---
title: 'MUSEQuBES: Quasar-field Blind Emitters Survey'
url: https://www.emergentmind.com/topics/muse-quasar-fields-blind-emitters-survey-musequbes
type: topic
---

# MUSEQuBES: Quasar-field Blind Emitters Survey

MUSEQuBES, the **MUSE Quasar-fields Blind Emitters Survey**, is a quasar-field program that combines **MUSE integral-field spectroscopy** with absorption spectroscopy of the same background quasars to study the circumgalactic and intergalactic gas around galaxies across both high and low redshift. Its defining feature is a **blind** galaxy census in quasar fields: foreground galaxies are identified spectroscopically from the MUSE datacubes rather than from photometric preselection, and their gaseous environments are then constrained with H I, C IV, and O VI absorption seen in the quasar spectra. In practice, the survey has developed into two closely linked branches: a high-redshift program centered on \(z\approx3.3\) Ly\(\alpha\) emitters (LAEs), and a low-redshift program targeting mostly sub-\(L_*\) galaxies at \(0.1<z<0.7\) with HST/COS absorption-line spectroscopy [2105.05260][2409.15423].

## 1. Survey definition and program structure

In the high-\(z\) branch, MUSEQuBES uses **8 MUSE fields of \(1'\times1'\)** centered on **8 bright background quasars** to identify faint LAEs and connect them to H I and C IV absorption in high-resolution quasar spectra. In the low-\(z\) branch, the survey uses **16 quasar fields** with deep MUSE observations and archival high-S/N HST/COS far-UV spectroscopy to build a blind census of foreground galaxies and connect them to H I and O VI absorption [2105.05260][2409.15423].

The survey architecture is deliberately symmetric between emission and absorption. MUSE provides the galaxy catalog, environment, morphology, and in many cases star-formation diagnostics, while the background quasar provides a pencil-beam probe of diffuse gas along the same field. This design is especially important for low-mass galaxies, because it reduces the incompleteness that affects targeted galaxy surveys and makes it possible to extend CGM studies well below the usual \(\sim L_*\) regime [2409.15423][2602.17593].

| Survey component | Core sample | Representative result |
|---|---|---|
| High-\(z\) LAE CGM | 96 LAEs in 8 MUSE fields | Excess H I and C IV out to \(\approx250\) pkpc and \(\approx500\) km s\(^{-1}\) [2105.05260] |
| High-\(z\) absorber catalogs | 86 LAEs for C IV; 96 LAEs for H I | C IV covering fraction \(\approx60\%\) at \(\log N({\rm C\,IV})=12.5\); H I covering fraction \(\approx88\%\) at \(\log N({\rm H\,I})=15\) [2304.04788][2411.11959] |
| Low-\(z\) H I mapping | 4595 \(z<0.5\) galaxies, 184 quasars, 5054 pairs | Excess absorption out to at least \(\sim15\,R_{\rm vir}\) and \(\sim600\) km s\(^{-1}\) [2303.16933] |
| Low-\(z\) O VI mapping | 247 galaxies suitable for O VI analysis | \(\log_{10}\langle N({\rm OVI})/{\rm cm}^{-2}\rangle = 14.14^{+0.09}_{-0.10}\) within \(R_{\rm vir}\) [2409.15423] |

## 2. Observational design and analysis framework

MUSEQuBES is methodologically built around blind source finding in three-dimensional quasar-field datacubes. In the high-\(z\) program, LAEs were extracted from MUSE cubes after empirical quasar PSF subtraction with **CubePSFSub** and continuum subtraction with **CubeBKGSub**, then identified with **CubEx/CubExtractor** using connected-voxel criteria. In the low-\(z\) program, galaxy redshifts were measured with **MARZ** and refined with a modified **PLATEFIT**, stellar masses were derived with **FAST**, and absorber catalogs were built with **VPFIT** from the quasar spectra [2105.05260][2409.15423].

A distinctive feature of the survey is that absorber catalogs are often constructed in a **galaxy-blind** manner and only then cross-matched to galaxies. At \(z\approx3.3\), the C IV analysis used a blind catalog of **489 C IV absorption components** in **152 systems** over \(2.900\le z\le3.835\), later matched to **86 LAEs** [2304.04788]. The corresponding H I work around the same LAEs used Voigt-profile fitting of all H I absorbers within \(\pm500\) km s\(^{-1}\) of the LAEs and compiled a catalog of **800 H I absorption components** [2411.11959]. At low redshift, the O VI catalog contains **118 O VI components** over \(0.13<z<0.71\), grouped into **67 systems**, and the resolved H I study identifies **227 H I components** in **103 unique H I systems** around **256 galaxies** [2409.15432][2507.22283].

The program also developed morphology-aware analyses that go beyond simple impact-parameter scaling. In the low-\(z\) anisotropy study, MUSEQuBES combined MUSE-based galaxy properties with \(HST\) imaging, a Bayesian wrapper around **GALFIT**, and posterior-based propagation of azimuthal-angle uncertainties. The azimuthal angle was defined from the galaxy position angle and the galaxy–quasar direction, folded into \(0^\circ\)–\(90^\circ\), so that \(\phi=0^\circ\) corresponds to the projected major axis and \(\phi=90^\circ\) to the projected minor axis [2602.17593].

## 3. High-redshift branch: Ly\(\alpha\) emitters, H I, C IV, and filamentary structure

The first statistical high-\(z\) characterization focused on **96 LAEs** at median redshift **\(z=3.3\)**, with typical \(L({\rm Ly}\alpha)\sim10^{42}\) erg s\(^{-1}\), dust-uncorrected SFRs of \(\sim1\,M_\odot\,{\rm yr}^{-1}\), a characteristic stellar mass of \(\sim10^{8.6}\,M_\odot\), and median LAE–quasar impact parameter **165 pkpc**. Stacking of the high-resolution quasar spectra showed significant excess H I and C IV absorption near the LAEs out to **500 km s\(^{-1}\)** and at least **\(\approx250\) pkpc**, corresponding to about **\(\approx7\)** virial radii. At \(\lesssim30\) km s\(^{-1}\) from the galaxies, the median H I and C IV optical depths are enhanced by an order of magnitude [2105.05260].

That stacked picture was sharpened by absorber-by-absorber analyses. The C IV study showed that C IV components cluster around LAEs within roughly **\(\pm400\) km s\(^{-1}\)**, that the covering fraction is **\(\approx60\%\)** for a threshold \(N({\rm C\,IV})=10^{12.5}\,{\rm cm}^{-2}\), and that the covering fraction remains at **\(\approx50\%\)** for impact parameters **150–250 pkpc**, or about **\(3\)–\(6\,R_{200}\)**. Using the covering-fraction profile, the LAE–C IV absorber two-point correlation function was constrained to **\(r_0=3.4\) cMpc** and **\(\gamma=1.2\)** for a threshold \(N({\rm C\,IV})=10^{13.0}\,{\rm cm}^{-2}\) [2304.04788]. The H I component analysis reached a complementary conclusion: H I absorption is enhanced near the LAEs compared to the IGM, but no trend is found between the H I column densities and impact parameters over \(\approx54\) to 260 pkpc. The LAEs show an overall H I covering fraction of **\(\approx88\%\)** for a threshold \(\log N({\rm H\,I})=15\), while pairs/group LAEs exhibit **100%** H I covering fraction out to \(\approx250\) pkpc and isolated LAEs remain near **\(\approx80\%\)** [2411.11959].

Environment is a recurrent result in the high-\(z\) branch. The 2021 stacking work found that about one-third of the LAEs classified as “group” systems have significantly stronger H I and marginally stronger C IV than isolated LAEs, which was attributed to the larger-scale structures in which they are embedded [2105.05260]. The later absorber-level analyses converged on the same interpretation: pair/group LAEs have higher C IV covering fractions than isolated LAEs, and H I covering fractions in differential LOS-velocity bins remain systematically higher for pairs/groups up to \(\approx300\) km s\(^{-1}\) [2304.04788][2411.11959].

MUSEQuBES has also produced a filament-scale case study at **\(z\approx3.577\)** in the Q1317\(-\)0507 field. There, the survey identified a group of **seven LAEs** with a statistically unusual projected alignment, a set of low-metallicity H I absorbers including an extremely metal-poor partial Lyman limit system with \([X/H]=-3.69^{+0.08}_{-0.08}\) for HM05 and \(\log_{10} n_{\rm H}/{\rm cm^{-3}}=-3.95^{+0.20}_{-0.20}\), and a giant Ly\(\alpha\)-emitting nebula with surface brightness \(\ge 10^{-19}\) erg cm\(^{-2}\) s\(^{-1}\) arcsec\(^{-2}\) and projected size \(\approx260\) pkpc aligned with the LAEs. The authors interpret this as the first detection of giant Ly\(\alpha\) emission tracing cosmic filaments and associated with normal LAEs rather than a luminous quasar or quasar pair [2412.04546].

## 4. Low-redshift branch: neutral hydrogen, O VI, and CGM anisotropy

The low-\(z\) H I program was first developed through statistical stacking. A sample of **4595 \(z<0.5\) galaxies** with median stellar mass \(\log(M_*/M_\odot)=10.0\) was probed by **184 background quasars**, producing **5054 quasar–galaxy pairs** with median impact parameter **\(b=1.5\) pMpc** and median \(b/R_{\rm vir}=10.4\). Excess H I absorption was detected out to at least **\(\sim15\,R_{\rm vir}\)** in the transverse direction and **\(\sim600\) km s\(^{-1}\)** along the line of sight. The median stacked profile for the full sample was described by a galaxy–absorber two-point correlation function with **\(r_0=7.6\) pMpc** and **\(\gamma=-1.57\)**, while the inner regions were better explained by a log-linear or Gaussian relation and the outer regions by a power law [2303.16933].

Resolved component analysis then moved this branch from stacks to direct \(N({\rm H\,I})\), covering-fraction, mass, and kinematic measurements. Using **256 galaxies** with median stellar mass \(\log_{10}(M_*/M_\odot)=8.7\), median redshift \(z=0.48\), and median impact parameter \(D\sim140\) pkpc, the survey showed that the H I profile around **isolated, star-forming galaxies** is well fit by a power law with slope **\(\sim -2.3\)** as a function of \(D/R_{\rm vir}\). For a threshold \(\log_{10}(N({\rm H\,I})/{\rm cm}^{-2})=14\), the H I covering fraction within the virial radius is significantly lower for high-mass passive galaxies than for isolated star-forming counterparts, while the covering-fraction profile of isolated star-forming galaxies implies a characteristic H I-rich CGM size of **\(\sim1.5\,R_{\rm vir}\)** across the stellar-mass range. The mean H I mass in the outer CGM, \(0.3\)–\(1\,R_{\rm vir}\), rises from **\(\sim10^5\)** to **\(\sim10^{6.6}\,M_\odot\)** with stellar mass, and non-isolated galaxies show an H I-rich environment that extends about **three times further** than for isolated galaxies [2507.22283].

The O VI branch established an equally strong mass dependence. In the main low-\(z\) O VI census, **247 low-mass galaxies** with median \(\log(M_*/M_\odot)\sim8.7\) were searched for O VI, and **60** showed associated absorption. For star-forming galaxies, the area-weighted average column density within the virial radius is \(\log_{10}\langle N({\rm OVI})/{\rm cm}^{-2}\rangle = 14.14^{+0.09}_{-0.10}\), significantly below the value for \(L_*\) galaxies. After combining **176 MUSEQuBES star-forming galaxies** with **253 star-forming galaxies from the literature**, both the average O VI column density and the average covering fraction were found to peak near \(\log_{10}(M_*/M_\odot)\approx9.5\). The characteristic normalized impact parameter where the O VI covering fraction falls to half its peak value is largest, about **\(1.1\,R_{\rm vir}\)**, at the same stellar mass. For dwarf galaxies with \(\log(M_*/M_\odot)<9\), the average O VI mass within the virial radius is **\(10^{5.2^{+0.1}_{-0.1}}\,M_\odot\)**, and stacked spectra suggest a highly ionized metal floor of \(\log_{10}(N({\rm OVI})/{\rm cm}^{-2})=13.2\) outside the virial radius [2409.15423].

The accompanying O VI kinematics paper showed that most O VI within \(R_{\rm vir}\) is consistent with being gravitationally bound, even for low-mass halos. The LOS velocity distribution of O VI components has mean \(\langle v_{\rm LOS}\rangle \approx 18\pm7\) km s\(^{-1}\) and dispersion \(\sigma_{v_{\rm LOS}}\approx86\pm7\) km s\(^{-1}\), and the upper-limit bound fraction within \(R_{\rm vir}\) is **\(<0.94\)** for all galaxies. The kinematic spread, as measured by the pixel-velocity two-point correlation function, is larger around higher-mass galaxies in absolute units, but this difference largely disappears for isolated galaxies when pixel velocities are normalized by the halo circular velocity [2409.15432].

A further extension of the low-\(z\) program addressed CGM anisotropy. Using **113 isolated galaxies** over \(0.12<z<0.75\), including **91 new measurements from MUSEQuBES**, the survey found that the H I covering fraction within \(R_{\rm vir}\) for low-mass galaxies with \(7<\log_{10}(M_*/M_\odot)<9\) and threshold \(\log_{10}(N({\rm HI})/{\rm cm}^{-2})=14.5\) is enhanced both along the disk plane, \(\phi<20^\circ\), and in the polar direction, \(\phi>70^\circ\). A similar, though tentative, bimodality appears for O VI in low-mass star-forming galaxies, whereas higher-mass galaxies show much weaker or absent azimuthal dependence. In contrast, the O VI pixel-velocity TPCF is significantly narrower along the disk plane than along the polar direction, with a reported difference of about **\(6\sigma\)** [2602.17593].

## 5. Relation to quasar-nebula studies and related MUSE programs

MUSEQuBES emerged within a broader MUSE quasar-field tradition. Several earlier or closely related MUSE studies established the observational logic later central to the survey: blind quasar-field IFU mapping of Ly\(\alpha\) nebulae, blind line-emitter searches, dark-galaxy candidate identification, and absorber-environment studies. A foundational example is the blind MUSE survey of **17 bright radio-quiet quasars** at \(3<z<4\), which found a **100% detection rate** of giant Ly\(\alpha\) nebulae with projected sizes \(>100\) pkpc and surface-brightness profiles consistent with power laws of slope about **\(-1.8\)** [1605.01422]. Another closely related study used 6 medium-deep MUSE quasar fields at \(z>3\) to identify fluorescent dark-galaxy candidates, finding **6** continuum-undetected sources with \(EW_0({\rm Ly}\alpha)>240\) Å at \(z>3.5\) and deriving a lower limit of about **60 Myr** on quasar lifetime if the most distant candidate is fluorescently illuminated [1709.03522]. Absorber-targeted MUSE work around very metal-poor LLSs also provided a direct precursor to the MUSEQuBES absorption–emission strategy, including the discovery of **five LAEs** around a pristine \(z\sim3.1\) LLS where only **\(\sim0.4\)** were expected [1607.03893].

A parallel MUSE quasar-nebula effort, **QSO MUSEUM**, extended this approach to a homogeneous sample of **61 quasars** at \(3.03<z<3.46\). It found extended Ly\(\alpha\) emission around **all 61** quasars, with average maximum projected extent of about **80 kpc**, bulk emission within **\(R<50\) kpc**, average exponential scale length **\(r_h=15.7\pm0.5\) kpc**, and relatively quiescent kinematics with \(\langle \sigma_{\rm Ly\alpha}\rangle<400\) km s\(^{-1}\). Only one system qualified as an ELAN, implying an occurrence rate of roughly **\(\sim1\%\)** when combined with the Borisova et al. sample [1808.10857]. A more targeted MUSE follow-up of Q2059-360 then showed how a proximate damped Ly\(\alpha\) absorber can act as a natural coronagraph, revealing a LAB with total size of about **120 pkpc** and two probable LAEs at projected separations of **265 kpc** and **207 kpc** [1705.05728].

MUSEQuBES also overlaps with the intermediate-redshift quasar-nebula regime. In a study combining **CUBS** and **MUSEQuBES**, deep MUSE observations of **30 UV-luminous quasars** at \(z=0.4\)–1.4 found [O II]-emitting circumgalactic nebulae around the majority of them: **7** with major-axis sizes \(>100\) kpc, **20** \(>50\) kpc, and **27** \(>20\) kpc. The authors interpreted these nebulae as evidence that cool, dense, and metal-enriched circumgalactic gas is common in the halos of luminous quasars at intermediate redshift [2404.00088].

## 6. Interpretation, limitations, and scientific significance

A central strength of MUSEQuBES is that it does not treat galaxies, absorbers, and environment as separable problems. Across both redshift branches, the survey repeatedly finds that gas properties depend not only on halo-scale distance but also on galaxy type, stellar mass, and local overdensity. At \(z\approx3.3\), pairs/group LAEs show systematically stronger H I and C IV than isolated LAEs, and a dedicated case study links metal-poor absorbers, LAE overdensity, and giant Ly\(\alpha\) emission to a filamentary structure [2105.05260][2412.04546]. At low redshift, non-isolated galaxies show H I-rich environments extending about three times farther than isolated galaxies, while rich groups often lack detectable O VI altogether [2507.22283][2409.15432]. This suggests that the survey’s most durable result is not simply the measurement of halo profiles, but the demonstration that the CGM and adjacent IGM are strongly structured by environment.

The survey also makes clear that absorber–host association is intrinsically ambiguous. In the high-\(z\) H I analysis, all galaxies associated with LLSs have impact parameters \(>50\) pkpc, which led to the suggestion that the true absorber hosts may be too faint to detect [2411.11959]. In the low-\(z\) O VI kinematic work, the “bound fraction” is explicitly described as an upper limit because \(D\) is only a projected radius and \(v_{\rm LOS}\) is only one component of the true three-dimensional velocity [2409.15432]. In the H I component study, the use of fixed \(\pm300\) or \(\pm600\) km s\(^{-1}\) association windows likewise admits two-halo and projection effects, especially at large \(D/R_{\rm vir}\) [2507.22283]. A plausible implication is that MUSEQuBES has shifted CGM interpretation away from one-galaxy/one-absorber language and toward a halo–environment–structure continuum.

Ly\(\alpha\)-based analyses carry their own interpretive limits. The high-\(z\) program depends on an empirical correction from Ly\(\alpha\) peak to systemic redshift, \(V_{\rm offset}=0.89\times{\rm FWHM}-58\) km s\(^{-1}\), precisely because resonant transfer shifts and broadens the line [2304.04788]. The 2021 LAE-CGM paper cautions that the measured stacked line widths include residual redshift scatter, spectral resolution, and radiative-transfer effects [2105.05260]. More generally, related MUSE quasar-nebula work emphasizes that the central \(1''\) around bright quasars is often unreliable after empirical PSF subtraction [1605.01422][1808.10857]. These are not incidental technicalities; they define what can and cannot be inferred kinematically from Ly\(\alpha\) maps in quasar fields.

The scientific significance of MUSEQuBES lies in the scale it brings to low-mass galaxies and to joint emission–absorption analysis. At \(z\approx3.3\), it extends CGM work from relatively massive LBGs to LAEs with characteristic stellar masses of only \(\sim10^{8.6}\,M_\odot\) [2105.05260]. At low redshift, it turns mostly sub-\(L_*\) galaxies with median \(\log(M_*/M_\odot)\sim8.7\)–8.9 into a statistically useful CGM sample for H I and O VI [2409.15423][2409.15432]. This suggests that the survey’s broader legacy is methodological as much as empirical: it shows that quasar-field integral-field spectroscopy can connect faint galaxies, diffuse halo gas, and larger-scale structure in a single framework, and that the resulting CGM picture is fundamentally multiphase, mass dependent, and environment dependent.

Source: https://www.emergentmind.com/topics/muse-quasar-fields-blind-emitters-survey-musequbes