinsically Luminous z$\gtrsim$10 Galaxy Candidates Test Early Stellar Mass Assembly

**Authors:** Caitlin M. Casey, Hollis B. Akins, Marko Shuntov, Olivier Ilbert, Louise Paquereau, Maximilien Franco, Christopher C. Hayward, Steven L. Finkelstein, Michael Boylan-Kolchin, Brant E. Robertson, Natalie Allen, Malte Brinch, Olivia R. Cooper, Xuheng Ding, Nicole E. Drakos, Andreas L. Faisst, Seiji Fujimoto, Steven Gillman, Santosh Harish, Michaela Hirschmann  
**arXiv:** [2308.10932](https://arxiv.org/abs/2308.10932)  
**Date:** 2023-08-21  
**Summary page:** https://www.summarizepaper.com/en/paper/2308.10932v1/

## Key points

- Discovery of 15 exceptionally luminous candidate galaxies at redshifts between 10 and 14
- Identified in the first 0.28 deg$^2$ of JWST/NIRCam imaging from the COSMOS-Web Survey
- Rest-frame UV magnitudes ranging from -20.5 to -22, making them the most intrinsically luminous candidates at redshifts above 10 detected by JWST so far
- NIRCam imaging with Hubble ACS/F814W and deep ground-based observations used to constrain photometric redshifts
- Analysis of spectral energy distributions using multiple open-source codes to evaluate low-redshift solutions
- 12 out of 15 candidates (80%) likely genuine sources at redshifts above 10, while 3 out of 15 (20%) likely contaminants at lower redshifts
- Three candidates at a redshift around 12 with estimated stellar masses around $5\times10^{9}\,M_\odot$, implying an effective stellar baryon fraction ($\epsilon_{\star}$) in the range of 0.2 to 0.5
- Suggests these galaxies reside in halos of comparable mass and may be forming stars rapidly on timescales shorter than 100 million years within $\Lambda$CDM cosmology framework
- Similar volume densities inferred for galaxies with stellar masses around $10^{10}\,M_\odot$ and $10^{9}\,M_\odot$, both approximately $10^{-6}$ Mpc$^{-3}$
- Supports the idea that starbursts occur frequently with a duty cycle close to unity at high redshifts, explaining the change in shape from double power law to Schechter function for UV luminosity function around redshift 8
- Spectroscopic redshift confirmation and further constraints on masses crucial for understanding how these early massive galaxies push the limits of galaxy formation within $\Lambda$CDM cosmology framework

## Summary

In this study, the authors report the discovery of 15 exceptionally luminous candidate galaxies at redshifts between 10 and 14. These galaxies were identified in the first 0.28 deg$^2$ of JWST/NIRCam imaging from the COSMOS-Web Survey. The sources have rest-frame UV magnitudes ranging from -20.5 to -22, making them the most intrinsically luminous candidates at redshifts above 10 detected by JWST so far. The identification of these galaxies was done using NIRCam imaging with Hubble ACS/F814W and deep ground-based observations that helped constrain their photometric redshifts. To evaluate the probability of low-redshift solutions, the authors analyzed the spectral energy distributions of these galaxies using multiple open-source codes. Based on their analysis, they concluded that 12 out of 15 candidates (80%) are likely genuine sources at redshifts above 10 while 3 out of 15 (20%) are likely contaminants at lower redshifts. Of particular interest are three candidates at a redshift around 12 which push the limits of early stellar mass assembly; they have estimated stellar masses around $5\times10^{9}\,M_\odot$, implying an effective stellar baryon fraction ($\epsilon_{\star}$) in the range of 0.2 to 0.5. This suggests that these galaxies reside in halos of comparable mass and may be forming stars rapidly on timescales shorter than 100 million years where star formation rate may outpace halo growth rate within $\Lambda$CDM cosmology framework. The similar volume densities inferred for galaxies with stellar masses around $10^{10}\,M_\odot$ and $10^{9}\,M_\odot$, both approximately $10^{-6}$ Mpc$^{-3}$, suggest that these galaxies reside in halos of comparable mass; this finding supports the idea that at such high redshifts, starbursts occur frequently with a duty cycle close to unity which could explain why there is a change in shape from double power law to Schechter function for UV luminosity function at around redshift 8. The authors emphasize that spectroscopic redshift confirmation and further constraints on their masses will be crucial for understanding how these early massive galaxies push the limits of galaxy formation within $\Lambda$CDM cosmology framework.