MQN01: A Multiphasic Cosmic Web Node at z~3.25

• Cosmic-Web Node MQN01 is a gas-rich, high-redshift structure at z ~ 3.25 characterized by a giant Lyα nebula, significant galaxy overdensity, and multiple tracers including CO emissions and X-ray AGN.
• Multiwavelength observations from VLT/MUSE, ALMA, Chandra, and JWST reveal its filamentary morphology, rapid mass assembly, and extreme AGN concentration that challenge simple quasar field interpretations.
• Extensive spectroscopic and imaging studies indicate accelerated stellar mass buildup, enhanced chemical enrichment, and a coexistence of multiphase gas components, marking MQN01 as a dynamic protocluster core.

MQN01, usually expanded as MUSE Quasar Nebula 01, is a high-redshift massive node of the cosmic web at $z \simeq 3.25$, centered on the hyperluminous quasar CTS G18.01. In the current literature it is described variously as a cosmic-web node, a protocluster, and a protocluster core, reflecting the fact that the system is identified through several convergent tracers rather than a single formal catalog definition. Those tracers include a giant Ly$\alpha$ nebula, a spectroscopically confirmed galaxy overdensity, a large excess of CO-emitting and dust-obscured galaxies, an extreme concentration of X-ray AGN, and extended X-ray emission from hot gas around the central quasar (Galbiati et al., 2024, Pensabene et al., 2024, Travascio et al., 2024, Travascio et al., 27 Aug 2025).

1. Identification and large-scale environment

MQN01 was first associated with the bright quasar environment of CTS G18.01 and a giant Ly$\alpha$-emitting structure. The ALMA survey paper describes the nebula as exceptionally extended, $\sim 30''$ in size, corresponding to $\sim 230$ physical kpc at $z \simeq 3.25$, and explicitly notes its filamentary morphology (Pensabene et al., 2024). The X-ray follow-up further characterizes MQN01 as a region containing one of the brightest Ly$\alpha$ nebulae and, in that paper’s phrasing, the largest galaxy overdensity discovered so far at $z>3$ (Travascio et al., 27 Aug 2025).

Within the inner MUSE-defined core, MQN01 contains 21 galaxies with high-confidence spectroscopic redshifts inside $4\times 4\,{\rm cMpc}^2$ and $|\Delta v|\le 1000\,{\rm km\,s^{-1}}$ from the quasar. The corresponding overdensity is reported as

$\alpha$0

for galaxies with $\alpha$1 mag (Galbiati et al., 2024). On larger scales, the FORS2 Lyman-break-galaxy mapping covers $\alpha$2 and shows that the photometric overdensity is aligned with the inner spectroscopic structure and with the extended Ly$\alpha$3 morphology, suggesting that the dense inner region may continue for a few tens of comoving Mpc (Galbiati et al., 2024).

This environment is therefore not presented as a compact isolated clump. The recurring interpretation is that MQN01 marks a dense junction of filamentary gas and galaxy structure, with a highly overdense inner core embedded in a larger anisotropic overdensity (Galbiati et al., 2024).

2. Observational basis and membership definition

MQN01 has been studied through a deliberately multiwavelength program. The core spectroscopic map is a VLT/MUSE $\alpha$4 mosaic of four $\alpha$5 arcmin$\alpha$6 pointings, with $\alpha$7 hr per pointing on average and a final footprint of $\alpha$8 arcmin$\alpha$9, corresponding to $\alpha$0 at $\alpha$1 (Galbiati et al., 2024). A companion MUSE description for the X-ray work refers to a 40 hr mosaic of four 10 hr pointings over roughly $\alpha$2 (Travascio et al., 2024). These data provide the conservative spectroscopic backbone: membership requires at least two identifiable rest-UV spectral features, so that Ly$\alpha$3 fluorescence from the quasar does not by itself define a galaxy redshift (Galbiati et al., 2024).

The wider-field optical context comes from VLT/FORS2 $\alpha$4, $\alpha$5, and $\alpha$6 imaging over $\alpha$7, yielding 535 LBG candidates through a calibrated $\alpha$8 selection (Galbiati et al., 2024). Near-infrared constraints come from VLT/HAWK-I and HST/ACS, while JWST/NIRCam pre-imaging and JWST/NIRSpec MSA spectroscopy provide rest-frame optical morphology, stellar masses, SFRs, and nebular diagnostics for selected members (Galbiati et al., 2024, Wang et al., 24 Nov 2025).

The sub-mm and molecular-gas census is based on ALMA Bands 3 and 6. The broad mosaic covers essentially the MUSE field, targeting CO(4–3), 3-mm continuum, and 1.2-mm continuum (Pensabene et al., 2024). A later high-angular-resolution ALMA Band 3 pointing with $\alpha$9 resolution focuses on the central $\sim 30''$0 around the quasar (Pensabene et al., 22 Jul 2025).

The X-ray view is based on a 634 ks Chandra ACIS-I survey. In the AGN census, the central MQN01 science region is tied to the MUSE mosaic and to spectroscopic or ALMA-confirmed members within $\sim 30''$1 of the quasar (Travascio et al., 2024). In the diffuse-halo analysis, the same Chandra dataset is re-aligned on the bright quasar itself to isolate extended soft X-ray emission after detailed PSF subtraction (Travascio et al., 27 Aug 2025).

3. Molecular gas, dust, and obscured assembly

The ALMA survey established that MQN01 is not merely overdense in UV-selected galaxies. In the $\sim 30''$2 field around the quasar, the survey identifies a robust sample of eleven CO line-emitting galaxies within $\sim 30''$3 of the quasar systemic redshift, including a closely separated quasar companion, and also eleven 1.2-mm continuum sources (Pensabene et al., 2024). A fraction of these galaxies are explicitly stated to be missed in previous deep rest-frame optical/UV surveys, which is one reason the paper treats mm imaging as essential to the baryon census of the node (Pensabene et al., 2024).

The environmental contrast relative to blank fields is quantified through the CO luminosity function and continuum number counts. Above the survey limiting CO luminosity, the cumulative overdensities are reported as

$\sim 30''$4

and the abstract summarizes the result as a galaxy overdensity of $\sim 30''$5 (Pensabene et al., 2024). The paper also finds evidence of a systematic flattening at the bright end of the CO luminosity function with respect to blank fields, implying that the excess is not only numerical but also weighted toward very luminous CO emitters (Pensabene et al., 2024).

The aggregate molecular-gas density is correspondingly large. Using the adopted CO(4–3) to CO(1–0) conversion framework, the authors derive

$\sim 30''$6

for the broader $\sim 30''$7 sample, and

$\sim 30''$8

for the secure $\sim 30''$9 sample without completeness correction (Pensabene et al., 2024). The paper interprets this as roughly an order-of-magnitude enhancement over the expected cosmic molecular-gas density at similar redshift (Pensabene et al., 2024).

Within the apparent central core, the CO-member velocity distribution inside $\sim 230$0 is approximately Gaussian, with

$\sim 230$1

although the same paper cautions that any virial interpretation is uncertain and likely conservative (Pensabene et al., 2024).

4. AGN concentration and the hot phase

MQN01 is also exceptional as an AGN environment. Combining Chandra, MUSE, and ALMA data, the X-ray AGN census identifies six X-ray AGN within a region of $\sim 230$2 and $\sim 230$3 around the quasar, corresponding to an X-ray AGN overdensity of $\sim 230$4 (Travascio et al., 2024). The same study reports an AGN fraction

$\sim 230$5

with the fraction increasing with stellar mass and reaching

$\sim 230$6

in the observed sample (Travascio et al., 2024).

The AGN study also characterizes the accretion intensity through the specific black-hole accretion rate

$\sim 230$7

with $\sim 230$8 (Travascio et al., 2024). The MQN01 AGN population is described as having a higher average $\sim 230$9 than the field and than the comparison protoclusters Spiderweb and SSA22, with evidence that $z \simeq 3.25$0 generally increases toward the center of the overdensity (Travascio et al., 2024). For the combined MQN01+Spiderweb sample, the radial trend yields mean Pearson and Spearman coefficients of $z \simeq 3.25$1 and $z \simeq 3.25$2, respectively (Travascio et al., 2024).

A distinct but related result is the detection of extended soft X-ray emission around the central quasar. After PSF and background subtraction, the diffuse-halo paper reports $z \simeq 3.25$3 net counts in the observed 0.5–2 keV band, at $z \simeq 3.25$4 significance, extending to at least $z \simeq 3.25$5 kpc from the QSO (Travascio et al., 27 Aug 2025). A joint spatial-spectral MCMC analysis favors thermal plasma in CIE with

$z \simeq 3.25$6

a steep $z \simeq 3.25$7-model profile,

$z \simeq 3.25$8

and a central electron density

$z \simeq 3.25$9

(Travascio et al., 27 Aug 2025).

Under the virialized-halo interpretation, the same analysis gives

$\alpha$0

and a hot-gas mass

$\alpha$1

equivalent to $\alpha$2 of $\alpha$3, or $\alpha$4 of the theoretical cosmological baryon budget of the halo (Travascio et al., 27 Aug 2025). The inferred X-ray luminosity is extremely high for this temperature, and the system is described as a clear outlier in the $\alpha$5-$\alpha$6 plane (Travascio et al., 27 Aug 2025).

The hot phase is also dynamically interesting. The cooling times are reported as

$\alpha$7

while

$\alpha$8

placing the inner halo in the commonly discussed precipitation regime (Travascio et al., 27 Aug 2025). This suggests a multiphase medium in which dense cool clumps and hot pressure support coexist.

5. Galaxy populations, chemical enrichment, and quenching

Despite the extreme environment, MQN01 does not appear to host a universal SFR enhancement at fixed stellar mass. The galaxy-growth study concludes that MQN01 members are forming stars at rates consistent with the main sequence at $\alpha$9, and that SFR is regulated primarily by local properties correlated with stellar mass rather than by environment alone (Galbiati et al., 2024). The environmental signature instead appears most strongly in the stellar-mass distribution: the high-mass end of the stellar mass function is significantly elevated relative to the field above

$z>3$0

(Galbiati et al., 2024). Relative to the overdensity-scaled field expectation, the paper reports $z>3$1 and $z>3$2 times more sources in the two highest stellar-mass bins (Galbiati et al., 2024). This is interpreted as evidence that massive galaxies in overdense regions build up their stellar mass earlier or more efficiently than galaxies in average environments (Galbiati et al., 2024).

The metallicity study sharpens that picture. Using 9 star-forming galaxies with JWST/NIRSpec detections of $z>3$3, [OIII], $z>3$4, and [NII], spanning $z>3$5 to $z>3$6, the paper finds that MQN01 galaxies have higher [NII]$z>3$7/$z>3$8 and lower [OIII]$z>3$9/$4\times 4\,{\rm cMpc}^2$0 than typical field galaxies at similar redshift, implying a metallicity enhancement of about

$4\times 4\,{\rm cMpc}^2$1

with respect to the field mass-metallicity relation (Wang et al., 24 Nov 2025). The same paper fits the MQN01 MZR with

$4\times 4\,{\rm cMpc}^2$2

and for the full MQN01 sample reports $4\times 4\,{\rm cMpc}^2$3, $4\times 4\,{\rm cMpc}^2$4, $4\times 4\,{\rm cMpc}^2$5, corresponding to

$4\times 4\,{\rm cMpc}^2$6

(Wang et al., 24 Nov 2025). When the fundamental metallicity relation is written through

$4\times 4\,{\rm cMpc}^2$7

the discrepancy with field galaxies becomes less significant, which the authors interpret as broadly consistent with accelerated or earlier stellar assembly rather than radically different regulator physics (Wang et al., 24 Nov 2025).

MQN01 also contains a striking quiescent counterexample to the otherwise gas-rich, star-forming node. The passive galaxy MQN01 J004131.9-493704, nicknamed the Red Potato, lies within the MQN01 structure at

$4\times 4\,{\rm cMpc}^2$8

and has

$4\times 4\,{\rm cMpc}^2$9

placing it more than 1 dex below the star-forming main sequence (Wang et al., 28 Jan 2026). ALMA gives no detectable molecular gas, with

$|\Delta v|\le 1000\,{\rm km\,s^{-1}}$0

for $|\Delta v|\le 1000\,{\rm km\,s^{-1}}$1, and a molecular-gas fraction

$|\Delta v|\le 1000\,{\rm km\,s^{-1}}$2

(Wang et al., 28 Jan 2026). Yet the galaxy is embedded in a large cool-gas reservoir traced by Ly$|\Delta v|\le 1000\,{\rm km\,s^{-1}}$3, H$|\Delta v|\le 1000\,{\rm km\,s^{-1}}$4, and [OIII], with an Ly$|\Delta v|\le 1000\,{\rm km\,s^{-1}}$5 halo diameter of $|\Delta v|\le 1000\,{\rm km\,s^{-1}}$6 kpc at a surface-brightness limit of $|\Delta v|\le 1000\,{\rm km\,s^{-1}}$7 (Wang et al., 28 Jan 2026). The favored interpretation is that inefficient gas accretion from the CGM, possibly maintained by a nearby AGN jet at projected distance $|\Delta v|\le 1000\,{\rm km\,s^{-1}}$8 kpc, has kept the galaxy passive despite its gas-rich environment (Wang et al., 28 Jan 2026).

6. Central dynamics and physical interpretation

The central $|\Delta v|\le 1000\,{\rm km\,s^{-1}}$9 kpc around the quasar contain a particularly informative galaxy pair. High-resolution ALMA imaging resolves a close companion, MQN01-QC, at approximately $\alpha$00 projected distance and $\alpha$01 from the quasar host (Pensabene et al., 22 Jul 2025). The companion is a massive rotating disk with a clear south–north CO velocity gradient and best-fit kinematic parameters

$\alpha$02

while an independent \textsc{GalPak}$\alpha$03 analysis gives a consistent $\alpha$04 (Pensabene et al., 22 Jul 2025). Its enclosed dynamical mass is

$\alpha$05

and the paper highlights it as the first quasar companion galaxy confirmed as a massive, dynamically cold rotating disk at such an early epoch (Pensabene et al., 22 Jul 2025).

The quasar host itself is more compact and kinematically less settled. Its CO(4–3) emission contains a narrow systemic component plus a broad blueshifted wing with

$\alpha$06

which the paper interprets either as a molecular outflow or as interaction-induced disturbance (Pensabene et al., 22 Jul 2025). Under the outflow interpretation, the estimated molecular outflow rate is

$\alpha$07

with an outflow depletion time of $\alpha$08 Myr (Pensabene et al., 22 Jul 2025).

At larger scales, the system is manifestly multiphase. Cold molecular gas, dust, UV-selected galaxies, X-ray AGN, giant Ly$\alpha$09 emission, and hot soft X-ray plasma all coexist within the same node. Different analyses, however, probe different scales and dynamical states. The ALMA core study reports a tentative virialized-core estimate of

$\alpha$10

from the central CO-member kinematics (Pensabene et al., 2024), whereas the diffuse X-ray halo interpretation yields

$\alpha$11

for the hot halo around the quasar (Travascio et al., 27 Aug 2025). This suggests that current mass estimates for MQN01 are strongly method- and aperture-dependent, and that the node is better understood as a forming, structured overdensity than as a single relaxed halo.

A recurrent misconception is that MQN01 is simply a quasar field or, conversely, that it is already a mature cluster in the low-redshift sense. The published evidence supports neither simplification. The system is quasar-centered, but its defining properties are environmental: a spectroscopic galaxy overdensity, a CO and dust excess, an extreme AGN concentration, and a hot halo embedded in a filamentary Ly$\alpha$12 structure (Galbiati et al., 2024, Pensabene et al., 2024, Travascio et al., 2024, Travascio et al., 27 Aug 2025). At the same time, its internal diversity is pronounced: ordinary main-sequence star formers, chemically advanced massive galaxies, a passive massive galaxy in bright cool CGM, a dynamically cold massive disk, and a disturbed quasar host all coexist within the same node (Wang et al., 24 Nov 2025, Wang et al., 28 Jan 2026, Pensabene et al., 22 Jul 2025).

Taken together, MQN01 is best characterized as a gas-rich, AGN-rich, chemically evolved, multiphase cosmic-web node at $\alpha$13, in which accelerated mass assembly is already evident at the high-mass end even though star formation at fixed stellar mass remains broadly consistent with the $\alpha$14 main sequence.