Before its time: a remarkably evolved protocluster core at z=7.88

Abstract

Protoclusters represent the most extreme environments in the very early Universe. They form from large-scale dark matter overdensities, harbouring an overabundance of galaxies fed by large gas reservoirs. Their early and accelerated evolution results in a distinct difference in the properties of galaxies resident in protoclusters versus the field, which is known to be in place by $z\sim 5-6$. We utilise JWST NIRCam observations of the A2744-z7p9OD protocluster at $z=7.88$ to constrain the properties of resident galaxies. We identify seven new protocluster members, bringing the total number to 23 and the total stellar mass of the protocluster to in excess of $10^{10}\ \rm{M_{\odot}}$. These galaxies are remarkably evolved just 650 Myr after the Big Bang, preferentially showing redder UV-slopes and stronger Balmer breaks than is typical of field galaxies. We use the PROSPECTOR spectral energy distribution fitting code to derive key galaxy properties, finding distinct populations in the core versus the outskirts of the protocluster. The core is largely composed of dusty, massive galaxies which can be characterised as undergoing a synchronised lulling phase, while galaxies in the protocluster outskirts are undergoing recent bursts of star formation. Finally, a strong suppression of the continuum around the Ly$α$-break evidences extreme neutral hydrogen column densities in many resident galaxies ($N_{\rm HI}\gtrsim10^{22.5}\ {\rm cm^{-2}}$). The A2744-z7p9OD system is the most extreme, evolved overdensity yet observed at $z>7$, with higher stellar masses, gas densities, and dust attenuation, revealing the intersection of local environment and high-redshift galaxy formation at their extremes.

Figures (14)

• Figure 1: An RGB image using the F090W, F277W and F444W filters, with overlaid points indicating the positions of spectroscopically confirmed galaxies (squares, solid line) and photometric candidates (circles). The dashed, large squares indicate the most clustered core regions. The left square includes four PRGs, while the right square contains seven PRGs, these objects compose the "core" regions (as defined in Section \ref{['sec:proto_props']}). Two of the 23 galaxies are not shown in this figure as they lie at a larger separation than the chosen field-of-view. The axis ticks are intended to indicate the approximate distances between galaxies after correcting for magnification (assuming a constant $\mu = 1.9$, while this does vary across the RGB image, this effect is relatively small).
• Figure 2: The UV luminosity function (LF) of PRGs in our sample (red squares) compared to the "nominal" UVLF measured by harikane_pure_2024 for field galaxies (FGs; black solid line). We additionally compare to the UVLF from a semi-analytical model of an extremely overdense region at $z=8$ from kulkarni_reionization_2011 (grey dotted line) and the central $R=2$cMpc region of the Obelisk simulation at $z=7.9$trebitsch_obelisk_2021 (orange dot-dash line). At the UV-bright end of the LF, the A2744-PC-z7p9 region appears to be 200 times overdense relative to the nominal UVLF (black dashed line). There are indications of a mild turnover at $M_{\rm UV}>-18$, however our sample will become highly incomplete near the $5\sigma$ depth (indicated by the shaded grey region), as discussed in Section \ref{['sec:selection']}, so this remains uncertain.
• Figure 3: The UV continuum slopes of our sample (red squares) as a function of UV magnitude. We calculate the UV-slope by fitting the photometric fluxes in the filters covering the wavelength range of $0.13 \mu {\rm m}<\lambda_{\rm rest}<0.31 \mu {\rm m}$ with a power-law. We compare these to literature values of field galaxies (FGs) measured from photometry austin_epochs_2024 and stacked spectroscopy roberts-borsani_between_2024. At the UV-bright end, our protocluster-resident sample lies above the $M_{\rm UV} - \beta$ relation seen in the field, and is largely inline with the relation seen in PRGs in simulations morokuma-matsui_forever22_2025. This appears to indicate that our protocluster sample is more dust enriched than field galaxies in the literature, potentially evidencing their more evolved nature.
• Figure 4: The Balmer break strength measured as the ratio of $(f_{\rm \nu,460M}+f_{\rm \nu,480M})/(2\times f_{\rm \nu,277W})$ for our sample. In the main panel, the full sample is indicated by red circular points, while in the inset, the spectroscopic and photometric samples are differentiated by circles and squares, respectively. Note here that we assume a redshift of $z=7.88$ for the photometric data points. We include, for comparison, Balmer break measurements from galaxies that host strong Balmer breaks in their SEDs (black circle, looser_recently_2024; black star, weibel_rubies_2025; black squares, kuruvanthodi_strong_2024), and from stacked spectra, representing the general galaxy population (grey squares) and a subsample of galaxies with $M_{\star}> 10^{8.6}\ {\rm M_{\odot}}$ (transparent grey squares) from roberts-borsani_between_2024. We additionally show the Balmer break strengths seen in the sphinx$_{20}$ simulations rosdahl_sphinx_2018rosdahl_lyc_2022katz_sphinx_2023, with the the black dashed line (taken from witten_rising_2025). The Balmer break strengths seen in many of the PRGs sit above those of typical galaxies at these redshifts, and are consistent with some of the strongest Balmer breaks strengths seen in the literature.
• Figure 5: The ratio of the flux observed in the F115W filter to the expected F115W from our PROSPECTOR SED-fitting, described in Section \ref{['sec:sed-fitting']}. The data points are colour coded by the inferred neutral hydrogen column density, discussed in Section \ref{['sec:sed-fitting']}. We note here the one circular data point is simultaneously a strong LAE and DLA, and hence we utilise the spectroscopically measured $N_{\rm HI}$ from Witten et al. (in prep.). The majority of galaxies show a strong suppression in the F115W filter that is best explained by high neutral hydrogen column densities ($N_{\rm HI}>10^{22}\ [{\rm cm^{-2}}]$), which has been seen in the galaxies for which deep NIRSpec prism observations are available chen_jwst_2024witten_rising_2025mason_constraints_2025.