# Unveiling the distant Universe: Characterizing $z\ge9$ Galaxies in the first epoch of COSMOS-Web

**Authors:** Maximilien Franco, Hollis B. Akins, Caitlin M. Casey, Steven L. Finkelstein, Marko Shuntov, Katherine Chworowsky, Andreas L. Faisst, Seiji Fujimoto, Olivier Ilbert, Anton M. Koekemoer, Daizhong Liu, Christopher C. Lovell, Claudia Maraston, Henry Joy McCracken, Jed McKinney, Brant E. Robertson, Micaela B. Bagley, Jaclyn B. Champagne, Olivia R. Cooper, Xuheng Ding  
**arXiv:** [2308.00751](https://arxiv.org/abs/2308.00751)  
**Date:** 2023-08-01  
**Summary page:** https://www.summarizepaper.com/en/paper/2308.00751v1/

## Key points

- 15 galaxy candidates identified at redshifts greater than or equal to 9 ($z\ge9$)
- Initial observations conducted using COSMOS-Web JWST over an area of 77 square arcminutes
- Observations conducted through four NIRCam filters (F115W, F150W, F277W, F444W) and partial overlap with MIRI in the F770W filter over an area of 8.7 square arcminutes
- Several publicly-available SED fitting and photometric redshift codes used for analysis
- Derived redshifts range from $z=9.3$ to $z=10.9$, with a mean redshift of $\langle z\rangle=10.0$
- UV-magnitudes range between M$_{\rm UV}=-21.2$ and M$_{\rm UV}=-19.5$, with a mean magnitude of $\langle $M$_{\rm UV}\rangle=-20.2$
- Mean rest-frame UV slope value is $\langle \beta\rangle=-2.4$
- Galaxies are more luminous on average compared to most $z\ge9$ candidates discovered by JWST in previous studies
- Rest-frame UV colors suggest that the underlying stellar population is not yet fully enriched in metals, consistent with early stages of galaxy formation models
- Stellar masses range from $\log_{\rm 10}($M$_\star/$M$_\odot)\approx8-9$, consistent with standard $\Lambda$CDM model
- Volume density aligns well with previously reported values at redshifts around $z\sim9-10$
- Compact galaxies but significantly resolved in observations, providing insights into their structure and formation processes
- Findings contribute to understanding early galaxy formation and provide constraints for theoretical models

## Summary

In this study, the authors report the identification of 15 galaxy candidates at redshifts greater than or equal to 9 ($z\ge9$) using initial observations from the COSMOS-Web JWST (James Webb Space Telescope) over an area of 77 square arcminutes. The observations were conducted through four NIRCam filters (F115W, F150W, F277W, F444W), with a partial overlap with MIRI (Mid-Infrared Instrument) in the F770W filter over an area of 8.7 square arcminutes. The authors applied several publicly-available SED (spectral energy distribution) fitting and photometric redshift codes to analyze the sample and determine their redshifts. The derived redshifts range from $z=9.3$ to $z=10.9$, with a mean redshift of $\langle z\rangle=10.0$. The UV-magnitudes of these galaxies range between M$_{\rm UV}=-21.2$ and M$_{\rm UV}=-19.5$, with a mean magnitude of $\langle $M$_{\rm UV}\rangle=-20.2$. The rest-frame UV slopes, as determined from SED-fitting, have a mean value of $\langle \beta\rangle=-2.4$. Interestingly, these galaxies are found to be more luminous on average compared to most $z\ge9$ candidates discovered by JWST in previous studies. However, they exhibit similar blue colors in their rest-frame UV as those found in other studies at similar redshifts; this suggests that the underlying stellar population in these galaxies is not yet fully enriched in metals which is consistent with models predicting the early stages of galaxy formation. The derived stellar masses for these galaxies range from $\log_{\rm 10}($M$_\star/$M$_\odot)\approx8-9$, which are not in tension with the standard $\Lambda$CDM (Lambda Cold Dark Matter) model describing the evolution of the universe. Additionally, the authors measured the volume density of these UV luminous galaxies and found that it aligns well with previously reported values in literature at redshifts around $z\sim9-10$. Furthermore, despite being compact these galaxies are significantly resolved in the observations; this indicates that they have a non-negligible physical extent providing valuable insights into their structure and formation processes. Overall, this study presents new insights into distant universe by characterizing a sample of high-redshift ($z\ge9$) galaxies using JWST observations; findings contribute to our understanding of early galaxy formation and provide constraints for theoretical models.