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

TL;DR

This work leverages JWST COSMOS-Web imaging to study the environments of $z>3$ massive quiescent galaxies by constructing the SONG sample of 171 QGs and identifying 2,048 low-mass neighbors. Through multi-step photometric redshift and SED analyses, the authors assess clustering, conformity, and filamentary signals, finding that high-$z$ QGs are strongly clustered relative to the field but not more than mass-matched star-forming galaxies, and that only a small fraction lie in filamentary or protocluster-like structures. The study also expands to seven spectroscopically confirmed QGs, revealing four protocluster systems including the distant Saigon at $z=4.55$, a case of a low-mass QG in an overdense, active neighborhood; these results point to environmental diversity and complex quenching pathways in the early Universe. Overall, the findings demonstrate that high-$z$ QGs can inhabit varied environments, with limited evidence for strong conformity at $z>3$, highlighting the nuanced role of environment in early massive galaxy evolution and the power of JWST in probing these regimes.

Abstract

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.

Figures (8)

• Figure 1: (a) RGB image of S$\mathrm{\hat{O}}$NG-141, one QG at $z =$ 3.63 (circled green at the center). The bands chosen for R, G, and B are NIRCam F444W, F277W, and F150W, respectively. All neighbors within 15$"$ are circled white, their names abbreviated as numbers on their right. (b) $\chi^2$ detection images of S$\mathrm{\hat{O}}$NG-141 and its neighbors, constructed and reduced from all four NIRCam bands (F115W, F150W, F277W, and F444W). The image contrast has been manually adjusted for visibility. Each galaxy's AB magnitude in the reddest band available is annotated on the top left corner of its corresponding cutout.
• Figure 2: SFR vs. $\mathrm{M_*}$ for the S$\mathrm{\hat{O}}$NG QGs (red dots) and their neighbors (green density contours). The contour levels are iso-proportions, representing the fraction of neighbors outside each contour line; for example, a contour level of 0.05 means that 5% of the neighbors are outside that line. The red straight line denotes our quenching threshold ($\mathrm{log_{10}(sSFR)}$$=-9.8$ yr$^{-1}$); the purple and lavender straight lines denote the Rinaldi2022 parameterized best fits for starburst (SB) and main-sequence (MS) galaxies at $2.8 \leq z<4$. For all the straight lines, 1$\sigma$ noise is added as shading. The top and right histograms show the $\mathrm{M_*}$ and SFR distributions.
• Figure 3: NUV-r-J diagram for neighbors of the S$\mathrm{\hat{O}}$NG sample. The purple line delineates the QG region from Ilbert2013. The lavender dots denote the neighbors that pass both this threshold and our sSFR criterion.
• Figure 4: Neighbor count (also called clustering signal or strength) versus annulus for the S$\mathrm{\hat{O}}$NG sample (green), compared to other samples. From left to right, top to bottom: S$\mathrm{\hat{O}}$NG differentiated by $\mathrm{M_*}$ (left plot, top row) and sSFR (right plot, top row); 100 randomly-selected SFGs differentiated by $\mathrm{M_*}$ and 100 random field pointings in COSMOS-Web (left plot, middle row); 100 randomly-selected SFGs and field pointings from COSMOS-Web (right plot, middle row); 100 random QGs from DREaM differentiated by $\mathrm{M_*}$ (left plot, bottom row) and sSFR (right plot, bottom row). For all the COSMOS data (QGs, mass-matched SFGs, and field pointings), the error bars are the 16$^\mathrm{th}$ and 84$^\mathrm{th}$ percentiles of the counts. The top x-axis represents the annuli converted into kpc at $z\sim3.5$, based on concordant $\Lambda$CDM cosmology assumptions from $\S$\ref{['sec:intro']}. All samples are analyzed with the same annuli, but their graphs are shifted left by steps of 0.5$"$ for visual clarity, except S$\mathrm{\hat{O}}$NG which is unshifted.
• Figure 5: Temperature maps of the three exceptional cases ($q_{P95}$$\geq$ 2) in \ref{['subsec:filament']}, showing potential filamentary structures. The large, central crimson hexagon represents the QG, and the smaller ivory hexagons represent its neighbors. Purple represents density regions. Each QG's $z\mathrm{_{phot}}$ is indicated at the bottom left corner of its respective panel.