M3G: MUSE Survey of Massive Galaxies

• M3G Survey is a targeted, high-resolution spectroscopic program that investigates the stellar and gas properties of extremely massive early-type galaxies.
• Observations with MUSE yield detailed 2D maps of kinematics and population gradients out to ~2 effective radii, enabling the study of multi-spin phenomena and merger signatures.
• The survey supports a two-phase formation model by linking dry merger histories with slow rotation and extensive accreted halos in brightest cluster galaxies.

The MUSE Most Massive Galaxies (M3G) Survey is a targeted, high-resolution integral-field spectroscopic program designed to investigate the stellar, gaseous, and evolutionary properties of the most massive galaxies in the local Universe. Using the Multi Unit Spectroscopic Explorer (MUSE) at the ESO/VLT, M3G systematically maps stars and ionized gas in twenty-five early-type galaxies (ETGs) with stellar masses $M_*\gtrsim6\times10^{11}\ M_\odot$ and absolute magnitudes $M_{Ks}<-25.7$ mag, placing special emphasis on the brightest cluster galaxies (BCGs) in rich environments. The survey provides a unique two-dimensional kinematic and population census out to $\sim2R_e$ (effective radii), shedding new light on assembly histories, multi-spin phenomena, gas content, and the imprint of environment in the most massive galactic systems (Krajnovic et al., 4 Feb 2026, Spavone et al., 2021, Pagotto et al., 2020).

1. Survey Design and Sample Definition

The M3G sample comprises 25 galaxies, including 14 BCGs (rank 1) and 11 non-BCG cluster member galaxies (ranks 2–7). Target selection is strictly defined by $M_{Ks}<−25.7$ (2MASS $k\_m\_ext$), $M_*\gtrsim6\times10^{11}\,M_\odot$ (Cappellari 2013), and membership in the three richest Abell clusters of the Shapley supercluster (Abell 3556, 3558, 3562), supplemented by 11 BCGs in rich southern Abell clusters ($\delta<5^\circ$). The galaxy sample is distributed across $0.037$\sim2R_e$ for each object.

A table summarizing sample selection is given below:

Criterion	Value	Reference
K-band magnitude ($M_{Ks}$)	$< -25.7$ mag	2MASS
Stellar mass ($M_*$)	$\gtrsim6\times10^{11}\,M_\odot$	Eq. 2, Cappellari 2013
Environment	Richest Abell clusters (Shapley core)	(Krajnovic et al., 4 Feb 2026)
Galaxy types	14 BCGs, 11 non-BCGs (ranks 2–7)	(Krajnovic et al., 4 Feb 2026)
Redshift range	$0.037 < z < 0.054$	(Pagotto et al., 2020)

2. Observational Strategy and Data Processing

Observations utilize the MUSE Wide Field Mode (WFM, no AO), yielding a 1′×1′ field with 0.2″ spaxels. The spectral range spans 4800–9300 Å with $\Delta\lambda=1.25$ Å per pixel and instrumental FWHM $\sim$2.5 Å (velocity resolution $\sim$50 km s$^{-1}$). Typical on-source exposure times per galaxy are 2–6 hr, subdivided into observing blocks with alternating object and sky exposures (OSOOSO pattern), exploiting spatial dithers and rotator angle offsets to mitigate IFU systematics. Obtained data reach a limiting r-band surface brightness of $\langle23.2~\rm{mag~arcsec}^{-2}\rangle\pm0.4$ at map edges, with mean combined seeing of 0.93″.

The data reduction protocol consists of:

1. Standard calibrations: bias, flats, trace tables, wavelength and LSF calibration.
2. Illumination correction (using twilight flats when available).
3. Sky subtraction (dedicated blank field exposures, “model” refitting, local continuum removal).
4. Telluric correction via molecfit fits to foreground-star or nuclear spectra.
5. Nightly flux calibration (standard stars).
6. Spatial registration and combination; cosmic-ray rejection; composite datacube formation.
7. Masking of residual sky/telluric regions during later analysis.

Stellar and gas kinematic extraction is performed after Voronoi tessellation of data cubes to S/N = 50 per bin. The line-of-sight velocity distribution (LOSVD) is fit using pPXF, parameterizing with Gauss–Hermite moments ($V, \sigma, h_3, h_4$). When satellites are projected, two-component pPXF models are used for source separation (Krajnovic et al., 4 Feb 2026).

3. Stellar Kinematics, Spin Phenomena, and Orbital Structure

M3G reveals a striking dichotomy between BCGs and non-BCGs in their kinematic properties. Multi-spin features, quantified through velocity and Gauss–Hermite moment maps, are widespread:

• 13/25 galaxies show a single coherent spin (MS-1), 5 of which exhibit “long-axis rotation” (kinematic–photometric misalignment $\sim90^\circ$).
• 8/25 display two spin reversals (classical KDC plus outer twist; MS-2).
• 4/25 manifest three or more spin reversals; the BCG PGC 046832 presents five reversals within $2R_e$.

The spin parameter $\lambda_R$—where $\lambda_R/\sqrt{\epsilon}>0.31$ delineates fast rotators—shows that all fast rotators (6/25, i.e. 24%) are non-BCGs (ranks $\ge2$), while all BCGs and second/third-ranked objects are slow rotators. The $V/\sigma$–$h_3$ anti-correlation, quantified by $$\xi_3 = \sum_i F_i h_{3,i}(V_i/\sigma_i)/\sum_i F_i h_{3,i}^2$$, sharply separates disk-like fast rotators ($\xi_3\lesssim-4$) from slow and non-regular rotators ($\xi_3\approx0$ or positive).

A plausible implication is that only triaxial and prolate-like galaxies with rich orbit structure—short- and long-axis tubes, box orbits, prograde and retrograde contributions—can produce the multi-spin velocity signatures commonly seen in BCGs. Orbit families and kinematic misalignments suggest a substantial influence from gas-poor major and repeated minor mergers, particularly for the slow rotator population. Outer kinematic components, detected as systematic radial zeroth-order kinemetry variation and elevated dispersions toward cluster values, are interpreted as tidally accreted halo stars not yet in equilibrium with the main system (Krajnovic et al., 4 Feb 2026).

4. Deep Imaging Synergy: Accreted Mass Fractions and Population Gradients

A subset of the M3G sample overlaps with the VEGAS deep imaging campaign (PGC 007748, PGC 015524, PGC 049940), enabling joint constraints on light profile decomposition, accreted mass fraction, and stellar population gradients (Spavone et al., 2021). Surface photometry is fit with three components: inner Sérsic (in situ), intermediate Sérsic (relaxed accreted), and outer exponential (unrelaxed debris). Accreted mass fractions—the sum of the intermediate and outer components—reach $f_{h,T}=77$–89%, consistent with predictions from cosmological simulations for $M_*\sim10^{12}M_\odot$ early types.

Transition radii $R_{tr,1}$ and $R_{tr,2}$, where the accreted component exceeds the in-situ contribution and the “halo” dominates, respectively, are identified. MUSE IFS coverage typically reaches just beyond $R_{tr,1}$, sampling into the accreted-dominated regime. All three pilot galaxies are slow rotators with flat or mildly rising $\lambda_R(R)$, again supporting major-merger-dominated assembly. Metallicity gradients exhibit discrete changes at the structural transitions, indicative of mixed accretion histories and the dominance of metal-rich satellites in the outer halo build-up (Spavone et al., 2021).

These results strongly support a two-phase formation model—compact, slow-rotating, metal-rich remnants from in situ star formation, surrounded by a massive accreted envelope with slowly varying kinematic and population signatures.

5. Warm Ionized Gas: Incidence, Morphologies, and Origins

M3G systematically assesses the prevalence, spatial structure, kinematics, and excitation of warm ionized gas in the very high–mass ETG regime (Pagotto et al., 2020). Ionized-gas emission is detected in 5/14 BCGs and 6/11 non-BCG satellites, with [O I] and [N I] lines requiring multi-Gaussian modeling in several BCGs and one satellite.

Morphologies span centrally concentrated disks, filaments (in two BCGs), and extended rotating/warped features (in two satellites). Kinematic position angle alignment of gas and stars occurs in 60% of satellites and only 25% of BCGs; misalignments in BCGs and slow rotators suggest frequent external gas accretion (e.g., via cluster cooling flows, mergers, or filaments). Nevertheless, internal sources of gas such as stellar mass-loss cannot be definitively excluded, particularly in triaxial or prolate-like systems, which may produce apparent kinematic decoupling through orbital anisotropy.

Emission-line diagnostics (BPT and WHAN diagrams) consistently indicate LINER/LIER-like excitation, with negligible contribution from ongoing star formation. Outflows and shocks are indicated by multi-component profiles, high velocity dispersions ($\sigma$ up to 400–1000 km/s), and red/blueshifted sub-components (Pagotto et al., 2020).

6. The Role of Environment and Implications for Galaxy Formation

Analysis of the full M3G sample demonstrates that galaxy mass and spin cannot be interpreted in isolation. BCGs at cluster centers—sites of repeated dry mergers and angular momentum dilution—are universally slow rotators with multi-spin signatures and frequently display evidence of accreted, dynamically unmixed haloes. In contrast, non-BCGs in similar environments may retain fast rotation, provided they avoid major dry mergers or retain sufficient gas for disk regrowth.

A plausible implication is that the location within the cluster, and thus the dynamical history of interactions, is as fundamental as intrinsic mass in shaping the internal kinematics and gas content of extreme-mass galaxies. These findings reinforce the paradigm that dry mergers are the dominant channel for the formation of slow rotators at the top end of the mass function, as evidenced by comprehensive kinematic dissection and stellar population trends revealed by M3G (Krajnovic et al., 4 Feb 2026, Spavone et al., 2021).

7. Outlook and Further Directions

The M3G survey establishes the most detailed two-dimensional kinematic and gaseous census to date for $M_*\gtrsim10^{12}\ M_\odot$ galaxies. Its results align with those from the ATLAS$^{3D}$ and MASSIVE surveys but probe a more extreme mass/cluster regime. The survey sets a new benchmark for empirical tests of the two-phase assembly paradigm and the environmental modulation of galaxy evolution. Future analyses integrating full-sample spectro-photometric measures, extended halo photometry, and high-resolution simulations will refine constraints on major/minor merger rates, stellar mass assembly timelines, and the physical processes mediating angular momentum evolution at the uppermost end of the galaxy mass distribution (Krajnovic et al., 4 Feb 2026, Spavone et al., 2021).