# The true number density of massive galaxies in the early Universe revealed by JWST/MIRI

**Authors:** Tao Wang, Hanwen Sun, Luwenjia Zhou, Ke Xu, Cheng Cheng, Zhaozhou Li, Yangyao Chen, H. J. Mo, Avishai Dekel, Xianzhong Zheng, Zheng Cai, Tiacheng Yang, Y. -S. Dai, David Elbaz, J. -S. Huang  
**arXiv:** [2403.02399](https://arxiv.org/abs/2403.02399)  
**Date:** 2024-03-04  
**Summary page:** https://www.summarizepaper.com/en/paper/2403.02399v1/

## Key points

- A pressing challenge in galaxy formation is the early assembly of massive galaxies
- Recent observations show that the number density and maximum stellar mass of these galaxies in the early Universe exceed model predictions
- Previous studies have been limited by uncertainty in estimating stellar mass due to insufficient constraints in the near-infrared segment of the spectral energy distribution
- Data from a JWST/MIRI survey within the PRIMER program was used to analyze massive galaxies at redshifts around z ~ 3-8, with findings revealing a decrease in number and mass densities at redshifts beyond 5 compared to earlier results
- The outcomes suggest a need for an increase in baryon-to-star conversion efficiency towards higher redshifts and larger stellar masses within the standard ΛCDM cosmology framework
- For the most massive galaxies at approximately z ~ 8, an efficiency value of around 0.3 is required, contrasting with lower values observed in low-redshift galaxies
- Results align with models suggesting suppressed stellar feedback due to high gas density and short free-fall times expected for massive halos at elevated redshifts

## Summary

In the realm of galaxy formation, a pressing challenge has arisen concerning the early assembly of massive galaxies. Recent observations have revealed that the number density and maximum stellar mass ($M_{\star}$) of these colossal entities in the early Universe surpass model predictions. This poses a significant conundrum for the LCDM cosmology. A key limitation in previous studies has been the substantial uncertainty in estimating $M_{\star}$ due to insufficient constraints in the rest-frame near-infrared segment of the spectral energy distribution. This is crucial for precise determination of $M_{\star}$. To comprehensively address this issue, data from an extensive JWST/MIRI survey within the PRIMER program was utilized to conduct a systematic analysis of massive galaxies at redshifts around $z \sim 3-8$, leveraging photometric constraints at rest-frame wavelengths exceeding 1 $\mu$m. The findings stemming from this rigorous investigation unveil a notable decrease in both number and mass densities of massive galaxies at redshifts beyond 5 when compared to earlier results that did not incorporate MIRI photometry. Within the framework of the standard $\Lambda$CDM cosmology, these outcomes necessitate a moderate augmentation in baryon-to-star conversion efficiency ($\epsilon$) towards higher redshifts and larger $M_{\star}$. Notably, for the most massive galaxies at approximately $z \sim 8$, an $\epsilon$ value of around 0.3 is required. This contrasts with $\epsilon \approx 0.14$, which is typically observed in low-redshift galaxies. The implications drawn from these results align with models postulating suppressed stellar feedback attributable to high gas density and correspondingly short free-fall times anticipated for massive halos at elevated redshifts. This refined analysis sheds light on critical aspects influencing galaxy formation dynamics and underscores the intricate interplay between various factors shaping the evolution of massive galaxies in the early Universe.