An X-ray-Emitting Proto-Cluster at $z\approx5.7$ Reveals Rapid Structure Growth

TL;DR

The study targets the formation of the first massive structures by identifying a proto-cluster, JADES‑ID1, at $z \\approx 5.68$ through JWST overdensities and confirming it with extended X-ray emission detected by Chandra. It combines the JWST overdensity significance with a strong soft X-ray signal to infer a bolometric luminosity of $L_{bol} \\approx 1.5\\times 10^{44}$ erg s$^{-1}$ and a total mass of $M_{500} \\approx 1.8\\times 10^{13}$ M$_\\odot$, with an ICM temperature $kT \\gtrsim 2.5$ keV. The joint ~6.9σ detection indicates hot ICM in a halo already assembling its virialized gas within ~1 Gyr after the Big Bang, challenging simplistic ΛCDM expectations about the timing of large-scale structure growth. This work demonstrates the power of JWST–Chandra synergy to map the earliest clusters and motivates future high-redshift X-ray and SZ surveys of proto-clusters.

Abstract

Galaxy clusters are the most massive gravitationally bound structures in the universe and serve as tracers of the assembly of large-scale structure. Studying their progenitors, proto-clusters, sheds light on the earliest stages of cluster formation. Yet, detecting proto-clusters is demanding: their member galaxies are loosely bound and the emerging hot intracluster medium (ICM) may only be in the initial stages of virialization. Recent JWST observations located several proto-cluster candidates by identifying overdensities of $z\gtrsim5$ galaxies. However, none of these candidates was detected by X-ray observations, which offer a powerful way to unveil the hot ICM. Here, we report the combined Chandra and JWST detection of a proto-cluster, JADES-ID1, at $z\approx5.68$, merely one billion years after the Big Bang. We measure a bolometric X-ray luminosity of $L_{\rm bol} = (1.5^{+0.5}_{-0.6}) \times10^{44} \ \rm{erg \ s^{-1}}$ and infer a total gravitating mass of $M_{500}= (1.8^{+0.6}_{-0.7}) \times 10^{13} \ \rm{M_{\odot}}$, making this system a progenitor of today's most massive galaxy clusters. The detection of extended, shock-heated gas indicates that substantial ICM heating can occur in massive halos as early as $z\approx5.7$. In addition, given the limited survey volume, the discovery of such a massive cluster is statistically unlikely, implying that the formation of the large-scale structure must have occurred more rapidly in some regions of the early universe than standard cosmological models predict.

Figures (4)

• Figure 1: Multi-wavelength view of the JADES-ID1 protocluster and its surroundings. Panel (a) shows a composite JWST image of the JADES field with a $45\hbox{$^{\prime\prime}$}\times45\hbox{$^{\prime\prime}$}$ ($265\times265$ kpc) box marking JADES-ID1. Panel (b) zooms in on this region, while panel (c) displays the exposure-corrected $0.3-2$ keV band Chandra image of the same region. Panel (d) presents a JWST/Chandra overlay of JADES-ID1, where the Chandra image has been processed by filling point sources, subtracting the background, and applying Gaussian smoothing with a kernel size of 15 pixels. Panel (e) shows the same processed Chandra image, with the locations of likely cluster member galaxies highlighted. The Chandra image reveals extended X-ray emission that is spatially coincident with the galaxy overdensity identified by JWST.
• Figure 1: Chandra$\mathbf{0.3-2}$ keV band images of the JADES-ID1 field.Left: Merged Chandra image with detected X-ray point sources marked by cyan regions and candidate $z\approx5.7$ member galaxies indicated by white circles. The large white circle with $21\hbox{$^{\prime\prime}$}$ radius denotes the aperture used for ICM analysis. Right: The same Chandra image after all X-ray point sources have been excised.
• Figure 2: X-ray surface brightness profile of the JADES-ID1 proto-cluster. The $0.3-2$ keV band profile was extracted in concentric annuli centered on the X-ray peak. The solid line shows the best‐fit $\beta$-model. The profile is corrected for exposure variations using the exposure map. The background is subtracted based on the $40\hbox{$^{\prime\prime}$}-110\hbox{$^{\prime\prime}$}$ ($235-646$ kpc at $z=5.68$) annulus around the proto-cluster. The resulting background level is $8.03 \times 10^{-10} \ \mathrm{photons \ s^{-1} \ cm^{-2} \ arcsec^{-2}}$. Extended emission is detected out to $\approx21\hbox{$^{\prime\prime}$}$ ($\approx125$ kpc). The error bars represent statistical uncertainties derived using the Gehrels approximation 1986ApJ...303..336G.
• Figure 3: Combined detection likelihood of JADES-ID1 and its surroundings. White contours show the $3-7$ keV band emission. The map incorporates the probability of having a $0.3-2$ keV band detection, a $3-7$ keV band non-detection, and a radially declining surface brightness profile. The position of the proto-cluster is highlighted with a magenta circle with $7\hbox{$^{\prime\prime}$}$ radius.