QSO MUSEUM: Extended Lyα Nebulae Survey

• The QSO MUSEUM Campaign is a systematic VLT/MUSE survey that images extended Lyα nebulae around z~3 quasars to study the circumgalactic medium.
• It utilizes high-depth snapshot spectroscopy with precise PSF subtraction and sensitivity calibration to reveal faint, diffuse Lyα emission and its kinematics.
• The campaign links AGN radiative feedback to CGM properties by statistically analyzing spatial profiles, morphologies, and environmental impacts including quasar pair alignments.

The QSO MUSEUM campaign (Quasar Snapshot Observations with MUse: Search for Extended Ultraviolet eMission) constitutes a comprehensive multi-cycle effort exploiting the VLT/MUSE integral field spectrograph to systematically survey the extended Ly$\alpha$ emission—the so-called "Ly$\alpha$ nebulae"—surrounding $z\sim3$ quasars. The project delivers both a statistical census and a physical analysis of the circumgalactic medium (CGM) and its kinematics, morphology, and ionization state, probing feedback, fueling, and environmental factors in high-redshift quasar hosts. Driven by the discovery of rare, enormous Ly$\alpha$ nebulae (ELAN) at $z\sim2$, the QSO MUSEUM campaign innovates via high-depth snapshot spectroscopy, meticulous sample selection, and extension to luminous and faint quasar populations, quasar pairs, and links to molecular reservoirs and strong lensing cosmography.

1. Scientific Objectives and Survey Framework

The primary scientific goal of the QSO MUSEUM campaign is to directly uncover the astrophysics of circumquasar gas by imaging faint, extended Ly$\alpha$ emission in the CGM. The approach is motivated by the need for a quantitative, homogeneous, and statistical measurement of the frequency, scale, morphology, and kinematics of Ly$\alpha$ nebulae across a representative $z\sim3$ quasar population, enabling tests of models for the origin and fate of baryons in galaxy formation and AGN fueling/feedback cycles (Battaia et al., 2018, Lobos et al., 22 Jul 2025). Key aims include:

• Determining how widespread bright, extended Ly$\alpha$ halos are and how their extent, surface brightness (SB), and kinematical state depend on host quasar luminosity, radio properties, and environmental density.
• Discriminating the dominant Ly$\alpha$ emission mechanisms (photoionization, resonant scattering, cooling, or shock excitation).
• Linking the instantaneously illuminated CGM properties to AGN radiative/mechanical feedback and black hole mass/accretion rate.
• Assessing the connection between Ly$\alpha$-bright regions, molecular gas reservoirs, and galaxy or structure formation on larger scales.

Benefiting from a systematic snapshot strategy, the initial survey observed 61 quasars over $3.03$\langle z \rangle=3.17$), absolute $i$-magnitudes $-29.67 \leq M_i(z=2) \leq -27.03$, and a mix of radio-loud/quiet hosts. Subsequent extensions increased the sample to 120 quasars, scaling to cover both the bright and faint SDSS quasar populations and incorporating diverse environments, including physically associated quasar pairs at a range of separations (Lobos et al., 22 Jul 2025, Herwig et al., 29 Aug 2024).

2. Observational Methodology and Sample Properties

Observations were conducted using the VLT/MUSE integral field spectrograph in snapshot mode, with $\sim$45-min exposures per target. This approach delivers spatially resolved, flux-calibrated spectral cubes covering $\sim1'$ fields. Key methodological steps include:

• Empirical point-spread function (PSF) modeling and subtraction to remove compact quasar and star/LAE continuum emission, isolating the diffuse Ly$\alpha$ nebular signal.
• Masking of continuum and companion sources and background subtraction using tools such as ZAP to maximize sensitivity to low SB features.
• Consistent surface brightness sensitivity (SB$>8.8\times10^{-19}$ erg s$^{-1}$ cm$^{-2}$ arcsec$^{-2}$, 2$\sigma$ in 1.25Å; typically $\sim4.2\times10^{-18}$ erg s$^{-1}$ cm$^{-2}$ arcsec$^{-2}$ in a 30Å narrow-band).
• Uniform data reduction fuels robust stacking and enables power-law and exponential profile fitting across the sample (Battaia et al., 2018, Lobos et al., 22 Jul 2025).

Table 1 summarizes the sample evolution (quantities as reported):

Campaign Phase	N(QSOs)	Median $z$	$M_i(z=2)$ Range	Radio-quiet/Loud	Ly$\alpha$ Detection Rate
QSO MUSEUM I (Battaia et al., 2018)	61	3.17	$-29.7$ to $-27.0$	39/15 (+6 undefined)	61/61 (100%)
QSO MUSEUM III (Lobos et al., 22 Jul 2025)	120	3.13	$\sim$Full SDSS	N/A	110/120 (92%)
QSO Pairs (Herwig et al., 29 Aug 2024)	14	$\sim3$	$i$=18–22.75	N/A	12/14 (86%)

The inclusion of faint quasars and quasar pairs now samples a parameter space in bolometric luminosity ($10^{45.1}-10^{48.7}$ erg s$^{-1}$), black hole mass ($10^{7.9}-10^{10.3}\,M_\odot$), and Eddington ratio ($0.01<\lambda_{\rm Edd}<1.8$) (Lobos et al., 22 Jul 2025).

3. Phenomenology of Extended Ly$\alpha$ Nebulae

Detection Statistics and Morphologies

The vast majority of $z\sim3$ quasars exhibit extended Ly$\alpha$ emission with radial extents out to $\sim$80–90 kpc for single quasars and $\sim$90 kpc for quasar pairs, above the stated SB limits (Battaia et al., 2018, Herwig et al., 29 Aug 2024). Morphologies range from roughly symmetric (median axis ratio $b/a \approx 0.71$) to highly elongated, especially for nebulae in quasar pairs, where $\sim45\%$ show emission extending towards the companion, resulting in a displaced flux-weighted centroid (Herwig et al., 29 Aug 2024).

Radial SB profiles, determined from stacked narrow-band extractions, follow exponential or power-law forms. Single quasar nebula profiles are well described by an exponential law with a scale length $r_h\approx 15.7$ kpc (Battaia et al., 2018), while quasar pair nebulae exhibit a shallower power-law SB profile of the form $\mathrm{SB}_{\mathrm{Ly}\alpha}\propto r^{\alpha}$ with $\alpha=-1.57\pm0.17$, as compared to typical slopes near $-2$ for single quasars (Herwig et al., 29 Aug 2024).

Kinematics

The Ly$\alpha$ emission is kinematically "quiescent" across all campaigns, with average velocity dispersions $\langle \sigma_{\mathrm{Ly}\alpha} \rangle < 400$ km s$^{-1}$ ($\mathrm{FWHM}<940$ km s$^{-1}$) and centroid shifts typically $\sim$100–200 km s$^{-1}$ relative to the quasar Ly$\alpha$ peak, but $\sim$700–800 km s$^{-1}$ compared to systemic redshifts. The majority of nebulae show no evidence for broad, high-velocity outflows; quasar pairs exhibit similar quiescent kinematics but occasionally display velocity gradients or offsets of several hundred km s$^{-1}$, suggesting large-scale structure-induced flows (Battaia et al., 2018, Herwig et al., 29 Aug 2024).

4. Emission Mechanisms and Physical Interpretation

The predominance of modest velocity dispersion, smooth broad-band profiles, and similarities between nebular and intrinsic quasar Ly$\alpha$ line profiles indicate that neither shock-heated outflows nor cooling radiation are the dominant sources of nebular emission (Battaia et al., 2018). The favored interpretation is a mix of:

• Photoionization in the optically thin regime ($N_\mathrm{HI} \ll 10^{17.2}$ cm$^{-2}$): The incident quasar UV continuum reprocesses into Ly$\alpha$ via recombination in spatially extended, metal-poor, $T\sim10^4$ K halo gas. The SB in this regime is largely insensitive to the exact quasar luminosity, scaling as $n_\mathrm{H}N_\mathrm{H}$.
• Resonant scattering of Ly$\alpha$ photons: Broad-line region photons and star-formation-produced Ly$\alpha$ are scattered in the neutral CGM, extending both the spatial reach and spectral imprint of the central source, explaining the velocity proximity of nebular and quasar Ly$\alpha$ peaks.
• Suppression or modification by host properties: High dust content or geometric alignment (e.g., high-inclination disks) can reduce the escape of both ionizing and Ly$\alpha$ photons, lowering observed SB without implying a deficit of gas (Muñoz-Elgueta et al., 2022).

Stacking results, profile fitting, and the modest dependence of SB on $L_{\rm bol}$ for optically thin conditions reinforce the dominance of unobscured photoionization, though instantaneous AGN "mode" can rapidly modulate SB and line widths, particularly in the central CGM (Lobos et al., 22 Jul 2025).

5. Environmental Effects: Quasar Pairs and Cosmic Web Filaments

The expansion to quasar pairs at $z\sim3$ directly probes the impact of environmental density and inter-halo structure on CGM illumination (Herwig et al., 29 Aug 2024). Principal findings include:

• Extended Ly$\alpha$ nebulae in pairs are as common and luminous as those around singles, with SB and scale size comparable at intermediate radii.
• Nebulae in pairs exhibit greater asymmetry, with a significant fraction ($\sim45\%$) extending preferentially towards their companion, and the SB centroid offset along the inter-quasar axis.
• The maximum nebular extent ($d_{\mathrm{QSO,max}}$) is anti-correlated with the misalignment angle ($\phi$) between nebula and inter-quasar axis (Spearman $r\approx-0.78$), indicating alignment with cosmic web filaments.
• The radial SB profile in pairs is shallower ($\alpha\approx-1.57$), indicating both increased cool gas density at large radii and/or enhanced intergalactic medium (IGM) contributions compared to isolated quasars.

These facts are interpreted as observational evidence that the cool CGM in quasar pairs is organized along the cosmic web, with increased mass and spatial connectivity facilitating both AGN-driven and environmental illumination. Quasar pair fields serve as unique "signposts" for mapping the IGM in emission.

6. Scaling Relations: AGN Feedback, Black Hole Properties, and Ionization Cones

The extension of the survey facilitates the first robust statistical paper of how the CGM's Ly$\alpha$ properties depend on quasar luminosity, black hole mass, and Eddington ratio (Lobos et al., 22 Jul 2025). Key findings:

• The Ly$\alpha$ surface brightness (SB$_{\mathrm{Ly}\alpha}$) scales with the quasar bolometric luminosity ($L_{\mathrm{bol}}$): brighter quasars yield more luminous CGM nebulae (SB$$_{\mathrm{Ly}\alpha}\propto L_{\mathrm{bol}}^{\alpha}$$).
• The velocity dispersion ($\sigma_\mathrm{Ly\alpha}$) in the innermost CGM (projected $R<40$ kpc or $\lesssim0.4R_{\mathrm{vir}}$) increases systematically with AGN power, indicating more dynamic perturbation and/or scattering in the presence of stronger feedback.
• When binning by black hole mass, sources with higher Eddington ratios—implying higher accretion rates—exhibit both higher SB and broader central Ly$\alpha$ lines, even at constant mass.
• The data support a scenario in which the opening angle of the ionization cone also increases with quasar luminosity, illuminating a greater fraction of the cool CGM.

These relationships collectively provide evidence that the CGM's instantaneous state is primarily regulated by the current AGN activity, not only by accumulated black hole mass (Lobos et al., 22 Jul 2025). Scaling relations (e.g., $\sigma_{50}\propto L_{\mathrm{bol}}^{\alpha}$) can be used to calibrate and constrain physical models of AGN feedback and galaxy evolution.

7. Molecular Gas Reservoirs and Multi-phase ISM/CGM Links

Follow-up observations of nine QSO MUSEUM targets with APEX/SEPIA180 yielded insights into the connection between molecular gas and the illuminated CGM (Muñoz-Elgueta et al., 2022):

• Massive molecular gas reservoirs were identified, with $M_\mathrm{H_2}$ spanning $(0.4-6.9)\times10^{11}M_\odot$ for CO/[CI] detected systems, and upper limits $<1.1\times10^{11}M_\odot$ otherwise.
• Significant velocity offsets ($-423$ to $+1236$ km s$^{-1}$) between molecular transitions and Ly$\alpha$ peaks signal complex dynamics: inflows, outflows, and turbulence on galactic to halo scales.
• Fainter, more compact Ly$\alpha$ nebulae sometimes coincide with the hosts of the strongest molecular lines, suggesting that high dust or edge-on disk geometry can obscure Ly$\alpha$/ionizing photons otherwise capable of illuminating the CGM.
• A plausible implication is that the most massive hosts (with highest molecular masses and velocity dispersions) may sit in more massive dark matter halos, where the fraction of cool CGM able to emit Ly$\alpha$ is reduced due to heating or geometric effects.

This establishes the importance of host galaxy and ISM properties—not just the AGN itself—in regulating the observable extent of CGM Ly$\alpha$ halos.

8. Cosmological and Future Directions

The QSO MUSEUM campaign also extends its reach to time-delay cosmography with strong lensing systems, providing independent Hubble constant (H$_0$) measurements through quadruply lensed quasar monitoring (Queirolo et al., 2023):

• Detailed modeling using multi-band HST images and ground-based lightcurves yields $H_0=76.6^{+7.7}_{-7.0}$ km s$^{-1}$ Mpc$^{-1}$ with a 9.6% uncertainty from a single lens, demonstrating the precision achievable with careful lens modeling and time-delay analysis.
• The methodology underscores the importance of data quality control (e.g., discarding contaminated filters), filter-by-filter lens light/mass modeling, and high cadence photometric campaigns.

Looking forward, future QSO MUSEUM directions include:

• Extending emission diagnostics to non-resonant lines (He II, C IV) to refine photoionization/scattering contributions.
• Improving systemic redshift determinations through near-IR and molecular follow-up.
• Employing adaptive optics (e.g., GALACSI on MUSE) and complementary IFUs for inner-kpc probing of feedback.
• Statistical binning across quasar properties to calibrate AGN feedback constraints.
• Targeting quasar pairs and specially aligned systems for ultra-deep observations that could directly image IGM filaments and cosmic web structures.

These avenues will enable the isolation of physical drivers regulating CGM observables, refine feedback models, and provide templates for leveraging AGN as beacons for baryon cycle and cosmological investigations at high redshift.