MAGNUM: AGN Dynamics under MUSE Lens

• MAGNUM is a comprehensive, spatially resolved approach that combines MUSE integral field spectroscopy, photometric monitoring, and ancillary multiwavelength data to study active galactic nuclei and their hosts.
• It employs advanced methods to decompose nuclear, disk, and outflow components with resolutions down to ~10 pc, enabling detailed mapping of ionization, kinematics, and metallicity gradients.
• Quantitative analyses of AGN SED invariance, variability, and feedback mechanisms from MAGNUM data support refined unification models and offer insights into the coevolution of AGN and galaxies.

Measuring Active Galactic Nuclei Under MUSE Microscope (MAGNUM) refers to a comprehensive, spatially resolved, multi-method approach to the paper of AGN and their host galaxies via advanced optical/near-IR integral field spectroscopy, long-term photometric monitoring, mid-IR and X-ray diagnostics, and quantitative modeling. The term "MAGNUM" encompasses both specific high-cadence monitoring programs and, especially in the context of the MUSE (Multi Unit Spectroscopic Explorer) instrument on the VLT, a suite of detailed, high-resolution surveys targeting the gas and stellar components in the central kiloparsecs of nearby active galaxies. MAGNUM research is characterized by its focus on physical decomposition (nuclear vs. host vs. outflow), multidimensional mapping of ionization and kinematics, rigorous metallicity and excitation diagnostics, and the identification of detailed feedback mechanisms including outflow-driven star formation and the effect of central AGN on chemical enrichment.

1. Observational Strategies and Instrumentation

The MAGNUM project combines several observational modalities to disentangle AGN structure and feedback:

• Photometric Monitoring: The original MAGNUM telescope campaign (Sakata et al., 2010) implemented high-cadence, multi-band (B/V/I) photometry with precise PSF modeling and host/narrow-line decomposition, enabling isolation of the variable AGN continuum and assessment of spectral energy distribution (SED) constancy.
• Integral Field Spectroscopy: The adoption of VLT/MUSE provides wide-field (up to 1′×1′), high-spatial-resolution (down to ∼10 pc, or even ∼2 pc in adaptive optics Narrow Field Mode) cubes, allowing full 2D mapping of emission lines, stellar continuum, and kinematic components across extended host galaxies and nuclear regions (Venturi et al., 2018, Mingozzi et al., 2018, Kakkad et al., 2023, Robbins et al., 20 Jan 2025, Rodríguez-Ardila et al., 13 Mar 2025, Amiri et al., 15 Sep 2025).
• Ancillary Multiwavelength Data: The integration of high-resolution radio, MIR, and X-ray imaging (e.g., Chandra/ACIS-S, soft and hard bands), as well as existing HST and ground-based imaging and spectroscopy, provides cross-validation for outflow phase identification, alignment with radio jets, and quantification of multi-phase feedback [(Cresci et al., 2015); (Asmus et al., 2013); (Venturi et al., 2018)].
• Temporal/Spectral Analysis: Time-series methods include Fourier-domain modeling, multi-Gaussian/nonparametric profile decomposition for emission lines, and Bayesian inference for power spectral density characterization and model comparison (Li et al., 2018).

These complementary data enable physical separation of disc, narrow-line region (NLR), and outflowing components, and support the rigorous application of kinematic, ionization, and abundance diagnostics in both nuclear and host galaxy scales.

2. Physical Characterization of AGN Components

Spectral Energy Distribution (SED): MAGNUM's long-term multi-band data show that the optical SED of the variable AGN continuum remains nearly constant during flux changes, with tight linear flux-to-flux correlations in all monitored bands (Sakata et al., 2010). The relationships

$$f_Y = a \cdot f_X + b \, , \qquad \alpha_{XY} = \frac{\log a}{\log(\nu_X/\nu_Y)}$$

validate the SED's invariance and facilitate host/narrow-line decomposition, revealing that observed color variations ("bluer when brighter") are typically due to the declining fractional contribution of a redder, non-variable host galaxy and narrow emission line (HOST+NL) component at higher nuclear fluxes.

Emission Line and Continuum Decomposition: In volume-resolved MUSE data, emission lines (e.g., [O III], Hα, [N II], [S II], [Fe VII], [Fe X]) are fit per spaxel using multiple Gaussians, velocity binning, or nonparametric percentile velocities (e.g., $v_{10}$, $W_{80}$), distinguishing between:

• Rotationally supported disc gas (coincident with stellar rotation);
• Outflow wings (typically $|v| > 150{-}250$ km/s);
• Kinematically distinct high-ionization and low-ionization narrow-line regions (NLR and CLR) (Mingozzi et al., 2018, Rodríguez-Ardila et al., 13 Mar 2025).

Ionization Diagnostics: Spatially resolved BPT diagrams ([O III]/Hβ versus [N II]/Hα, [S II]/Hα, [O I]/Hα), combined with ratios such as [O III]/[S II] or [S III]/[S II], separate AGN, star formation, and shock-ionized/excited regions. High-ionization cones and coronal line regions aligned with radio jets or outflows are directly mapped (Mingozzi et al., 2018, Rodríguez-Ardila et al., 13 Mar 2025).

Metallicity Mapping: The gas-phase metallicity $Z_{\rm gas}$ is derived using strong-line methods, with specific calibrations for AGN-photoionized and star-forming regions (Amiri et al., 15 Sep 2025):

$$Z_{\rm gas} = 8.073 - 0.32 \cdot \mathrm{O3N2} \, ; \quad \mathrm{O3N2} = \frac{[\mathrm{O} III] \lambda5007 / \mathrm{H}\beta}{[\mathrm{N} II] \lambda6584 / \mathrm{H}\alpha}$$

(for H II regions)

$$Z_{\rm gas} = Z_{\rm int} - 0.1 \times \log_{10}\left(\frac{n_e}{300\,\text{cm}^{-3}}\right)$$

with $Z_{\rm int}$ empirically parameterized using [N II], [O III], and $n_e$ [Storchi-Bergmann et al. 1998], for AGN regions.

Negative radial metallicity gradients are detected in both NLR and disk components, implying that AGN-driven outflows do not efficiently redistribute metals to flatten the gradient; instead, enrichment follows inside-out star formation (Amiri et al., 15 Sep 2025).

3. AGN Feedback and Host Galaxy Evolution

Outflow Properties and Kinematics: MUSE data enable measurement of ionized outflows (via [O III] velocity wings, multi-component fits) on kpc scales. Mass outflow rates are computed using the continuity equation:

$\dot{M} = \Omega r^2 \rho v \simeq f \frac{M v}{r}$

with $f$ as a geometric factor (e.g., $f=3$ for cloud-filled volume), $M$ from Hα luminosity (corrected for extinction and electron density), and $v$ measured from noncircular velocity components (Venturi et al., 2020, Venturi et al., 2018).

Feedback Mechanisms: MUSE's spatial resolution allows isolation of noncircular outflowing motions distinct from disk rotation, and quantification of their mass, velocity, and energetics (kinetic energy and momentum flux). These outflows can show declining mass rates with radius (Venturi et al., 2018), indicating either wind deceleration or episodic nuclear activity.

An example of positive feedback is found in NGC 5643: AGN-driven outflows compress dense ISM at the edges of dust lanes, triggering star formation in circumnuclear clumps (Cresci et al., 2015). Young stellar populations, high EW(Hα), and diagnostic line ratios confirm in situ star formation induced by mechanical feedback.

Coronal Line and Multi-phase Feedback: Extended CLR ([Fe VII], [Fe X]) emission maps reveal sizes from several hundred parsecs to multiple kpc, strongly aligned with the radio jet axis. The luminosity and extension of CLR scale positively with radio jet power, and emission profiles and morphology (secondary off-nuclear peaks; shallow radial decline) imply that jet-driven shocks, alongside nuclear photoionization, are necessary to power the observed high-excitation lines (Rodríguez-Ardila et al., 13 Mar 2025).

Metallicity and Star Formation: Despite energetic outflows and feedback, negative metallicity gradients indicate inefficient metal mixing by AGN-driven winds. The circumnuclear rings, often showing ongoing star formation, are dynamically linked to inflow patterns set by large-scale stellar bars and gas dynamics (Robbins et al., 20 Jan 2025).

4. Quantitative and Theoretical Modeling

Time-series and Variability Analysis: MAGNUM monitoring uses flux-to-flux regression to demonstrate constant-variable AGN SEDs, supporting the use of single-component accretion disk models for variability analyses. Power spectral density (PSD) models (either power-law or damped random walk) for stochastic AGN light curves are inferred in the Fourier domain using Bayesian MCMC, with model selection via Bayes factors (Li et al., 2018). This approach robustly separates AGN intrinsic variability from measurement noise and accommodates irregularly sampled data, essential for reverberation mapping and disk parameter inference.

Emission State Statistics: Synthetic AGN light curves drawn from red-noise models predict Gaussian flux distributions for PSDs with index $\beta < 1$; observed multi-modal flux histograms (as in mm/radio monitoring) require multiple emission states or spatially distinct emission zones (Park et al., 2012). This implies that time-domain MUSE or other IFU surveys should jointly assess power spectra and flux distributions to probe AGN variability origins.

3D Tomographic and Geometric Modeling: Nonparametric and modeling approaches reconstruct observed line-of-sight velocity fields and spatial emission distributions to test geometric models of outflows (e.g., hollow cones, clumpy shells), correct for projection effects, and quantitatively connect observed properties to theoretical feedback models (Venturi et al., 2020).

5. Implications for Unification, Galaxy Evolution, and Future Surveys

AGN Unification: Mid-IR and X-ray data compiled at sub-arcsecond resolution confirm a near-linear MIR–X-ray luminosity correlation over six decades, invariant with viewing angle, obscuration (up to $N_H \sim 10^{23.3}$ cm⁻²), or AGN type, except at very low Eddington ratios. This emphasizes the universal nature of nuclear dust reprocessing and supports unified models where orientation, not intrinsic structure, drives observed diversity (Asmus et al., 2013).

Feedback and Coevolution: The ability to directly map AGN impact (photoionization, shocks, winds) on circumnuclear ISM demonstrates both negative (suppression of star formation, ISM clearing) and positive (star formation triggering) feedback modes. Quantitative analysis of outflow momentum/energy relative to AGN radiation, bolometric output, and host star formation rates is central to constraining evolutionary models.

Multi-wavelength and Multi-messenger Synergy: Findings from AGN monitored with MUSE and other modalities provide crucial context for gravitational-wave measurements of AGN disks, allowing mapping of spin alignment and disk age/density constraints onto resolved, optical/IR kinematic and excitation maps (Vajpeyi et al., 2021).

Survey Design and Data Products: Large IFU surveys (MAGNUM, MURALES, MaNGA) deploying robust spatially resolved diagnostics have revealed hidden AGN populations, clarified the role of stellar and bar-driven gas inflows in AGN fueling, and established benchmark datasets for outflow phase and metallicity modeling (Wylezalek et al., 2017, Balmaverde et al., 2019).

6. Limitations, Challenges, and Future Directions

• Density and Extinction Uncertainties: Electron density and dust extinction estimates, especially for outflowing and off-nuclear gas, are dominant sources of systematic error in mass, metallicity, and kinetic energy calculations (Mingozzi et al., 2018, Amiri et al., 15 Sep 2025).
• Projection and Geometric Complications: The complex, clumpy, and anisotropic morphology of AGN outflows (e.g., the “tuning-fork” structure resolved to 2 pc scales in Circinus) requires detailed 3D modeling to interpret observed line maps and velocities (Kakkad et al., 2023).
• Multi-phase Outflows and Global Impact: Ionized gas outflows are only one phase; molecular and neutral gas observations are needed for a complete mass and momentum budget. Observed ionized outflows alone often lack sufficient mass flux to quench star formation on short timescales, necessitating studies of total feedback efficacy (Venturi et al., 2020, Kakkad et al., 2023).
• AGN Diversity and Cosmic Evolution: While the local MAGNUM sample reveals predominantly inside-out metallicity gradients and limited metal redistribution by AGN outflows, the situation may differ at high redshift, requiring comparative studies spanning cosmic time and AGN luminosity regimes (Venturi et al., 2020).

The MAGNUM framework under the MUSE microscope thus establishes a physically rigorous, multi-scale, multi-wavelength paradigm for AGN and host galaxy paper that is influencing theories of feedback, coevolution, and the microphysics of the active nucleus environment.