Early JWST studies reporting an unexpected abundance of massive galaxies at z ∼ 5–8 challenge galaxy formation models in the ΛCDM framework. Previous stellar mass ( M _⋆ ) estimates suffered from large uncertainties due to the lack of rest-frame near-infrared data. Using deep JWST/NIRCam and Mid-Infrared Instrument (MIRI) photometry from Public Release IMaging for Extragalactic Research, we systematically analyze massive galaxies at z ∼ 3–8, leveraging rest-frame ≳1 μ m constraints. We find MIRI is critical for robust M _⋆ measurements for massive galaxies at z > 5: excluding MIRI overestimates M _⋆ by ∼0.4 dex on average for M _⋆ > 10 ^10 M _⊙ galaxies, with no significant effects at lower masses. This reduces number densities of M _⋆ > 10 ^10 M _⊙ (10 ^10.3 M _⊙ ) galaxies by ∼36% (55%). MIRI inclusion also reduces “Little Red Dot” (LRD) contamination in massive galaxy samples, lowering the LRD fraction from ∼32% to ∼13% at M _⋆ > 10 ^10.3 M _⊙ . Assuming pure stellar origins, LRDs exhibit M _⋆ ∼ 10 ^9–10.5 M _⊙ with MIRI constraints, rarely exceeding 10 ^10.5 M _⊙ . Within standard ΛCDM, our results indicate a moderate increase in the baryon-to-star conversion efficiency ( ϵ ) toward higher redshifts and masses at z > 3. For the most massive z ∼ 8 galaxies, ϵ ∼ 0.3, compared to ϵ ≲ 0.2 for typical galaxies at z < 3. This result is consistent with models where high gas densities and short free-fall times suppress stellar feedback in massive high- z halos.

Clusters and their progenitors (protoclusters) at z ∼ 2 − 4, the peak epoch of star formation, are ideal laboratories to study the formation process of both the clusters themselves and their member galaxies. However, a complete census of their member galaxies has been challenging due to observational difficulties. Here we present new JWST/NIRCam observations targeting the distant cluster CLJ1001 at z = 2.51 from the COSMOS-Web program, which, in combination with previous narrowband imaging targeting H α emitters and deep millimeter surveys of CO emitters, provide a complete view of massive galaxy assembly in CLJ1001. In particular, JWST reveals a population of massive, extremely red cluster members in the long-wavelength bands that were invisible in previous Hubble Space Telescope (HST)/F160W imaging (HST-dark members). Based on this highly complete spectroscopic sample of member galaxies, we show that the spatial distribution of galaxies in CLJ1001 exhibits a strong central concentration, with the central galaxy density already resembling that of low- z clusters. Moreover, we reveal a “top-heavy” stellar mass function for the star-forming galaxies (SFGs), with an overabundance of massive SFGs piled up in the cluster core. These features strongly suggest that CLJ1001 is caught in a rapid transition, with many of its massive SFGs likely soon becoming quiescent. In the context of cluster formation, these findings suggest that the earliest clusters form from the inside out and top to bottom, with the massive galaxies in the core assembling first, followed by the less massive ones in the outskirts.

The NOEMA formIng Cluster survEy (NICE) is a large program targeting 69 massive galaxy group candidates at z>2 in six deep fields. We report spectroscopic confirmation of eight groups at 1.65 <= z <= 3.61 in COSMOS. Homogeneously selected as significant overdensities of red IRAC sources with red Herschel colors, four groups are confirmed by CO and [CI] with NOEMA 3mm observations, three are confirmed with ALMA, and one is confirmed by H alpha from Subaru/FMOS. We constructed the integrated FIR SEDs for the eight groups, obtaining total IR SFR =260-1300 M-circle dot similar to yr(-1). We adopted six methods to estimate the dark matter masses, including stellar mass to halo mass relations, overdensity with galaxy bias, and NFW profile fitting to radial stellar mass density. We found the radial stellar mass density are consistent with a NFW profile, supporting that they are collapsed structures hosted by a single dark matter halo. The best halo mass estimates are log(M-h/M-circle dot)=12.8-13.7 with uncertainty of 0.3 dex. From halo mass estimates, we derive baryonic accretion rate BAR=(1-8)x10(3) M-circle dot/yr for this sample. We find a quasi-linear correlation between the integrated SFR/BAR and the theoretical halo mass limit for cold streams, M-stream/M-h, with SFR/BAR=10(-0.46 +/- 0.22)(M-stream/M-h)(0.71 +/- 0.16) with a scatter of 0.40dex. Further, we compare halo masses and stellar masses with simulations, and find all structures are consistent with being progenitors of M-h(z=0) > 10(14) M-circle dot galaxy clusters, and the most massive central galaxies have stellar masses consistent with brightest cluster galaxies (BCGs) progenitors in the TNG300 simulation. The results strongly suggest these structures are forming massive galaxy clusters via baryonic and dark matter accretion.

The study of distant galaxy groups and clusters at the peak epoch of star formation is limited by the lack of a statistically and homogeneously selected and spectroscopically confirmed sample. Recent discoveries of concentrated starburst activities in cluster cores have opened a new window to hunt for these structures based on their integrated IR luminosities. Here, we carry out a large NOEMA (NOrthern Extended Millimeter Array) program targeting a statistical sample of infrared-luminous sources associated with overdensities of massive galaxies at z > 2, the Noema formIng Cluster survEy (NICE). We present the first result from the ongoing NICE survey, a compact group at z = 3.95 in the Lockman Hole field (LH-SBC3), confirmed via four massive (M⋆ ≳ 1010.5 M⊙) galaxies detected in the CO(4–3) and [CI](1–0) lines. The four CO-detected members of LH-SBC3 are distributed over a 180 kpc physical scale and the entire structure has an estimated halo mass of ∼1013 M⊙ and total star formation rate of ∼4000 M⊙ yr−1. In addition, the most massive galaxy hosts a radio-loud active galactic nucleus with L1.4 GHz, rest = 3.0 × 1025 W Hz−1. The discovery of LH-SBC3 demonstrates the feasibility of our method to efficiently identify high-z compact groups or cluster cores undergoing formation. The existence of these starbursting cluster cores up to z ∼ 4 provides critical insights into the mass assembly history of the central massive galaxies in clusters.

Structural properties of cluster galaxies during their peak formation epoch, z ∼ 2–4 provide key information on whether and how the environment affects galaxy formation and evolution. Based on deep Hubble Space Telescope (HST)/Wide Field Camera 3 (WFC3) imaging toward the z = 2.51 cluster, J1001, we explore environmental effects on the structure, color gradients, and stellar populations of a statistical sample of cluster star-forming galaxies (SFGs). We find that the cluster SFGs are on average smaller than their field counterparts. This difference is most pronounced at the high-mass end ( M ⋆ > 10 10.5 M ⊙ ), with nearly all of them lying below the mass–size relation of field galaxies. The high-mass cluster SFGs are also generally old, with a steep negative color gradient, indicating an early formation time likely associated with strong dissipative collapse. For low-mass cluster SFGs, we unveil a population of compact galaxies with steep positive color gradients that are not seen in the field. This suggests that the low-mass compact cluster SFGs may have already experienced strong environmental effects, e.g., tidal/ram pressure stripping, in this young cluster. These results provide evidence on the environmental effects at work in the earliest formed clusters with different roles in the formation of low- and high-mass galaxies.