# CEERS Key Paper VII: JWST/MIRI Reveals a Faint Population of Galaxies at Cosmic Noon Unseen by Spitzer

**Authors:** Allison Kirkpatrick, Guang Yang, Aurelien Le Bail, Greg Troiani, Eric F. Bell, Nikko J. Cleri, David Elbaz, Steven L. Finkelstein, Nimish P. Hathi, Michaela Hirschmann, Benne W. Holwerda, Dale D. Kocevski, Ray A. Lucas, Jed McKinney, Casey Papovich, Pablo G. Perez-Gonzalez, Alexander de la Vega, Micaela B. Bagley, Emanuele Daddi, Mark Dickinson  
**arXiv:** [2308.09750](https://arxiv.org/abs/2308.09750)  
**Date:** 2023-08-18  
**Summary page:** https://www.summarizepaper.com/en/paper/2308.09750v1/

## Key points

- CEERS program conducted observations of the Extended Groth Strip using MIRI on JWST in 2022
- Four MIRI pointings utilized longer wavelength filters (F770W, F1000W, F1280W, F1500W, F1800W, and F2100W)
- MIRI is capable of observing significantly deeper than MIPS with shorter integration times due to larger aperture and improved sensitivity
- MIRI can detect faint galaxies ($L_{\rm IR}<10^{10} L_\odot$) at redshifts around $z\sim1-2$
- Some MIRI galaxies exhibit "mid-IR weakness" with strong near-IR emission but relatively weaker mid-IR emission
- New infrared templates were developed to understand how mid-IR to near-IR emission changes with increasing infrared luminosity
- Two color-color diagrams were presented to distinguish between different types of galaxies
- There are fewer than ten IR AGN per MIRI pointing, potentially due to challenges in distinguishing AGN from mid-IR weak galaxies
- MIRI is effective at observing moderate luminosity galaxies ($L_{\rm IR}=10^9 - 10^{10}L_\odot$) at redshifts $z=1 - 2$
- Photometry alone is not sufficient for identifying AGN within this faint population

## Summary

The Cosmic Evolution Early Release Science (CEERS) program conducted observations of the Extended Groth Strip using the Mid-Infrared Instrument (MIRI) on the James Webb Space Telescope (JWST) in 2022. This paper focuses on four MIRI pointings that utilized longer wavelength filters, including F770W, F1000W, F1280W, F1500W, F1800W, and F2100W. The main objective was to compare the MIRI galaxies with the Spitzer/MIPS 24μm population in the EGS field. The study found that MIRI is capable of observing significantly deeper than MIPS with much shorter integration times. This enhanced capability can be attributed to JWST's larger aperture and MIRI's improved sensitivity. Particularly noteworthy is MIRI's ability to detect faint galaxies ($L_{\rm IR}<10^{10} L_\odot$) at redshifts around $z\sim1-2$. One interesting finding is that a significant portion of MIRI galaxies exhibit "mid-IR weakness," characterized by strong near-IR emission but relatively weaker mid-IR emission. Additionally, most of the star formation in these galaxies appears to be unobscured. To better understand this phenomenon, new infrared templates were developed to capture how mid-IR to near-IR emission changes with increasing infrared luminosity. To distinguish between different types of galaxies, two color-color diagrams were presented. These diagrams proved most effective when used together to separate mid-IR weak galaxies and active galactic nuclei (AGN) from dusty star-forming galaxies. Furthermore, this study provides the first number counts of 10μm sources and reveals that there are fewer than ten IR AGN per MIRI pointing. This scarcity may be due to challenges in distinguishing AGN from intrinsically mid-IR weak galaxies, potentially caused by low metallicities or low dust content. In conclusion, this study demonstrates that MIRI is particularly effective at observing moderate luminosity galaxies ($L_{\rm IR}=10^9 - 10^{10}L_\odot$) at redshifts $z=1 - 2$. However, it is important to note that photometry alone is not sufficient for identifying AGN within this faint population. The findings of this study contribute to our understanding of galaxy evolution and provide insight into the capabilities of JWST's MIRI instrument.