All the Massive Galaxy Overdensities during Reionization: JWST Rest-Frame Optical Selection Reveals Young, Chemically Evolved Galaxies Embedded in Dense, Neutral Gas at z > 5

TL;DR

This work uses a blind rest-frame optical selection with JWST/NIRCam grism spectroscopy to identify five massive galaxy overdensities at $z\sim5.66$–$6.77$ in the Abell 2744 field, revealing halo masses of order $M_{\mathrm{halo}} \sim 10^{11}-10^{14}\,M_\odot$. Across these overdensities, member galaxies tend to have lower stellar masses but bluer UV continua and weaker Balmer breaks than field galaxies, with tentative evidence for enhanced chemical enrichment at $z>6$. The study further performs Ly$\alpha$ absorption modeling to map neutral gas content, finding a wide $N_{\mathrm{HI}}$ range ($10^{18}-10^{23}\,\mathrm{cm}^{-2}$) and DLAs in several systems, including a coherently high Hi reservoir in the $z=7.88$ overdensity. Together, these results show that early protoclusters can be gas-rich and dynamically diverse, with environmental effects influencing gas accretion and chemical enrichment rather than simply accelerating stellar mass growth; they underscore the importance of gas tomography for understanding reionization-era structure formation and motivate wider JWST surveys.

Abstract

The high-redshift progenitors of present-day galaxy clusters are believed to substantially contribute to the global star-formation rate density and drive the large-scale reionization of the Universe. Here we present a blind and unbiased search for and characterization of galaxy overdensities during the reionization epoch at redshifts $z\sim 5.5-7$, based on rest-frame optical JWST/NIRCam grism spectroscopy of the Abell\,2744 lensing field as part of the JWST-ALT survey. Using a physically-motivated, cosmological inference Friends-of-Friends (FoF) algorithm, we identify six galaxy overdensities, including five robust systems at $z=5.66$ to $6.77$. They are all characterized by total halo masses $M_{\rm halo} \gtrsim 10^{11}\,M_{\odot}$ inferred from a range of proxies. We find that the galaxy members in these overdense environments are on average less massive though equally metal-rich, and generally comprised of younger stellar populations as indicated from their bluer spectral slopes less prominent Balmer breaks, than field galaxies at similar redshifts. Further, we use this novel rest-frame optical selection of galaxy proto-clusters to infer the fraction and 3D distribution of strong Lyman-$α$ emitters (LAEs) and damped Lyman-$α$ absorbers (DLAs) in the overdensity environments. We find that two out of six galaxy overdensities have excess \hi\ absorption compared to the field-average, while the other four are consistent within their large scatter in density. These results present the first direct observational constraints on the tomography of the dense, neutral gas reservoirs in large-scale galaxy overdensities at $z>5$ and highlight the limitations of pre-JWST searches for reionization-era galaxy overdensities relying on the detection of strong LAEs alone.[Abridged]

Figures (9)

• Figure 1: Spatial and redshift distribution of galaxies in the ALT catalog with overdensities marked. Spatial and redshift distribution of galaxies from the ALT catalog in the range $5.5 < z < 7$. The horizontal axis shows the grism redshift ($z_{\rm grism, ALT}$), while the vertical axis indicates the comoving offset in declination ($\rm \Delta DEC ; [cMpc]$). Marker size reflects the comoving offset in right ascension ($\rm \Delta RA ; [cMpc]$), ranging from $-6.5$ to $5.3$ cMpc, with larger symbols corresponding to larger offsets. Galaxies belonging to the five most significant overdensities identified by the FoF algorithm are shown in color, while all field galaxies are displayed in gray.
• Figure 2: Projected spatial distribution of galaxies in the five largest overdensities identified in the ALT survey, plotted in comoving coordinates (RA vs. DEC). Each panel corresponds to a different overdensity, labeled by its mean redshift: (a) $z = 5.66$, (b) $z = 5.77$, (c) $z = 6.24$, (d) $z = 6.34$, and (e) $z = 6.77$. Individual galaxies are color-coded by their line-of-sight velocity in km/s relative to the group’s mean redshift and scaled based on their stellar mass.
• Figure 3: Dark matter halo mass versus redshift. Each overdensity is color-coded following the notation of Fig. \ref{['fig:ODs_introfig']}, with distinct symbols indicating the mass estimation method. Open color-coded circles at $z=0$ indicate the median descendant halo masses inferred from the UV-based halo masses, with error bars showing the standard deviation derived from tracing haloes in the IllustrisTNG simulations following the method described in Witten_2025_7_66_cluster. The gray shaded region shows the expected evolution of a Coma-like cluster Chiang_2013 assuming constant evolution beyond $z \sim 7$. The black horizontal dashed line marks the shock stability threshold separating cold accretion from hot ICM formation Dekel_2006, while the diagonal dashed line indicates the limit for penetrating cold flows. For comparison, overdensities from Helton_2024, fudamoto_2025, and Witten_2025_7_66_cluster are shown with black, gray, and light green markers, respectively.
• Figure 4: Example SED fits and photometric data for two galaxies in two of the overdense environments at $z = 5.64$ (top) and $z = 7.88$ (bottom). Filled squares with error bars show the HST (dark blue) and JWST/NIRCam (light blue) photometry. The dark gray step curve displays the NIRSpec/prism spectrum. The orange solid line shows the best-fitting SED model including a DLA (SED+DLA), while the semi-transparent grey curve indicates the corresponding intrinsic SED model without the Ly$\alpha$ region from ALT_paper_2024. Open orange squares represent the model-predicted photometry obtained from the SED+DLA model, and open grey squares show the photometry predicted by the intrinsic SED. The inferred $N_{\ion{H}{i}}$ values are indicated in the upper right of each panel.
• Figure 5: Normalized distributions of stellar masses ($\log_{10}(M_{\star}/M_{\odot})$) for overdensity galaxies (red) and field galaxies (gray) in each of the five largest overdensities. Insets show the corresponding empirical cumulative distribution function (ECDFs) along with the KS statistic and $p$-value.