The influence of environment on galaxy properties is well-established in the local universe, where massive, quiescent galaxies tend to reside in dense structures like galaxy cluster cores. Studying high-redshift massive structures helps understand their origin. Protoclusters, the progenitors of local clusters (log(Mhalo/M⊙) > 14), have been identified up to z ∼ 7-8, primarily through overdensities of star-forming galaxies. Recent surveys have revealed quiescent galaxies at high redshifts (z up to 4.01, with candidates at z > 4 detected by JWST), demonstrating suppressed star formation activity. However, the link between this quenching and environment remains unclear. While some studies have found individual quiescent galaxies in star-forming protoclusters or high quiescent fractions in protoclusters using both photometric and spectroscopic redshifts, no protocluster at z > 2.5 had been identified solely by the distribution of massive quiescent galaxies. This lack of knowledge hinders our understanding of the relationship between galaxy quenching and environment. Detecting overdense regions of quiescent galaxies is challenging due to their low number density compared to star-forming galaxies. The COSMOS survey, with its deep multi-band photometry, provides a solution to this issue.

Previous research has extensively explored protoclusters, identifying them through overdensities of star-forming galaxies characterized by UV continuum emission, Lyα, Hα, and infrared emission. These studies have revealed the existence of these structures at various redshifts, offering insights into the early stages of galaxy cluster formation. However, the understanding of the role of quiescent galaxies in protoclusters remained limited. Existing work on high-redshift quiescent galaxies showed their suppressed star formation activity even at high redshifts. Although some studies have touched upon the presence of quiescent galaxies in protoclusters, a comprehensive analysis of protoclusters based solely on the distribution of massive quiescent galaxies has been lacking. This research addresses this gap by focusing on the distribution of quiescent galaxies to uncover a protocluster.

This study searched for overdense structures of quiescent galaxies at z ∼ 3 in the COSMOS field (∼2 deg²) using the COSMOS2020 Classic catalog. Quiescent galaxies were selected based on three criteria: (1) a Kₛ < 24.8 mag cut to minimize depth inhomogeneity effects; (2) log(sSFR₁σ,upper/yr⁻¹) < -9.5, indicating suppressed star formation; and (3) log(M*/M⊙) > 10.3, ensuring completeness in the stellar mass regime. The overdensity map was constructed using Gaussian kernel density estimation with a bandwidth of 3 arcmin. Regions near bright stars and survey edges were masked. A significant overdense region (4.2σ) was found at 2.74 < zphot < 2.94, spanning ∼7 × 4 pMpc². This structure contained 14 quiescent galaxies, all with log(M*/M⊙) > 10.5. Keck/MOSFIRE H-band spectroscopy targeted 9 galaxies within this region. The data were reduced using MOSFIRE DRP, and redshifts were determined using SLINEFIT, fitting stellar continuum templates with a Gaussian velocity profile. Redshift probability distributions were used to assess the robustness of the spectroscopic redshifts. The number density of quiescent galaxies was compared between QO-1000 and the general COSMOS field, and the color-magnitude diagram was examined for the members of QO-1000 to check the red sequence. The halo mass of QO-1000 was estimated using two methods: converting the stellar mass of the most massive galaxy to its host halo mass using stellar-to-halo mass ratios, and converting the total stellar mass of spectroscopically confirmed members to the host halo mass using relations from z∼1 clusters. The IllustrisTNG simulation was used to find analogous structures and study their evolution.

The study identified QO-1000, an overdense structure containing 14 photometrically selected quiescent galaxies with log(M*/M⊙) > 10.5 in a 7×4 pMpc² area at 2.74 < zphot < 2.94. Keck/MOSFIRE spectroscopic observations confirmed four quiescent galaxies within a narrow redshift range (2.76 < z < 2.79). Three of these galaxies are remarkably close, located within 1 × 1 pMpc². These four galaxies are all massive (log(M*/M⊙) > 11.0) and have low specific star formation rates (-10.6 < log(sSFR/yr⁻¹) < -9.7), confirming their quiescent nature. QO-1000's number density of quiescent galaxies is at least 68 times higher than the general field, demonstrating its exceptional overdensity. The structure exhibits a clear red sequence in the color-magnitude diagram (J-Kₛ vs. Kₛ), reinforcing its concentration of quiescent galaxies. The estimated halo mass of QO-1000 is log(Mhalo/M⊙) > 13.2. Comparison with the IllustrisTNG simulation shows that similar structures at z = 2.73 are expected to evolve into massive galaxy clusters (log(Mhalo/M⊙) ≥ 14.8) by z = 0. This suggests QO-1000 is a mature protocluster, possibly representing a transition phase from star-forming to quenched galaxy clusters.

The discovery of QO-1000 provides strong evidence for the existence of protoclusters dominated by quiescent galaxies at z ∼ 3. The high overdensity, red sequence, and high quiescent fraction strongly support its protocluster nature. The massive stellar masses of the quiescent galaxies suggest a history of intense star formation followed by rapid quenching. This finding contrasts with previously known protoclusters dominated by star-forming galaxies, highlighting a more mature and evolved stage of protocluster development. The comparison with IllustrisTNG simulations strengthens the interpretation of QO-1000 as a mature protocluster on its evolutionary path towards a massive galaxy cluster. The structure's high density of quiescent galaxies implies efficient quenching mechanisms operating within the protocluster environment. Further investigations are needed to fully understand these mechanisms.

This study presents the discovery of QO-1000, a protocluster of massive quiescent galaxies at z = 2.77, demonstrating the existence of overdense regions of quenched galaxies at high redshift. The spectroscopic confirmation of multiple massive, quiescent galaxies and the comparison with simulations reveals a more evolved stage in protocluster formation than previously observed. Future research will focus on detailed analysis of the star formation history, morphology, and dynamical mass of QO-1000's member galaxies.

The study's limitations primarily stem from the relatively small sample size of spectroscopically confirmed galaxies, which could potentially underestimate the true density of the structure. The reliance on photometric redshifts for a large fraction of the galaxy sample introduces uncertainties that could influence the assessment of the overdensity. The chosen selection criteria for quiescent galaxies may inadvertently exclude some galaxies and limit the generalizability of the results.