# The FENIKS Survey: Spectroscopic Confirmation of Massive Quiescent Galaxies at z ~ 3-5

**Authors:** Jacqueline Antwi-Danso, Casey Papovich, James Esdaile, Themiya Nanayakkara, Karl Glazebrook, Taylor A. Hutchison, Katherine E. Whitaker, Z. Cemile Marsan, Ruben J. Diaz, Danilo Marchesini, Adam Muzzin, Kim-Vy H. Tran, David J. Setton, Yasha Kaushal, Joshua S. Speagle, Justin Cole  
**arXiv:** [2307.09590](https://arxiv.org/abs/2307.09590)  
**Date:** 2023-07-18  
**Summary page:** https://www.summarizepaper.com/en/paper/2307.09590v1/

## Key points

- Discovery of massive galaxies at high redshifts by JWST raises questions about their physical properties and implications for galaxy formation
- Potential systematic uncertainties in observations, including selection techniques, survey volume, and overestimated stellar masses
- Need for large and robust samples of massive galaxies at z > 4 with spectroscopic confirmation of redshifts
- Spectroscopic confirmation of quenched massive galaxies at early epochs provides insights into their progenitors during the epoch of reionization
- Recent study reported spectroscopic redshifts for three massive quiescent galaxy candidates at z ~ 3-5
- Candidates selected from FENIKS survey using deep imaging optimized for increased sensitivity to characteristic red colors associated with strong Balmer/4000 Å breaks in galaxies at z > 3
- Colors consistent with stellar populations aged between 1-2 billion years old challenge prevailing notion that all quiescent galaxies at z > 3 are recently-quenched "post-starburst" galaxies
- One candidate identified as a broad-line AGN with strong, redshifted Hβ+[O III] emission indicative of a powerful outflow; highlights diverse nature of objects classified as quiescent galaxies
- Progenitor already in place by z ~ 7-11 and reached mass approximately 10^11 solar masses by z ≈ 10; valuable insights into formation and evolution during early universe's structure and dynamics.

## Summary

The discovery of massive galaxies at high redshifts (z ∼ 10) by the James Webb Space Telescope (JWST) has raised questions about the accuracy of their derived physical properties and their implications for our understanding of galaxy formation. While these findings challenge predictions from the ΛCDM model, it is important to consider potential systematic uncertainties in our observations. One area of concern is the selection techniques used to identify these massive galaxies. Issues with photometric selection techniques, effective survey volume, and overestimated stellar masses may contribute to uncertainties in our understanding of these objects. To address these concerns, there is a pressing need for large and robust samples of massive galaxies at z > 4 with precise redshifts confirmed through spectroscopy. In particular, spectroscopic confirmation of already quenched massive galaxies at early epochs provides valuable insights into their probable progenitors. These progenitors were likely the most massive galaxies during the epoch of reionization. By studying quiescent galaxies at high redshifts, researchers can gain a better understanding of the processes that led to their formation and subsequent quenching. To contribute to this research effort, a recent study reported spectroscopic redshifts for three massive quiescent galaxy candidates at z ~ 3-5. One candidate was securely identified at z = 3.757, while two others were tentatively identified at z = 3.336 and z = 4.673. These candidates were selected from the FENIKS survey which utilized deep imaging optimized for increased sensitivity to characteristic red colors associated with strong Balmer/4000 Å breaks in galaxies at z > 3. The rest-frame UVJ and (ugi)_s colors of three out of four quiescent candidates were consistent with stellar populations aged between 1-2 billion years old; challenging the prevailing notion that all quiescent galaxies at z > 3 are recently-quenched "post-starburst" galaxies instead suggesting some may have formed earlier and remained quiescent for an extended period. Interestingly, spectroscopic analysis revealed that one of the quiescent galaxy candidates was actually a broad-line active galactic nucleus (AGN) with strong, redshifted Hβ+[O III] emission indicative of a powerful outflow; highlighting the diverse nature of objects identified as quiescent galaxies and emphasizing the importance of spectroscopic confirmation to accurately classify them. Furthermore, star-formation history analysis suggests that its progenitor was already in place by z ~ 7-11 and reached a mass approximately 10^11 solar masses by z ≈ 10; providing valuable insights into formation and evolution during early universe's structure and dynamics.