X-ray Protocluster at Reveals Rapid Structure Growth

By Dr. Noopur JainReviewed by Louis CastelFeb 11 2026

Astronomers have detected extended X-ray emission from JADES-ID1 at z≈5.68, confirming the presence of a hot intracluster medium just one billion years after the Big Bang.
The finding provides the earliest direct evidence that massive halos were already undergoing virial heating and rapid large-scale structure growth in the early universe. The article was published in the journal Nature.

Image Credit: Triff/Shutterstock.com

Background

Galaxy clusters, the largest gravitationally bound structures in the universe, trace the evolution of the large-scale structure, and their early progenitors, protoclusters, offer insights into the initial stages of this formation. The detection of protoclusters at such high redshifts is typically difficult because their member galaxies are loosely bound and the hot intracluster medium (ICM), which is a key signature of cluster formation, may be only beginning to virialize. Recent observations using the James Webb Space Telescope (JWST) have successfully located several protocluster candidates by identifying overdensities of z?5 galaxies. However, prior to this work, none of these candidates had been confirmed through X-ray detection, which is crucial for unveiling the hot ICM. This study combines deep Chandra X-ray observations with JWST data, leading to the first joint detection of a protocluster at this epoch. The presence of extended, shock-heated gas suggests that substantial ICM heating was already occurring in massive halos as early as z≈5.7, although the system is unlikely to be fully virialized at this stage.

The Current Study

To characterize JADES-ID1 using JWST data, the centroid was defined using the DETECTIFz algorithm on two-dimensional galaxy overdensity maps in narrow-redshift slices. The slice with the highest overdensity peak, z=5.68, determined the protocluster center. The statistical significance of the galaxy overdensity was quantified by comparing the observed values within a spherical volume of radius 410 kpc to 106 Monte Carlo realizations of the mean field density over 5.44 < z < 6.08. The absence of substantial foreground structures was verified by analyzing photometric galaxy catalogues from JWST and Hubble Space Telescope (HST) observations. Galaxy surface densities were measured within a 40′′×40′′ box centered on the X-ray peak and compared to a large background region across the JADES footprint. The analysis showed only a single significant overdensity peak in the z=5.25–6.23 redshift bin, ruling out comparable galaxy overdensities at other redshifts.

For the X-ray analysis, 99 Chandra Advanced CCD Imaging Spectrometer (ACIS-I) observations covering the CDFS were analyzed, representing the deepest X-ray field ever observed. The data processing included reprocessing individual observations using the chandra_repro tool and filtering high-background periods to yield a total cleaned exposure time of 6.55 Ms. Absolute astrometry was corrected to ensure accurate alignment of point sources, using the deepest observation as the reference coordinate system. The observations were merged to generate energy-filtered images in the standard 0.5–2.0-keV and 3–7-keV bands, with analysis focused on the 0.3–2.0-keV (soft) band and 3–7-keV (hard) bands. The soft band was selected because emission from a few-keV thermal plasma at z≈5.7 is expected to be redshifted predominantly into this energy range, whereas little emission is expected in the hard band. Exposure-corrected images were generated by dividing count images by exposure maps, which accounted for vignetting and molecular contamination, assuming an optically thin thermal plasma model (APEC) with kT=2 keV and Z=0.3Z?. X-ray point sources, primarily originating from the cosmic X-ray background, were identified and masked to prevent contamination of the extended emission. To better visualize the faint diffuse emission, the point-source-masked image was processed by filling the excluded regions, subtracting background components using an annular region, and applying Gaussian smoothing with a kernel size of 15 pixels.

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Results and Discussion

While initial inspection of the raw 0.3–2.0-keV Chandra image did not show bright X-ray emission cospatial with JADES-ID1, the processed image revealed large-scale diffuse X-ray emission near the JWST-derived centroid. The X-ray centroid was found to be offset by approximately 8'' (about 47 kpc) from the galaxy overdensity peak identified by JWST. Such separations are commonly observed in dynamically young or merging systems, comparable to offsets seen in high-redshift systems still undergoing collapse. The extended X-ray emission spatially coincident with the galaxy overdensity suggests the presence of a large-scale hot ICM. The quantitative analysis of the extended emission involved establishing its extended nature through a surface brightness profile, examining its presence across different X-ray energy ranges, and analyzing its spectral properties. Notably, the soft-band X-ray emission traces the gravitational potential of the protocluster and allows for a more precise determination of its centroid, luminosity, and total mass. The combined-likelihood analysis indicates that the probability of the detected signal arising from a random background fluctuation is approximately 2.6 × 10?7, corresponding to a significance of about 5σ. The inferred bolometric X-ray luminosity is approximately 1.5 × 1044 erg s?¹, implying a total mass of M500 ≈ 1.8 × 10¹³ M?.

Conclusion

The combined Chandra and JWST detection of the JADES-ID1 protocluster at z≈5.68 provides crucial confirmation of the onset of gravitational collapse at this early epoch. The detection of extended X-ray emission confirms the presence of a hot ICM, indicating that substantial virial heating is already occurring in massive halos barely a billion years after the Big Bang, even if the structure has not yet reached full virial equilibrium. Given the limited survey volume, the discovery of such a massive cluster so early in the universe is statistically unlikely according to standard cosmological models. This suggests that the formation of large-scale structure must have occurred more rapidly in some regions of the early universe than predicted.