--- title: 'MQN01: A Multiphasic Cosmic Web Node at z~3.25' url: https://www.emergentmind.com/topics/cosmic-web-node-mqn01 type: topic --- # MQN01: A Multiphasic Cosmic Web Node at z~3.25 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 [2410.03822], [2401.04765], [2410.03933], [2508.20074]. ## 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** [2401.04765]. 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 \(z>3\)** [2508.20074]. 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 \[ \frac{\rho}{\bar{\rho}} = 53 \pm 17, \] for galaxies with \(M_R\le -19.25\) mag [2410.03822]. On larger scales, the FORS2 Lyman-break-galaxy mapping covers \(\approx 24\times 24\,{\rm cMpc}^2\) and shows that the photometric overdensity is aligned with the inner spectroscopic structure and with the extended Ly\(\alpha\) morphology, suggesting that the dense inner region may continue for **a few tens of comoving Mpc** [2410.03822]. 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 [2410.03822]. ## 2. Observational basis and membership definition MQN01 has been studied through a deliberately multiwavelength program. The core spectroscopic map is a **VLT/MUSE** \(2\times 2\) mosaic of four \(1\times1\) arcmin\(^2\) pointings, with \(\sim 10\) hr per pointing on average and a final footprint of \(\approx 2\times2\) arcmin\(^2\), corresponding to \(4\times4\,{\rm cMpc}^2\) at \(z\approx 3.25\) [2410.03822]. A companion MUSE description for the X-ray work refers to a **40 hr mosaic of four 10 hr pointings** over roughly \(2\times2\,{\rm arcmin}^2\) [2410.03933]. These data provide the conservative spectroscopic backbone: membership requires at least two identifiable rest-UV spectral features, so that Ly\(\alpha\) fluorescence from the quasar does not by itself define a galaxy redshift [2410.03822]. The wider-field optical context comes from **VLT/FORS2** \(U\), \(B\), and \(R\) imaging over \(\approx 24\times24\,{\rm cMpc}^2\), yielding **535 LBG candidates** through a calibrated \(UBR\) selection [2410.03822]. 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 [2410.03822], [2511.19608]. 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 [2401.04765]. A later high-angular-resolution ALMA Band 3 pointing with \(\simeq 0.3''\) resolution focuses on the central \(\sim 150\,{\rm kpc}\) around the quasar [2507.16921]. 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 \(\pm 1000\,{\rm km\,s^{-1}}\) of the quasar [2410.03933]. 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 [2508.20074]. ## 3. Molecular gas, dust, and obscured assembly The ALMA survey established that MQN01 is not merely overdense in UV-selected galaxies. In the \( \sim 2'\times 2' \) field around the quasar, the survey identifies a **robust sample of eleven CO line-emitting galaxies** within \(\pm 4000\,{\rm km\,s^{-1}}\) of the quasar systemic redshift, including a closely separated quasar companion, and also **eleven** 1.2-mm continuum sources [2401.04765]. 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 [2401.04765]. 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 \[ \delta_{\rm CO}^{4000}(>L'_{\rm lim})=14^{+9}_{-6}, \qquad \delta_{\rm CO}^{1000}(>L'_{\rm lim})=18^{+14}_{-8}, \] and the abstract summarizes the result as a galaxy overdensity of \(\delta>10\) [2401.04765]. 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 [2401.04765]. The aggregate molecular-gas density is correspondingly large. Using the adopted CO(4–3) to CO(1–0) conversion framework, the authors derive \[ \rho({\rm H_2})=3.7^{+1.0}_{-0.9}\times10^8\ M_\odot\,{\rm cMpc^{-3}} \] for the broader \(\Delta v<4000\,{\rm km\,s^{-1}}\) sample, and \[ \rho({\rm H_2})=(1.0\pm0.3)\times10^9\ M_\odot\,{\rm cMpc^{-3}} \] for the secure \(\Delta v<1000\,{\rm km\,s^{-1}}\) sample without completeness correction [2401.04765]. The paper interprets this as roughly an order-of-magnitude enhancement over the expected cosmic molecular-gas density at similar redshift [2401.04765]. Within the apparent central core, the CO-member velocity distribution inside \(\Delta v_{\rm QSO}<1000\,{\rm km\,s^{-1}}\) is approximately Gaussian, with \[ v_0 = -137^{+84}_{-86}\ {\rm km\,s^{-1}}, \qquad \sigma = 208^{+45}_{-59}\ {\rm km\,s^{-1}}, \] although the same paper cautions that any virial interpretation is uncertain and likely conservative [2401.04765]. ## 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 \(16\,{\rm cMpc}^2\) and \(\pm 1000\,{\rm km\,s^{-1}}\) around the quasar, corresponding to an **X-ray AGN overdensity of \(\sim 1000\)** [2410.03933]. The same study reports an AGN fraction \[ f_{\rm AGN} > 20\%, \] with the fraction increasing with stellar mass and reaching \[ f_{\rm AGN}=1 \quad\text{for}\quad \log(M_\*/M_\odot)>10.5 \] in the observed sample [2410.03933]. The AGN study also characterizes the accretion intensity through the specific black-hole accretion rate \[ \lambda_{\text{sBHAR}} = \frac{k_{\text{bol}} L_X}{1.3 \times 10^{38} \times 0.002 \times M_\*}, \] with \(k_{\rm bol}=25\) [2410.03933]. The MQN01 AGN population is described as having a higher average \(\lambda_{\rm sBHAR}\) than the field and than the comparison protoclusters Spiderweb and SSA22, with evidence that \(\lambda_{\rm sBHAR}\) generally increases toward the center of the overdensity [2410.03933]. For the combined MQN01+Spiderweb sample, the radial trend yields mean Pearson and Spearman coefficients of \(-0.71\) and \(-0.76\), respectively [2410.03933]. 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 \(\approx 66\pm 8\) net counts in the observed 0.5–2 keV band, at \(\sim 8\sigma\) significance, extending to at least \(\sim 30\) kpc from the QSO [2508.20074]. A joint spatial-spectral MCMC analysis favors thermal plasma in CIE with \[ kT = 1.81^{+0.38}_{-0.37}\ {\rm keV}, \] a steep \(\beta\)-model profile, \[ \beta=2.04^{+1.35}_{-0.75}, \qquad r_{\rm core}=36^{+16}_{-13}\ {\rm kpc}, \] and a central electron density \[ n_{e,0}=0.86^{+0.48}_{-0.19}\ {\rm cm^{-3}} \] [2508.20074]. Under the virialized-halo interpretation, the same analysis gives \[ M_{\rm vir}\simeq (3\pm1)\times10^{13}\ M_\odot, \qquad R_{\rm vir}=190^{+19}_{-20}\ {\rm kpc}, \] and a hot-gas mass \[ M_{\rm hot}(