Not all protoclusters host evolved galaxies: Evidence for reduced environmental effects in a lower halo mass protocluster at $z = 7.66$

TL;DR

The paper addresses how environmental effects shape galaxy evolution in protoclusters at $z\gtrsim 7$ by exploiting JWST/NIRCam grism data for the $z=7.66$ SMACS-PC-z7p7 core. It combines spectroscopic confirmation of six [OIII] emitters with photometric candidates to measure a dense overdensity ${\delta \sim 200}$ in a compact region and derives a halo mass ${\log_{10}M_{200\rm c}[\mathrm{M_\odot}] = 11.4\pm0.2}$ using simulations to map stellar mass to halo mass; the system is predicted to evolve into a low-mass Fornax-like cluster (${\log_{10}M_{200\rm c,z=0}} = 13.7\pm0.6$). The galaxies in SMACS-PC-z7p7 show uniformly young, star-forming properties with rising SFHs and mostly field-like HI damping, in contrast to more massive $z>7$ protoclusters where environmental effects suppress star formation, suggesting halo-mass dependent environmental impact. This work demonstrates that vigorous star formation can persist in lower-mass protocluster cores, while feedback and virial heating become important at higher halo masses, highlighting the need for larger high-redshift protocluster samples and cosmological simulations to map the evolution of environmental effects across cosmic time.

Abstract

The progenitors of present-day galaxy clusters offer crucial insight into how galaxies and large-scale structure co-evolve in the early Universe. We present JWST/NIRCam grism spectroscopy of the photometrically identified $z=7.66$ protocluster core in the SMACS J0723.3-7327 lensing field, SMACS-PC-z7p7. Six [O III]-emitters and five additional photometric candidates are found within a 0.3 arcmin$^2$ ($1.5\ {\rm cMpc}^2$) region, corresponding to an overdensity of $δ\sim 200$. Despite the extreme overdensity, the resident galaxies exhibit star-formation histories, UV-slopes and neutral hydrogen column densities that are consistent with those of field galaxies at similar redshifts. This is in stark contrast with the consistently high neutral hydrogen column densities, old stellar populations and large dust masses of galaxies within a $z=7.88$ protocluster in the Abell 2744 field. Comparison with the TNG-Cluster and TNG300 simulations indicates a halo mass of ${\rm log_{10}}(M_{200{\rm c}}[{\rm M_{\odot}}]) = 11.4\pm0.2$, and implies that, on average, SMACS-PC-z7p7 will evolve into a present-day Fornax-like cluster (${\rm log_{10}}(M_{200{\rm c},\ z=0}[{\rm M_{\odot}}]) = 13.7\pm0.6$). The uniformly young, highly star-forming nature of the galaxy population of SMACS-PC-z7p7 suggests that environmental effects only become significant above halo masses of ${\rm log_{10}}(M_{200{\rm c}}[{\rm M_{\odot}}]) \gtrsim 11.5$. Comparison to other $z\gtrsim7$ protoclusters reveals that vigorous star formation persists in lower-mass protoclusters, whereas accelerated evolution and suppression of star formation emerge in more massive haloes. SMACS-PC-z7p7 therefore represents an early stage of protocluster assembly, where residence within an overdense environment still enhances star formation, and feedback processes have yet to exert a significant influence.

Figures (12)

• Figure 1: RGB image using the F090W, F115W and F444W filters, showing the protocluster region in the SMACS0723 field. The inset panels zoom on regions that contain spectroscopically confirmed $z=7.66$ galaxies. Within the inset panels ($\sim 5\times 5\ {\rm pkpc^2}$) the names correspond to the galaxies indicated by the markers. The red boxes indicate our two lower confidence photometric candidates. The axes have been corrected for lensing by taking the median magnification, $\mu = 1.43$.
• Figure 2: The rest-frame spectra of [OIII]-emitters identified in the SMACS0723 field at $z=7.66$. Each panel shows a different galaxy, identified by the text in the top left of each panel. The shaded red region around the red solid line shows the error of each spectrum. The dashed line shows the best-fit model spectrum of the H$\rm{\beta}$ , [OIII] $\lambda$4960Å and [OIII] $\lambda$5008Å emission lines. Despite the relative weakness of the emission lines seen from PW3 and PW1, their detection in the 2D spectra in both R and C directions, combined with their consistent photometric redshift, leads us to conclude that they are indeed emission lines at $z=7.66$.
• Figure 3: The UV luminosity function of galaxies within SMACS-PC-z7p7 (red squares), compared to the UVLF of another $z\sim8$ protocluster, A2744-PC-z7p9 witten_before_2025, calculated within the same central volume. The UVLF of other overdense environments from simulations are shown trebitsch_obelisk_2021kulkarni_reionization_2011, that have a similar shallow gradient to that seen in SMACS-PC-z7p7. The grey shaded region represents the $5\sigma$ depth in the UV-continuum. Relative to the nominal UVLF of $z\sim 8$ galaxies from harikane_pure_2024 (black solid line), the UVLF of SMACS-PC-z7p7 appears to be $\sim 300$ times higher (black dashed line).
• Figure 4: The ratio of the observed to expected flux in the F115W filter, where increasing neutral hydrogen column density (colorbar) produces excessive damping of the Lyman-$\alpha$ break, suppressing the observed flux in the F115W filter. The photometry of SMACS-PC-z7p7-resident galaxies are shown by squares, while A2744-PC-z7p9-resident galaxies are identified with circles witten_before_2025. While two galaxies in SMACS-PC-z7p7 show extreme column densities, this is a much smaller fraction of the sample than those with extreme column densities in A2744-PC-z7p9.
• Figure 5: The star-forming main sequence (SFMS) of galaxies in SMACS-PC-z7p7 (squares), A2744-PC-z7p9 witten_before_2025 and MACS0416-OD-z8p5 fudamoto_sapphires_2025, compared to the SFMS of field galaxies measured from observations simmonds_bursting_2025 and simulations mcclymont_thesan-zoom_2025. The SFMS is colored based on its virial mass, estimated using the method discussed in Section \ref{['sec:halo_mass']}. While the most massive galaxies in A2744-PC-z7p9 are below the SFMS, all galaxies in SMACS-PC-z7p7 and MACS0416-OD-z8p5 fall on or above the SFMS of field galaxies.