# Quiescent or dusty? Unveiling the nature of extremely red galaxies at $z>3$

**Authors:** L. Barrufet, P. Oesch, R. Marques-Chaves, K. Arellano-Cordova, J. F. W. Baggen, A. C. Carnall, F. Cullen, J. S. Dunlop, R. Gottumukkala, Y. Fudamoto, G. D. Illingworth, D. Magee, R. J. McLure, D. J. McLeod, M. J. Michałowski, M. Stefanon, P. G. van Dokkum, A. Weibel  
**arXiv:** [2404.08052](https://arxiv.org/abs/2404.08052)  
**Date:** 2024-04-11  
**Summary page:** https://www.summarizepaper.com/en/paper/2404.08052v1/

## Key points

- The James Webb Space Telescope (JWST) has revolutionized our understanding of high-redshift galaxies
- The NIRCam instrument on JWST discovered a population of previously undetectable Hubble Space Telescope (HST)-dark galaxies in optical wavelengths
- A recent study analyzed 23 HST-dark galaxies with $\mathrm{H-F444W>1.75}$ using low-resolution NIRSpec/PRISM spectra
- Results showed these galaxies have a mean spectroscopic redshift of $\mathrm{z_{spec} = 4.1 \pm 0.7}$, all lying at $\mathrm{z_{spec} \geq 3}$
- Dusty HST-dark galaxies have a dust attenuation from the Balmer decrement of $\mathrm{A_V = 3.1 \pm 1.2}$, higher than values obtained from continuum measurements ($\mathrm{A_V = 1.9 \pm 0.8}$)
- Dusty galaxies were found to be H$\alpha$ emitters with equivalent widths ranging from $\mathrm{68Å < EW_{H\alpha} < 550Å}$
- Approximately $\mathrm{13\%}$ of the sample comprised quiescent galaxies at $\mathrm{z_{spec} > 3.5}$
- Morphological analysis revealed size variations correlating with spectral types: extremely red galaxies were more extended while quiescent galaxies tended to be compact
- Around $\mathrm{85\%}$ of the HST-dark galaxies had stellar masses exceeding $\mathrm{log(M_{*}/M_{\odot}) > 9.8}$
- Using NIRSpec/PRISM spectroscopy is crucial for distinguishing between dusty and quiescent galaxies and accurately quantifying their physical properties
- Future studies utilizing higher-resolution NIRSpec data in conjunction with ALMA observations are expected to enhance our understanding further

## Summary

The advent of the James Webb Space Telescope (JWST) has revolutionized our understanding of high-redshift galaxies. The use of the NIRCam instrument on-board JWST has led to the discovery of a population of previously undetectable Hubble Space Telescope (HST)-dark galaxies in optical wavelengths. These galaxies may have been obscured by dust or were quiescent or at extreme redshifts. In a recent study analyzing 23 HST-dark galaxies with $\mathrm{H-F444W>1.75}$, low-resolution NIRSpec/PRISM spectra were used to unveil their nature and physical properties. The results showed that these HST-dark galaxies have a mean spectroscopic redshift of $\mathrm{z_{spec} = 4.1 \pm 0.7}$, with all galaxies in the sample lying at $\mathrm{z_{spec} \geq 3}$. Most of these galaxies are dusty, with a dust attenuation from the Balmer decrement of $\mathrm{A_V = 3.1 \pm 1.2}$, which is higher than values obtained from continuum measurements ($\mathrm{A_V = 1.9 \pm 0.8}$). Dusty HST-dark galaxies were found to be H$\alpha$ emitters with equivalent widths ranging from $\mathrm{68Å < EW_{H\alpha} < 550Å}$, indicating varying levels of recent star formation activity. Interestingly, hidden among these dusty galaxies were quiescent galaxies at $\mathrm{z_{spec} > 3.5}$, comprising approximately $\mathrm{13\%}$ of the sample. Morphological analysis revealed size variations correlating with spectral types: extremely red galaxies were more extended while quiescent galaxies tended to be compact. The study also highlighted that HST-dark galaxies are predominantly massive, with around $\mathrm{85\%}$ having stellar masses exceeding $\mathrm{log(M_{*}/M_{\odot}) > 9.8}$. This research demonstrates that HST-dark galaxies represent a diverse population and emphasizes the importance of using NIRSpec/PRISM spectroscopy for distinguishing between dusty and quiescent galaxies and accurately quantifying their physical properties. Future studies utilizing higher-resolution NIRSpec data in conjunction with ALMA observations are expected to further enhance our understanding of these enigmatic sources at high redshifts.