Early massive galaxy formation in the core of a galaxy protocluster 650 million years after the Big Bang

TL;DR

This study addresses how massive galaxies form within the dense core of a $z\sim7.9$ protocluster by integrating JWST rest-frame UV–optical data with ALMA $[$\mathrm{CII}$]$158 $\mathrm{\mu m}$ and dust continuum maps of the A2744- Quintet. The authors reveal gas-rich mergers, tidal bridges, and diverse star-formation histories among five members in a $\sim10$ kpc region, including recent quenching in some components and bursts in others, consistent with a merger-driven growth scenario. Cosmological zoom-in simulations from the FirstLight project reproduce analogous gas stripping, bridges, and burst-like SFHs, predicting rapid growth to $M_{\ast}>10^{10}\,M_{\odot}$ within $\sim150$ Myr, aligning with the observational findings. Together, the results provide direct evidence that protocluster environments can drive efficient galaxy formation and rapid evolution during the epoch of reionization.

Abstract

Rest-frame optical observations with the James Webb Space Telescope (JWST) have uncovered a population of massive galaxies, exceeding $10^{10}$ solar masses, present less than a billion years after the Big Bang. The large stellar masses of these galaxies require an efficient conversion of baryons into stars, which may exceed theoretical expectations. However, the formation process of massive galaxies so early in the Universe's history is perplexing, as observations provide limited information to constrain their evolutionary pathways. Here, we present multi-wavelength observations of a galaxy complex consisting of at least five galaxies within a $\sim10\,{\rm kpc}$ region, referred to as the \quintet, using JWST and the Atacama Large Millimeter/submillimeter Array. This system, located in the core of a galaxy protocluster at approximately 650 million years after the Big Bang, reveals the detailed physical processes involved in the formation of massive galaxies. These processes include a dynamic cycles of merger induced gas stripping, leading the temporal termination of star formation, and recycling of the stripped gas, with subsequent enhancement of star formation in other galaxies of the system, which is expected to evolve into massive galaxies that host more than $10^{10}$ solar masses of stars. The new observations represent the first comprehensive evidence of a massive galaxy formation through gas-rich, multiple-galaxy mergers induced by a dense protocluster environment in the $650\,{\rm Myrs}$ after the Big Bang. Our results suggest that the protocluster core is indeed one of the main drivers of efficient galaxy formation and rapid evolution in the early Universe, as predicted by theoretical studies.

Figures (9)

• Figure 1: JWST images of the A2744- Quintet at $\mathbf{z=7.9}$: Upper image shows a false color image of a $60^{\prime\prime}\times90^{\prime\prime}$ region around the A2744- Quintet using F444W (red), F277W (green), and F150W images (blue). White circles indicate galaxies belong to A2744-z7p9OD, a galaxy protocluster at $z=7.9$Witten2025-2. The white square indicate the A2744- Quintet.Lower left and middle panels show the stacked images of the A2744- Quintet obtained by JWST's short and long wavelength filters, respectively. Lower right panel shows the ALMA [C ii] $158\,{\rm \mu m}$ emission line map. The white contours show the $2,3,5\,\sigma$ levels of the [C ii] $158\,{\rm \mu m}$ emission line, and black dashed contours show $6,12,24,48\,\sigma$ signal in the stacked JWST's long wavelength image. Labels show names of each member galaxies of the A2744- Quintet and labels in parentheses show names of sub-components. The white bar in the lower left corner shows physical $2.5\,{\rm kpc}$ scale in the image plane without lensing corrections. The white ellipses in the lower right corners in each panel show full width at half maximums of the point spread functions and the synthesized beam size of JWST and ALMA observations, respectively.
• Figure 2: Observed [C ii] $158\,{\rm \mu m}$ emission lines and the gas structure of the A2744-the Quintet. 3-D visualization of the [C ii] $158\,{\rm \mu m}$ emission line obtained in our observations. The bluest color starts from $3\,\sigma$ signal and reddest color ends at $10\,\sigma$ signal. The velocity reference is the strongest signal in the data, which corresponds to the [C ii] emission from YD4. The 2-D image on the back shows moment-0 image of the [C ii] $158\,{\rm \mu m}$ emission line overlaying contours of a stacked image using JWST's long wavelength data ($6\sigma$ -- $48\,\sigma$). An interactive version of the 3-D visualized [C ii] emission map is available as a html format file in https://github.com/yfudamoto/data-the_Quintet.
• Figure 3: Star formation histories of member galaxies in A2744- Quintet: Middle panel shows a false color image of the A2744- Quintet using the stacked images of the observed wavelengths of $1.2$ -- $2.1\,{\rm \mu m}$ in red, $2.5$ -- $3.6\,{\rm \mu m}$ in green, and $4.1$ -- $4.8\,{\rm \mu m}$ in blue. In other panels, spectral energy distributions (SEDs) modeling show distinct star formation histories (SFHs) of galaxies in the A2744- Quintet, showing the outcomes of gas transportation within the system (see Methods for SED fitting and the definition of star formation modes). The recent quenching of of YD7-E and ZD1, the continuous star-formation of YD4 and YD6, and the starbursts of YD1, S1, and YD7-W show the results of gas stripping, compression, and recycling as seen from the spatial distribution of the [C ii] $158\,{\rm \mu m}$ emission lines.
• Figure S1: Velocity channel map of [C ii] $158\,{\rm \mu m}$ emission lines of A2744- Quintet.The background image shows the stacked image of JWST's long wavelength filters. The velocity range is labeled at the top of each panel. Clumps and bridges are labeled in the maps. The synthesized beam is shown at the lower left corner of each map. Contours start from $2\,\sigma$ and end at $9\,\sigma$ with a step of $1\,\sigma$.
• Figure S2: The projected velocity and velocity dispersion field of the [C ii] $158\,{\rm \mu m}$. Left panel shows the line of sight velocity dispersion of [C ii] emission, while the line of sight velocity of [C ii] emission is shown in right panel. The black solid contour shows the 2 sigma level of [C ii] $158\,{\rm \mu m}$ emission. Dashed contours show $6, 12, 24, 48\,\sigma$ of JWST LW stacked image. Black ellipses at the lower right corners show the synthesized beam size of ALMA observations.