XLSSC 122: Distant Galaxy Cluster Insights

• XLSSC 122 is a high-redshift galaxy cluster at z≈2 characterized by extensive multiwavelength observations that detail both its mature intracluster medium and active assembly state.
• It exhibits a hot, metal-enriched ICM with measured parameters (kT≈5 keV, significant SZ signal) that underscore its thermodynamic properties and disturbed morphology.
• Multiwavelength analyses reveal a diverse galaxy population with a pronounced red-sequence, rapid quenching in the core, and merger-induced dynamics influencing its structure.

XLSSC 122 is a galaxy cluster at redshift $z=1.98$ in the XMM-LSS/XXL program, also designated XLSSU J021744.1-034536. It is among the best-characterized clusters known at such an early epoch because it has been studied through extended X-ray emission, the thermal Sunyaev-Zel’dovich (SZ) effect, HST and JWST imaging and spectroscopy, weak lensing, radio continuum data, and intracluster-light measurements. The literature presents it simultaneously as a mature system, with a hot metal-enriched intracluster medium (ICM) and a substantial quenched red-sequence population, and as a dynamically active halo still undergoing significant assembly (Mantz et al., 2017, Willis et al., 2020, Marrewijk et al., 2023).

1. Discovery, designation, and survey setting

XLSSC 122 was first identified as a faint extended X-ray source in the XMM Large Scale Structure survey and was found to coincide with a compact overdensity of galaxies with photometric redshifts $1.9\pm0.2$. One cited SZ study uses the designation XLSSU J021744.1-034536 for the same object, and the cluster center is placed near $\alpha \simeq 34.43^\circ$, $\delta \simeq -3.76^\circ$, with the brightest cluster galaxy at $(34.43422^\circ,-3.75880^\circ)$ (Willis et al., 2020).

Within XXL data products, XLSSC names are cluster designations rather than the IAU-style source names used in the 3XLSS X-ray source catalog. In that catalog framework, confirmed clusters are cross-linked through a field named XLSSC, and the cluster-oriented release to consult for confirmed systems is XXL_365_GC rather than the point-source-style 3XLSS table (Chiappetti et al., 2018).

The cluster’s importance is partly historical. A deep XMM-Newton study described it as the first massive cluster discovered through its X-ray emission at $z\approx2$, and later work emphasized that it is a rare case in which a $z\sim2$ cluster can be studied as a resolved thermodynamic, dynamical, and galaxy-evolution system rather than merely as a candidate overdensity (Mantz et al., 2017).

2. Intracluster medium: X-ray and SZ measurements

A 100 ks XMM-Newton observation provided the first precise bulk ICM constraints for XLSSC 122. From spectra extracted within $35''\approx r_{500}$, the cluster was measured to have an average temperature $kT=5.0\pm0.7$ keV, metallicity $Z/Z_\odot=0.33^{+0.19}_{-0.17}$, and an X-ray spectroscopic redshift $1.9\pm0.2$0. Using the gas-density profile and an assumed $1.9\pm0.2$1, the study derived $1.9\pm0.2$2 kpc and $1.9\pm0.2$3, with $1.9\pm0.2$4. The corresponding rest-frame luminosities within $1.9\pm0.2$5 were $1.9\pm0.2$6 and $1.9\pm0.2$7 (Mantz et al., 2017).

The same study combined CARMA 30 GHz data with the X-ray analysis and measured the spherically integrated Compton parameter within $1.9\pm0.2$8 as $1.9\pm0.2$9, or $\alpha \simeq 34.43^\circ$0. It also reported a large X-ray/SZ centroid offset of $\alpha \simeq 34.43^\circ$1, equivalent to $\alpha \simeq 34.43^\circ$2 kpc, and interpreted that as evidence for a disturbed configuration (Mantz et al., 2017).

Higher-resolution resolved SZ imaging substantially refined that picture. Joint ALMA+ACA plus ACT analysis detected the SZ signal at $\alpha \simeq 34.43^\circ$3 in ALMA+ACA alone and about $\alpha \simeq 34.43^\circ$4 when ACT large-scale information was added. The pressure profile was found to have a flattened core, consistent with a non-cool-core or morphologically disturbed classification at $\alpha \simeq 34.43^\circ$5. The evidence-weighted SZ mass from the empirical ALMA+ACA+ACT fits was $\alpha \simeq 34.43^\circ$6, and the integrated SZ signal was $\alpha \simeq 34.43^\circ$7 (Marrewijk et al., 2023).

That later SZ study also revised the earlier centroid-offset interpretation. Rather than a single badly misaligned SZ peak, it found much better agreement between the main SZ decrement, the X-ray emission, and the galaxy distribution, with a residual SZ displacement of $\alpha \simeq 34.43^\circ$8 from the BCG/X-ray peak. At the same time, it identified excess SZ flux to the south, where little or no X-ray emission is detected, and obtained tentative evidence for a second pressure component or filamentary-like structure. The cluster was therefore described as dynamically active but already containing a large reservoir of thermalized gas (Marrewijk et al., 2023).

3. Spectroscopic confirmation and galaxy populations in the core

HST/WFC3 imaging and slitless G141 spectroscopy delivered the first direct spectroscopic confirmation of the member population. Using integrated redshift probabilities, “gold” members were defined by

$\alpha \simeq 34.43^\circ$9

with the interval chosen to correspond to $\delta \simeq -3.76^\circ$0 for a 5 keV cluster. This yielded 33 gold members; four additional “silver” members with $\delta \simeq -3.76^\circ$1 brought the total spectroscopically identified membership to 37, and the mean cluster redshift to $\delta \simeq -3.76^\circ$2. The same data showed a physically distinct foreground structure at $\delta \simeq -3.76^\circ$3, separated from the cluster by 76 comoving Mpc along the line of sight (Willis et al., 2020).

The cluster already exhibits a clear color-magnitude bimodality. Red-sequence members were defined by

$\delta \simeq -3.76^\circ$4

and 19 gold red-sequence members were used for stellar-population fitting. The resulting red sequence coexists with a blue cloud and a minority of emission-line galaxies, indicating that the system is not fully passive even though its central galaxy population is already substantially evolved (Willis et al., 2020).

The same study reported a quenched fraction $\delta \simeq -3.76^\circ$5. For the 19 gold red-sequence members, luminosity-weighted age fitting gave $\delta \simeq -3.76^\circ$6 Gyr for the $\delta \simeq -3.76^\circ$7 model, implying a mean formation redshift $\delta \simeq -3.76^\circ$8 with spread 10.9–13.3. For dustier models the inferred ages and formation redshifts shifted to $\delta \simeq -3.76^\circ$9 Gyr with $(34.43422^\circ,-3.75880^\circ)$0 for $(34.43422^\circ,-3.75880^\circ)$1, and $(34.43422^\circ,-3.75880^\circ)$2 Gyr with $(34.43422^\circ,-3.75880^\circ)$3 for $(34.43422^\circ,-3.75880^\circ)$4. The paper explicitly cautioned that the highest formation redshifts are model-dependent and sensitive to dust and metallicity assumptions (Willis et al., 2020).

A later 12-band BAGPIPES analysis refined the member-level star formation histories for the 37 spectroscopic members. Of these, 26 passed the photometric-quality cuts used for SFH analysis. The oldest stars in the red-sequence galaxies were found to span roughly 0.5 Gyr to $(34.43422^\circ,-3.75880^\circ)$5 Gyr, with characteristic decline times $(34.43422^\circ,-3.75880^\circ)$6–0.3 Gyr, and the oldest members tended to have the longest $(34.43422^\circ,-3.75880^\circ)$7. The BCG itself was fit with age 2.24 Gyr, $(34.43422^\circ,-3.75880^\circ)$8 Gyr, and $(34.43422^\circ,-3.75880^\circ)$9; several other very old members had ages 2.2–2.5 Gyr and $z\approx2$0 near 11 (Trudeau et al., 2022).

4. Outskirts, radial structure, and environmental transformation

The outskirts of XLSSC 122 were subsequently mapped with HST from the core to 3 Mpc, corresponding to about $z\approx2$1 for $z\approx2$2 kpc. This program combined the original core observations with eight outer fields and expanded the photometric and spectroscopic census to 63 gold, 11 silver, and 8 bronze members; the confirmed gold+silver sample therefore rose to 74 spectroscopic members, with 35 new member galaxies added in the outer regions (Todd et al., 20 Jan 2026).

Using the incompleteness-corrected projected galaxy-number-density profile, the study fit a projected NFW-like profile within $z\approx2$3 and obtained a scale radius $z\approx2$4 kpc. The radial profile was described as smoothly declining and approximately spherically symmetric, with no clear evidence for infalling groups or cosmic filaments in the spectroscopically confirmed galaxy distribution. A small overdensity near 500 kpc was only at the $z\approx2$5–$z\approx2$6 level and was not taken as secure substructure (Todd et al., 20 Jan 2026).

The outer-field data strengthened the evidence for strong environmental processing. The member population shows a clear color bimodality, and the red fraction rises from $z\approx2$7 in the outskirts to $z\approx2$8 in the core for galaxies with $z\approx2$9. The same work quotes an overall cluster red fraction of $z\sim2$0, while a comparable CANDELS field sample gives $z\sim2$1. Because the bimodality remains sharp and the intermediate-color population is sparse, the authors interpreted the radial change as indicative of rapid quenching upon infall, consistent with a fast mechanism such as ram-pressure stripping (Todd et al., 20 Jan 2026).

The red-member luminosity function was fit with a Schechter form and yielded $z\sim2$2 and $z\sim2$3. The faint-end slope was described as similar, within large uncertainties, to those of $z\sim2$4 clusters of comparable temperature, while the characteristic magnitude was about 1 mag fainter. The paper treated that as evidence that XLSSC 122 already has a cluster-like red-galaxy population, but not one that is simply a passively evolved analog of lower-redshift systems (Todd et al., 20 Jan 2026).

5. Weak lensing, mass distribution, and merger state

The first weak-lensing measurement of XLSSC 122 with HST/WFC3-IR used 592 background galaxies, corresponding to $z\sim2$5, and reconstructed a $z\sim2$6 mass peak. Assuming a spherical NFW halo centered on the X-ray peak, that analysis obtained

$z\sim2$7

The same study reported that the lensing peak is spatially consistent with the X-ray peak, BCG, and SZ peak within the quoted centroid uncertainties, with offsets of $z\sim2$8 from the X-ray peak, $z\sim2$9 from the BCG, and $35''\approx r_{500}$0 from the newer SZ peak (Kim et al., 22 Jul 2025).

JWST/NIRCam then transformed the quality of the lensing constraints. Using deep F090W, F200W, F277W, and F356W imaging, the JWST study assembled a background sample of 4944 galaxies, including 2463 in module A, at a density of about $35''\approx r_{500}$1. The weak-lensing mass reconstruction produced a $35''\approx r_{500}$2 unsmoothed peak and a $35''\approx r_{500}$3 smoothed peak. Modeling the cluster with a single spherical NFW halo centered on the BCG and comparing several mass–concentration prescriptions, the preferred solution gave

$35''\approx r_{500}$4

with the Prada et al. relation favored over several alternatives by $35''\approx r_{500}$5–7.7 (Scofield et al., 11 Dec 2025).

The JWST analysis also established that the mass peak coincides with both the X-ray peak and the BCG, while the mass map is elongated along a NE–SW axis. The same axis is seen in the X-ray emission, intracluster light, and cluster-member distribution. In contrast, the SZ pressure peak is offset from the mass/X-ray/BCG complex by $35''\approx r_{500}$6–$35''\approx r_{500}$7, equivalent to $35''\approx r_{500}$8–117 kpc, along the same general axis. A tentative $35''\approx r_{500}$9 MeerKAT 1.28 GHz signal is also spatially consistent with the central region, though its classification as diffuse cluster radio emission remains uncertain (Scofield et al., 11 Dec 2025).

These lensing and multiwavelength data underpin the present dynamical picture of XLSSC 122. The cluster is no longer described simply as an X-ray/SZ offset system. Rather, the main mass peak, dense X-ray core, and BCG remain coincident, while the thermal-pressure distribution is displaced and asymmetric. The resolved SZ work and the JWST weak-lensing analysis therefore converge on an active merger interpretation, with the JWST paper favoring a post-merger or post-core-passage state rather than a simple pre-merger superposition (Marrewijk et al., 2023, Scofield et al., 11 Dec 2025).

6. Assembly history and use as a probe of early structure formation

The link between galaxy ages and halo assembly has been analyzed explicitly for XLSSC 122-like haloes using MultiDark Planck 2 simulations. Weighting the assembly histories by the age posteriors of the oldest members, one study found that 74% of analogous haloes were less than 10% assembled at the onset of star formation in the oldest galaxies, 67% were less than 10% assembled when those galaxies had formed 50% of their $kT=5.0\pm0.7$0 stellar masses, and 75% were less than 30% assembled when they had formed 90% of their stellar masses. On that basis, the oldest red-sequence members were interpreted as having formed and quenched during the protocluster stage, before XLSSC 122 itself had fully virialized (Trudeau et al., 2022).

XLSSC 122 has also been used more speculatively as a fossil record of the first generations of galaxies. An exploratory Press–Schechter analysis treated the old stellar populations of its post-starburst members as constraints on star formation in progenitor haloes at $kT=5.0\pm0.7$1–13. Under the adopted no-dust age model, it inferred a rapid rise in halo star formation efficiency from about $kT=5.0\pm0.7$2 to $kT=5.0\pm0.7$3 over that interval, with representative values $kT=5.0\pm0.7$4 at $kT=5.0\pm0.7$5, $kT=5.0\pm0.7$6 at $kT=5.0\pm0.7$7, $kT=5.0\pm0.7$8 at $kT=5.0\pm0.7$9, and $Z/Z_\odot=0.33^{+0.19}_{-0.17}$0 at $Z/Z_\odot=0.33^{+0.19}_{-0.17}$1 (Liu et al., 2020).

The same exploratory framework used XLSSC 122 to test fuzzy-dark-matter suppression of early halo formation. In its fiducial conservative setup, the requirement that the cluster contain a substantial very old stellar component implied a lower allowed boson mass of approximately

$Z/Z_\odot=0.33^{+0.19}_{-0.17}$2

The authors were explicit that this result is model-dependent, relies on the $Z/Z_\odot=0.33^{+0.19}_{-0.17}$3 stellar-population solution, and is best understood as a proof of concept rather than as a precision cosmological bound (Liu et al., 2020).

Taken together, these studies explain why XLSSC 122 has become a benchmark object. It is not merely a very distant cluster; it is a system in which the ICM, dark matter, red sequence, outskirts population, and merger geometry can all be studied in detail at $Z/Z_\odot=0.33^{+0.19}_{-0.17}$4. The persistent tension between its mature baryonic content and its still-active dynamical state is therefore not a contradiction but a defining property: XLSSC 122 is a cluster whose galaxies, gas, and halo have reached different stages of evolution by cosmic noon (Willis et al., 2020, Marrewijk et al., 2023).