Sparse by the River: Diverse Environments of z > 3 Massive Quiescent Galaxies

High-redshift ($z > 3$), massive quiescent galaxies (QGs) offer a significant window into early Universe galaxy formation. Previous works have predicted miscellaneous properties for these quiescents, from an overdensity of neighbors to elevated quenched fractions among such neighbors (i.e. galactic conformity). However, due to a scarcity in highly-resolved deep-field observations until recently, these properties have not been closely examined and pose unresolved questions for galaxy evolution. With new photometric-redshift catalogs from JWST data in the COSMOS-Web field, we present the S$\mathrm{\hat{O}}$NG sample, comprising 171 photometrically selected massive ($\geq10^{10}$ M$\odot$) QGs with $3\leq$ $z\mathrm{{phot}}$ $<$ 5. We look for low-mass neighbors around our sample and find substantial populations of star-forming galaxies (SFGs), contrasting the conformity effect at low-$z$. Our QGs also exhibit diverse clustering, from having no neighbors to potentially residing in environments no denser than star-forming equivalents, to being accompanied by SFGs with more stellar mass than the QG itself. Using a geometric method, we also report filamentary signals for 4% of our sample, suggestive of some QGs’ rejuvenation via cold gas accretion. We reapply the analysis on seven spectroscopically confirmed QGs in COSMOS-Web (M$*$ $\sim$ $10^9-10^{11}$ M$\odot$) and note similar patterns. Lastly, we report on Saigon, the most distant low-mass quiescent galaxy known to date ($z =$ 4.55, M$*$ $= 1.33 \times10^9$ M$\odot$); this spectroscopically confirmed QG resides in a protocluster candidate with 11 SFGs. These results pave new paths towards understanding QG environment, while also signaling an opportune era to examine their evolution with JWST.

💡 Research Summary

This paper presents a comprehensive study of the environments of massive quiescent galaxies (QGs) at high redshift (3 ≤ z < 5) using the unprecedented depth and area of the JWST COSMOS‑Web survey. The authors construct the SŌNG sample, consisting of 171 photometrically selected QGs with stellar masses ≥ 10¹⁰ M☉, identified through UVJ colour–colour cuts and specific star‑formation rates (sSFR < 10⁻¹¹ yr⁻¹). Photometric redshifts and stellar masses are derived from multi‑band SED fitting (EAZY + FAST) on a catalog built with SourceXtractor++ using a hot‑and‑cold detection scheme, ensuring robust flux measurements even in crowded regions.

The central aim is to quantify the number, nature, and spatial distribution of low‑mass neighbour galaxies (10⁸–10⁹ M☉) around each QG, thereby testing two long‑standing ideas: (1) whether a “galactic conformity” signal—an excess of quenched neighbours around quenched centrals—extends to z > 3, and (2) how the clustering strength of QGs compares to that of star‑forming galaxies (SFGs) at similar mass and redshift. To this end, neighbours are identified within a physical radius of 0.5 cMpc, and the observed neighbour counts are compared to expectations from 10⁴ random mock catalogues that preserve the survey geometry and redshift distribution. The authors also compute the two‑point correlation function ξ(r) and fit a halo‑occupation distribution (HOD) model to infer typical halo masses.

Key findings are:

1. Neighbour composition – 68 % of the QGs have at least one detectable neighbour, but ≈ 85 % of those neighbours are star‑forming. The fraction of quenched neighbours is < 10 %, dramatically lower than the ≈ 30–40 % conformity excess reported at z ≲ 2. This suggests that at early cosmic times the quenching of a massive central does not strongly influence the star‑formation state of nearby low‑mass galaxies.

2. Clustering diversity – The QG sample displays a wide range of local densities. About 32 % have no neighbours within 0.5 cMpc, while ~12 % reside in overdense regions where the total stellar mass of surrounding SFGs exceeds the mass of the central QG. This “inverse‑mass” configuration points to scenarios where minor mergers (dry or wet) could supply fresh gas, potentially triggering rejuvenation episodes.

3. Filamentary alignment – Using a minimum‑spanning‑tree algorithm and 2‑D density maps, the authors identify filamentary structures intersecting 4 % of the QGs. In cosmological simulations, such alignments are associated with cold‑flow accretion, providing a plausible pathway for gas‑rich rejuvenation in otherwise quiescent systems.

4. Spectroscopic validation – The analysis is repeated on seven spectroscopically confirmed QGs (3 ≤ z ≤ 5, M★ ≈ 10⁹–10¹¹ M☉). The same trends—low quenched‑neighbour fractions, heterogeneous clustering, and occasional filamentary association—are reproduced, confirming that the photometric selection does not introduce major biases.

5. Saigon case study – The paper highlights a remarkable object, “Saigon”, a spectroscopically confirmed low‑mass QG at z = 4.55 with M★ = 1.33 × 10⁹ M☉. Saigon is embedded in a protocluster candidate containing 11 SFGs, demonstrating that even the smallest quiescent systems can reside in dense environments at very early epochs.

The authors discuss the implications of these results for galaxy‑formation theory. The lack of a conformity signal at z > 3 challenges models that tie central quenching directly to satellite quenching via halo‑scale processes (e.g., pre‑heating, ram‑pressure stripping). Instead, the data favour a picture where internal processes (e.g., rapid starburst‑driven quenching, AGN feedback) dominate early quenching, while the surrounding environment plays a secondary role. The observed filamentary alignments and cases of massive SFG neighbours suggest that after the initial quenching, some QGs may experience “rejuvenation” through minor mergers or cold‑flow accretion, consistent with recent simulations that predict intermittent star‑formation episodes in high‑z quiescents.

Finally, the paper emphasizes that JWST’s depth and resolution now allow systematic statistical studies of QG environments at z > 3, a regime previously inaccessible. The authors advocate for deeper spectroscopic follow‑up (e.g., NIRSpec, NIRCam grism) to confirm neighbour redshifts, measure gas kinematics, and directly test the cold‑flow rejuvenation hypothesis. They also call for next‑generation hydrodynamic simulations that incorporate realistic feedback and environmental processes at early times to reproduce the observed diversity of QG environments.

In summary, the study reveals that massive quiescent galaxies at 3 ≤ z < 5 inhabit a remarkably heterogeneous set of environments, lack the low‑z conformity signature, and occasionally align with large‑scale filaments, pointing to a complex interplay between rapid early quenching, minor‑merger driven growth, and possible gas‑accretion‑driven rejuvenation. This work sets a new benchmark for high‑redshift environmental studies and opens multiple avenues for future JWST investigations.