The Bigfoot: A footprint of a Coma cluster progenitor at z=3.98

Abstract

Protoclusters, galaxy clusters' high redshift progenitors, hold the keys to understanding the formation and evolution of clusters and their member galaxies. However, their cosmological distances and spatial extensions (tens of Mpc) have inhibited complete mapping of their structure and constituent galaxies, which is key to robustly linking protoclusters to their descendants. Here we report the discovery of the Bigfoot, a tridimensional structure at $z = 3.98$ including 11 subgroups traced by 55 (700) spectroscopic (photometric) redshifts with JWST, extending over $15\times 37$ $\times 49{\rm{cMpc^3}}$ in the PRIMER-UDS field. Bigfoot's large-scale and mass function of member galaxies closely match constrained simulations' predictions for the progenitors of today's most massive clusters (${M_0} > 10^{15} {M_{_ \odot }}$). All subgroups with ${M_{\rm{h}}} > {10^{12.5}}{M_{_ \odot }}$ exhibit enhanced fractions of massive galaxies ($>{10^{10.0} {M_{_ \odot }}}$) compared to lower-mass halos and the field, demonstrating the accelerated formation of massive galaxies in massive halos. The presence of this massive protocluster with a large central halo (${10^{13.0} {M_{_ \odot }}}$) in a JWST deep field bears important cosmological implication that favors high ${σ_8}$ of PLANCK cosmology over low-redshift probes.

Figures (7)

• Figure 1: The weighted number overdensity map of PCL0217--"the Bigfoot". The upper left panel shows the distribution of the weighted number overdensity of galaxies with $3.73 < {z_{{\rm{phot}}}} < 4.23$ in PRIMER-UDS. The red open circles show the position of the 8 confirmed subgroups observed by the JWST/PRIMER survey Dunlop2021, the pink circle shows a previously reported overdensity of quiescent galaxies Tanaka2024 covered by the JWST/BEACON survey Morishita2025. The upper right panel shows the 3-dimensional distribution of the 11 subgroups. An animated version of this figure is available at https://box.nju.edu.cn/f/d6e29ce33a704cd29439/?dl=1. The lower figures show the enlarged overdensity map of the eight confirmed subgroups covered by the PRIMER survey. The purple and red crosses show the location of massive (${M_{\rm{*}}} > {10^{9.5}}{M_{_ \odot }}$) SFGs and quiescent galaxies (QGs). The overdensities shown in the upper and lower panels are measured in circular areas with radius 1' and 15", respectively.
• Figure 2: Example of Spectroscopically confirmed members of the Bigfoot. The upper panel presents the redshift distribution of 55 spectroscopically confirmed galaxies. As examples, the lower panels show the JWST spectroscopy of four confirmed member galaxies in 0217A, their best-fit SED from Bagpipes, and the $4"\times4"$ RGB cutouts from the JWST/NIRCam images.
• Figure 3: The total SMF of the Bigfoot compared with massive clusters with ${M_0} \sim {10^{15}}{M_ \odot }$. The left figure shows that the Bigfoot has a comparable number of galaxies and occupied volume to simulated progenitors of Coma-type massive galaxy clusters, where the red line shows the total SMF of the 9 subgroups of PCL0217 at ${z_{{\rm{spec}}}} = 3.97 - 4.00$ subtracted by a predicted contribution from the field SMF. The dashed lines show the total SMF of the progenitors ($z = 4.04$) of the 7 most massive galaxy clusters (${M_0} \sim {10^{15}}{M_ \odot }$) from the ELUCID simulation WangHY2014. The right panel compares the total SMF of the Bigfoot with the observed mass functions of red galaxies in local clusters from DESI and SDSS Meng2023.
• Figure 4: Stellar mass functions with massive galaxies excess in the Bigfoot. In the first 11 panels, the blue lines show the SMFs of SFGs in each subgroup of the Bigfoot, while the green dotted lines show the SMF of field galaxies at $3.5<z<4.5$WangT2025, which is based on the same catalog as this work and has been re-normalized to match the SMFs of the Bigfoot. All of the 4 massive subgroups show a top-heavy SMF of SFGs compared with the re-normalized field SMF and the less-massive subgroups, suggesting the accelerated formation of massive galaxies in massive overdensities. The last panel shows the total SMF of massive subgroups with $\log ({M_{\rm{h}}}/{M_ \odot }) > 12.5$, the blue line and the red line show the distribution of SFGs and QGs, respectively. The orange line shows the total SMF, which is the sum of the blue line and red line. The QGs, including both passive galaxies and post-starburst galaxies, are classified using an updated UVJ criterion Cutler2024. Due to the limited number of QGs in each subgroup, we can only plot the total SMF of QGs instead of the SMF of QGs in each subgroup.
• Figure 5: The stellar mass and number density profile of 0217A: the left panel shows the projected stellar mass density profile, while the right panel shows the projected number density profile. The surface overdensities of 0217A are shown as the red lines, which have been subtracted by the field level (estimated as the density of field galaxies at $3.73<z<4.23$ in PRIMER-COSMOS). For comparison, the orange dash-dotted lines show the density profile of cluster J1001 at $z = 2.51$Sun2024, the purple dashed lines show the result of the GCLASS clusters at $z \sim 1$vanderBurg2014, and the blue dotted lines show the results of local clusters at $z \sim 0.15$vanderBurg2015. All of these archival results have been corrected to the density down to ${M_{\rm{*}}} = {10^{8.5}}{M_{_ \odot }}$ using their total SMF.