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JWST shakes up the hunt for earliest galaxy cluster

February 02, 2026 • 5 min read • views
JWST shakes up the hunt for earliest galaxy cluster
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View our Twitter (X) feed View our Youtube channel View our Instagram feed View our Substack feed Search Popular SearchesCritical thinkingPhilosophyEmotional IntelligenceFree Will Latest Videos Latest Articles JWST shakes up the hunt for earliest galaxy cluster

The Universe formed stars, galaxies, and even galaxy clusters extremely early on in our cosmos. This new marvel is one more JWST surprise.

by Ethan Siegel February 2, 2026 Image of a star field with numerous galaxies; several objects are circled in white, and one object is marked with an “X” near the center. This protocluster of galaxies, known as JADES-ID1, is the 4th earliest, most distant protocluster of galaxies ever identified, as its member galaxies are shown with circles in this JWST image. Its hot, X-ray emitting gas, as found complementarily with Chandra, marks the earliest cluster to show this characteristic X-ray glow.

Credit: Á. Bogdán et al., Nature, 2026

Key Takeaways
  • Although our Universe was born relatively uniform, with only tiny density imperfections, those imperfections eventually grew into stars, galaxies, galaxy clusters, and the grand cosmic web.
  • While JWST has revealed a surprisingly large number of early, bright galaxies, it’s also revealed a few surprises about the earliest galaxy clusters: finding a few very early, very massive ones that defy expectations.
  • With the power of JWST’s infrared views and Chandra’s X-ray capabilities, we’ve just found the earliest protocluster of galaxies with significant X-ray emissions, indicating more than a trillion solar masses worth of hot gas. Here’s what it means.

An astrophysics column on big questions and our universe.

Newsletter

The Hubble Space Telescope displayed what the Universe looks like.

Hubble eXtreme Deep field XDFOver the course of 50 days, with a total of over 2 million seconds of observing time (the equivalent of 23 complete days), the Hubble eXtreme Deep Field (XDF) was constructed from a portion of the prior Hubble Ultra Deep Field image. Combining light from ultraviolet through visible light and out to Hubble’s near-infrared limit, the XDF represents humanity’s deepest view of the cosmos: a record that stood until the JWST’s first deep field was released on July 11, 2022. Credit: NASA, ESA, G. Illingworth, D. Magee, and P. Oesch (University of California, Santa Cruz), R. Bouwens (Leiden University), and the HUDF09 Team

Its successor, JWST, now reveals how the Universe grew up.

This tiny fraction of the JADES survey area, taken with JWST’s NIRCam instrument, showcases relatively nearby galaxies in detail, galaxies at intermediate distances that appear grouped together, and even ultra-distant galaxies that may be interacting or forming stars, despite their faint nature and red appearance. Even though we’ve been performing JWST science for over two years, we are only beginning to probe the full richness of the cosmos with JWST. Credit: NASA, ESA, CSA, STScI, B. Robertson (UC Santa Cruz), B. Johnson (CfA), S. Tacchella (Cambridge), P. Cargile (CfA)

Galaxies formed and grew massive swiftly: requiring under 300 million years.

A colored pixelated grid with rectangular outlines; a legend in the top right labels blue as F115W, green as F200W, and red as F277W—capturing data from the JWST to record a distant galaxy.This image shows a three-filter NIRCam view of galaxy MoM-z14: the new record holder (as of May 16, 2025) for the most distant galaxy ever discovered. Invisible at wavelengths below 1.8 microns, JWST has measured its spectrum and detected several emission lines, cementing its status as arising from when the Universe was a mere 282 million years old. Credit: R.P. Naidu et al., Open Journal of Astrophysics (submitted)/arXiv:2505.11263, 2025

Larger-scale, more massive structures, like galaxy clusters, take longer.

This image shows the huge galaxy cluster MACS J1149.5+223, whose light took over 5 billion years to reach us. The huge mass of the cluster is bending the light from more distant objects. The light from these objects has been magnified and distorted due to gravitational lensing. The same effect is creating multiple images of the same distant objects. Meanwhile, the central location of the cluster clearly shows intracluster light: a remarkable tracer of dark matter. Credit: NASA, ESA, S. Rodney (John Hopkins University, USA) and the FrontierSN team; T. Treu (University of California Los Angeles, USA), P. Kelly (University of California Berkeley, USA) and the GLASS team; J. Lotz (STScI) and the Frontier Fields team; M. Postman (STScI) and the CLASH team; and Z. Levay (STScI)

The earliest mature, fully-fledged cluster is CL J1001+0220.

Mature galaxy cluster CL J1001This X-ray/infrared composite image shows galaxy cluster CL J1001+0220, the earliest known mature, X-ray emitting galaxy cluster. Although this was the earliest known galaxy cluster of any type in 2016, several younger protoclusters have since been identified. It remains the earliest mature galaxy cluster known today, even in the JWST era. Credits: X-ray: NASA/CXC/Université Paris/T.Wang et al; Infrared: ESO/UltraVISTA; Radio: ESO/NAOJ/NRAO/ALMA

Simulations predict such clusters to appear late: after 2-3 billion years.

Astronomical image showing a galaxy cluster with multiple bright glowing sources and smaller points against a deep blue and purple space background.The SIBELIUS project, which simulates galaxies and structures beyond the local Universe, is part of the Virgo Consortium that attempts to use cosmological simulations to reproduce features of galaxies, groups, and clusters that are seen all across the Universe. By using a mix of theory, observations, and simulations, astrophysicists can better understand the nature of dark matter in our cosmos. The formation of large galaxy clusters generally requires long timescales and initially large overdensities, with the largest initial overdensities requiring less time to form clusters. Credit: Virgo Consortium/SIBELIUS project

However, protoclusters, or still-forming galaxy clusters, appear far earlier.

Two 3D space maps show galaxy distributions at redshifts z=6.6 and z=5.7, each highlighting an overdense region—potentially among the jwst earliest galaxy cluster detections—labeled "z66OD" and "z57OD" with red arrows.These two galaxy protoclusters, z66OD (at a redshift of 6.6) and z57OD (at a redshift of 5.7) are overdense collections of galaxies on large, cluster-like scales, with 12 and 44 galaxies (at least) inside of them, respectively. The blue blobs show the reconstructed mass distribution of these protoclusters, whose light comes from 830 million and 1.0 billion years after the Big Bang. Credit: Y. Harikane et al., Astrophysical Journal, 2019

A 2019 study revealed protoclusters z66OD and z57OD: with at least 12 and 44 member galaxies, respectively.

most distant pre-Webb proto-clusterThis galaxy protocluster, known as z66OD, contains 12 independent galaxies all at the same redshift: z=6.6. Its light comes to us from just 830 million years after the Big Bang. Prior to JWST, this protocluster was the earliest collection of galaxies in the same region of space ever known. The blue shading shows the estimated extent of the protocluster. Credit: NAOJ/Y. Harikane et al., Astrophysical Journal, 2019

Then, in 2023, JWST spotted the earliest known protocluster of galaxies assembling: A2744z7p9OD.

JWST most distant galaxy clusterThe galaxies that are members of the identified protocluster A2744z7p9OD are shown here, outlined atop their positions in the JWST view of galaxy cluster Abell 2744. At just 650 million years after the Big Bang, it’s the oldest protocluster of galaxies ever identified. This is early, but is consistent with simulations of when the earliest protoclusters should emerge from the most initially overdense regions. Credit: NASA, ESA, CSA, Takahiro Morishita (IPAC); Processing: Alyssa Pagan (STScI)

With seven large, bright member galaxies just 650 million years after the Big Bang, it remains the youngest protocluster identified.

JWST most distant protoclusterThis image shows the view of JWST’s NIRCam instrument as it looked at galaxy cluster Abell 2744 and revealed a number of galaxies that are members of a protocluster. The red squares show several of the galaxies for which spectroscopic measurements were obtained; the orange circles are photometric galaxy candidates that may yet turn out to be part of this cluster. Small, low-mass galaxies form earlier; larger, evolved galaxies and galaxy clusters only appear at later times. Credit: T. Morishita et al., ApJL, 2023

However, one hallmark of mature clusters is missing from these protoclusters: hot, X-ray emitting gas.

A dense field of distant galaxies and stars scattered across the dark expanse of space.This image composite shows the full-field of a large galaxy cluster within the COSMOS-Web survey, using a combination of JWST NIRCam and Hubble infrared data, with X-ray data from the Chandra X-ray telescope overlaid in violet. The X-rays are evidence from the heated gas that occurs when galaxy clusters merge or experience major disruptive events. Credit: ESA/Webb, NASA & CSA, G. Gozaliasl, A. Koekemoer, M. Franco, and the COSMOS-Web team

However, the discovery of the fourth earliest galaxy protocluster, JADES-ID1, changes all of that.

A split image shows a wide-field view of distant galaxies on the left, with a zoomed-in section highlighting faint circled objects on the right—captured by JWST while studying an earliest galaxy cluster. White scale bars and text annotations are present.A whopping 66 potential member galaxies have been identified in galaxy protocluster JADES-ID1, identified with data obtained with deep JWST imagery. A large overdensity this rare and this early, just 1 billion years after the Big Bang (at z = 5.68), is a remarkable find. Credit: Á. Bogdán et al., Nature, 2026

Alongside 66 potential member galaxies, X-ray emitting gas was spotted by NASA’s Chandra.

An image of deep space captured by JWST shows numerous galaxies, with a highlighted square area containing a blue haze, indicating the presence of dark matter in one of the earliest galaxy clusters.This animation shows a more full field of the early protocluster JADES-ID1 from within the JWST Advanced Deep Extragalactic Survey field. The X-ray data, from NASA’s Chandra X-ray observatory, is overlaid in this animation in blue, showcasing the characteristic X-ray emission of hot gas in galaxy clusters. Credits: X-ray: NASA/CXC/CfA/Á Bogdán; Infrared: NASA/ESA/CSA/STScI; Image Processing: NASA/CXC/SAO/P. Edmonds and L. Frattare; Animation: E. Siegel

It’s definitively associated with the distant protocluster, with the right spectrum of X-ray energies.

A heatmap with contour lines and a color bar labeled "Combined likelihood σ" from 0 to 5, showing coordinates and a dashed magenta circle—illustrates the search for the JWST earliest galaxy cluster. North and East directions are indicated.The white contours show higher-energy X-rays, which shouldn’t come from a protocluster, while the yellow contours show the likelihood of low-energy X-rays, which are present, based on the locations of individually identified protocluster members. The violet circle, indicating the central location of the emitted X-rays, is shown superimposed atop the yellow likelihood map. Credit: Á. Bogdán et al., Nature, 2026

These X-ray emissions mark the earliest cosmic detection of intracluster heating and virialization.

A composite image of a galaxy cluster, including the JWST earliest galaxy cluster, with a blue overlay showing X-ray data and many background galaxies; some objects are circled in orange. “X-RAY & INFRARED” appears at the corner.This figure represents an X-ray (from Chandra) and infrared (from JWST) composite of the protocluster of galaxies known as JADES-ID1, the earliest galaxy cluster showing signs of heated, virialized gas that emits low-energy (but not high-energy) X-rays. The Universe, as revealed by this protocluster, can grow up remarkably fast. Credit: X-ray: NASA/CXC/CfA/Á Bogdán; Infrared: NASA/ESA/CSA/STScI; Image Processing: NASA/CXC/SAO/P. Edmonds and L. Frattare

Containing trillions of solar masses, such evolved, early protoclusters must be cosmically rare.

lensing and X-ray map of El Gordo JWSTHere, the El Gordo galaxy cluster is shown in two unusual ways: where the colors represent the mass density of the diffuse mass component as inferred by gravitational lensing data (colors), while the black contours show the X-ray emissions as measured via the Chandra X-ray telescope. Note that the two maps are very different from one another, indicating that overall mass does not follow the same locations that the normal matter does. Late-time galaxy clusters often emit X-rays, but only one early protocluster, JADES-ID1, has been seen to do so thus far.

Credit: J.M. Diego et al. (PEARLS collaboration), A&A, 2023

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