MUSEQuBES: Quasar-field Blind Emitters Survey

• The survey MUSEQuBES innovatively maps quasar fields by detecting galaxies blindly with MUSE and linking them to circumgalactic absorption features in quasar spectra.
• It employs advanced techniques like CubePSFSub, MARZ, and VPFIT to quantify H I, C IV, and O VI properties, revealing detailed covering fractions and gas profiles.
• Findings indicate that gas characteristics depend on galaxy mass, environment, and anisotropic structure, bridging low-mass galaxy studies with large-scale cosmic mapping.

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 $0.1Muzahid et al., 2021, Dutta et al., 2024).

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 (Muzahid et al., 2021, Dutta et al., 2024).

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 (Dutta et al., 2024, Dutta et al., 19 Feb 2026).

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 $\alpha$0 km s$\alpha$1 (Muzahid et al., 2021)
High-$\alpha$2 absorber catalogs	86 LAEs for C IV; 96 LAEs for H I	C IV covering fraction $\alpha$3 at $\alpha$4; H I covering fraction $\alpha$5 at $\alpha$6 (Banerjee et al., 2023, Banerjee et al., 2024)
Low-$\alpha$7 H I mapping	4595 $\alpha$8 galaxies, 184 quasars, 5054 pairs	Excess absorption out to at least $\alpha$9 and $L_*$0 km s$L_*$1 (Dutta et al., 2023)
Low-$L_*$2 O VI mapping	247 galaxies suitable for O VI analysis	$L_*$3 within $L_*$4 (Dutta et al., 2024)

2. Observational design and analysis framework

MUSEQuBES is methodologically built around blind source finding in three-dimensional quasar-field datacubes. In the high-$L_*$5 program, LAEs were extracted from MUSE cubes after empirical quasar PSF subtraction with CubePSFSub and continuum subtraction with CubeBKGSub, then identified with CubEx/CubExtractor using connected-voxel criteria. In the low-$L_*$6 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 (Muzahid et al., 2021, Dutta et al., 2024).

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 $L_*$7, the C IV analysis used a blind catalog of 489 C IV absorption components in 152 systems over $L_*$8, later matched to 86 LAEs (Banerjee et al., 2023). The corresponding H I work around the same LAEs used Voigt-profile fitting of all H I absorbers within $L_*$9 km s$0.1800 H I absorption components (Banerjee et al., 2024). At low redshift, the O VI catalog contains 118 O VI components over $0.167 systems, and the resolved H I study identifies 227 H I components in 103 unique H I systems around 256 galaxies (Dutta et al., 2024, Dutta et al., 29 Jul 2025).

The program also developed morphology-aware analyses that go beyond simple impact-parameter scaling. In the low-$0.1GALFIT, 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 $0.1Dutta et al., 19 Feb 2026).

3. High-redshift branch: Ly$0.1

The first statistical high-$0.196 LAEs at median redshift $z$0, with typical $z$1 erg s$z$2, dust-uncorrected SFRs of $z$3, a characteristic stellar mass of $z$4, and median LAE–quasar impact parameter 165 pkpc. Stacking of the high-resolution quasar spectra showed significant excess H I and C IV absorption near the LAEs out to 500 km s$z$5 and at least $z$6 pkpc, corresponding to about $z$7 virial radii. At $z$8 km s$z$9 from the galaxies, the median H I and C IV optical depths are enhanced by an order of magnitude (Muzahid et al., 2021).

That stacked picture was sharpened by absorber-by-absorber analyses. The C IV study showed that C IV components cluster around LAEs within roughly $1'\times1'$0 km s$1'\times1'$1, that the covering fraction is $1'\times1'$2 for a threshold $1'\times1'$3, and that the covering fraction remains at $1'\times1'$4 for impact parameters 150–250 pkpc, or about $1'\times1'$5–$1'\times1'$6. Using the covering-fraction profile, the LAE–C IV absorber two-point correlation function was constrained to $1'\times1'$7 cMpc and $1'\times1'$8 for a threshold $1'\times1'$9 (Banerjee et al., 2023). 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 $z$0 to 260 pkpc. The LAEs show an overall H I covering fraction of $z$1 for a threshold $z$2, while pairs/group LAEs exhibit 100% H I covering fraction out to $z$3 pkpc and isolated LAEs remain near $z$4 (Banerjee et al., 2024).

Environment is a recurrent result in the high-$z$5 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 (Muzahid et al., 2021). 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 $z$6 km s$z$7 (Banerjee et al., 2023, Banerjee et al., 2024).

MUSEQuBES has also produced a filament-scale case study at $z$8 in the Q1317$z$90507 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 $\sim L_*$0 for HM05 and $\sim L_*$1, and a giant Ly$\sim L_*$2-emitting nebula with surface brightness $\sim L_*$3 erg cm$\sim L_*$4 s$\sim L_*$5 arcsec$\sim L_*$6 and projected size $\sim L_*$7 pkpc aligned with the LAEs. The authors interpret this as the first detection of giant Ly$\sim L_*$8 emission tracing cosmic filaments and associated with normal LAEs rather than a luminous quasar or quasar pair (Banerjee et al., 2024).

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

The low-$\sim L_*$9 H I program was first developed through statistical stacking. A sample of 4595 $z$0 galaxies with median stellar mass $z$1 was probed by 184 background quasars, producing 5054 quasar–galaxy pairs with median impact parameter $z$2 pMpc and median $z$3. Excess H I absorption was detected out to at least $z$4 in the transverse direction and $z$5 km s$z$6 along the line of sight. The median stacked profile for the full sample was described by a galaxy–absorber two-point correlation function with $z$7 pMpc and $z$8, while the inner regions were better explained by a log-linear or Gaussian relation and the outer regions by a power law (Dutta et al., 2023).

Resolved component analysis then moved this branch from stacks to direct $z$9, covering-fraction, mass, and kinematic measurements. Using 256 galaxies with median stellar mass $\approx250$0, median redshift $\approx250$1, and median impact parameter $\approx250$2 pkpc, the survey showed that the H I profile around isolated, star-forming galaxies is well fit by a power law with slope $\approx250$3 as a function of $\approx250$4. For a threshold $\approx250$5, 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 $\approx250$6 across the stellar-mass range. The mean H I mass in the outer CGM, $\approx250$7–$\approx250$8, rises from $\approx250$9 to $\alpha$00 with stellar mass, and non-isolated galaxies show an H I-rich environment that extends about three times further than for isolated galaxies (Dutta et al., 29 Jul 2025).

The O VI branch established an equally strong mass dependence. In the main low-$\alpha$01 O VI census, 247 low-mass galaxies with median $\alpha$02 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 $\alpha$03, significantly below the value for $\alpha$04 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 $\alpha$05. The characteristic normalized impact parameter where the O VI covering fraction falls to half its peak value is largest, about $\alpha$06, at the same stellar mass. For dwarf galaxies with $\alpha$07, the average O VI mass within the virial radius is $\alpha$08, and stacked spectra suggest a highly ionized metal floor of $\alpha$09 outside the virial radius (Dutta et al., 2024).

The accompanying O VI kinematics paper showed that most O VI within $\alpha$10 is consistent with being gravitationally bound, even for low-mass halos. The LOS velocity distribution of O VI components has mean $\alpha$11 km s$\alpha$12 and dispersion $\alpha$13 km s$\alpha$14, and the upper-limit bound fraction within $\alpha$15 is $\alpha$16 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 (Dutta et al., 2024).

A further extension of the low-$\alpha$17 program addressed CGM anisotropy. Using 113 isolated galaxies over $\alpha$18, including 91 new measurements from MUSEQuBES, the survey found that the H I covering fraction within $\alpha$19 for low-mass galaxies with $\alpha$20 and threshold $\alpha$21 is enhanced both along the disk plane, $\alpha$22, and in the polar direction, $\alpha$23. 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 $\alpha$24 (Dutta et al., 19 Feb 2026).

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$25 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 $\alpha$26, which found a 100% detection rate of giant Ly$\alpha$27 nebulae with projected sizes $\alpha$28 pkpc and surface-brightness profiles consistent with power laws of slope about $\alpha$29 (Borisova et al., 2016). Another closely related study used 6 medium-deep MUSE quasar fields at $\alpha$30 to identify fluorescent dark-galaxy candidates, finding 6 continuum-undetected sources with $\alpha$31 Å at $\alpha$32 and deriving a lower limit of about 60 Myr on quasar lifetime if the most distant candidate is fluorescently illuminated (Marino et al., 2017). 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 $\alpha$33 LLS where only $\alpha$34 were expected (Fumagalli et al., 2016).

A parallel MUSE quasar-nebula effort, QSO MUSEUM, extended this approach to a homogeneous sample of 61 quasars at $\alpha$35. It found extended Ly$\alpha$36 emission around all 61 quasars, with average maximum projected extent of about 80 kpc, bulk emission within $\alpha$37 kpc, average exponential scale length $\alpha$38 kpc, and relatively quiescent kinematics with $\alpha$39 km s$\alpha$40. Only one system qualified as an ELAN, implying an occurrence rate of roughly $\alpha$41 when combined with the Borisova et al. sample (Battaia et al., 2018). A more targeted MUSE follow-up of Q2059-360 then showed how a proximate damped Ly$\alpha$42 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 (North et al., 2017).

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 $\alpha$43–1.4 found [O II]-emitting circumgalactic nebulae around the majority of them: 7 with major-axis sizes $\alpha$44 kpc, 20 $\alpha$45 kpc, and 27 $\alpha$46 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 (Johnson et al., 2024).

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 $\alpha$47, pairs/group LAEs show systematically stronger H I and C IV than isolated LAEs, and a dedicated case study links metal-poor absorbers, LAE overdensity, and giant Ly$\alpha$48 emission to a filamentary structure (Muzahid et al., 2021, Banerjee et al., 2024). 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 (Dutta et al., 29 Jul 2025, Dutta et al., 2024). 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-$\alpha$49 H I analysis, all galaxies associated with LLSs have impact parameters $\alpha$50 pkpc, which led to the suggestion that the true absorber hosts may be too faint to detect (Banerjee et al., 2024). In the low-$\alpha$51 O VI kinematic work, the “bound fraction” is explicitly described as an upper limit because $\alpha$52 is only a projected radius and $\alpha$53 is only one component of the true three-dimensional velocity (Dutta et al., 2024). In the H I component study, the use of fixed $\alpha$54 or $\alpha$55 km s$\alpha$56 association windows likewise admits two-halo and projection effects, especially at large $\alpha$57 (Dutta et al., 29 Jul 2025). 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$58-based analyses carry their own interpretive limits. The high-$\alpha$59 program depends on an empirical correction from Ly$\alpha$60 peak to systemic redshift, $\alpha$61 km s$\alpha$62, precisely because resonant transfer shifts and broadens the line (Banerjee et al., 2023). The 2021 LAE-CGM paper cautions that the measured stacked line widths include residual redshift scatter, spectral resolution, and radiative-transfer effects (Muzahid et al., 2021). More generally, related MUSE quasar-nebula work emphasizes that the central $\alpha$63 around bright quasars is often unreliable after empirical PSF subtraction (Borisova et al., 2016, Battaia et al., 2018). These are not incidental technicalities; they define what can and cannot be inferred kinematically from Ly$\alpha$64 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 $\alpha$65, it extends CGM work from relatively massive LBGs to LAEs with characteristic stellar masses of only $\alpha$66 (Muzahid et al., 2021). At low redshift, it turns mostly sub-$\alpha$67 galaxies with median $\alpha$68–8.9 into a statistically useful CGM sample for H I and O VI (Dutta et al., 2024, Dutta et al., 2024). 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.