Massive Black Hole Feedback

Updated 5 September 2025

Massive black hole feedback is the process where energy from SMBHs, via radiative and mechanical channels, regulates star formation and gas dynamics in galaxies.
Observations—such as X-ray cavities and radio jets—combined with simulations provide insights into how SMBH feedback shapes the intracluster medium and galaxy morphology.
Theoretical frameworks indicate that both continuous and episodic feedback can significantly alter gas cooling rates, quench star formation, and define scaling relations in cosmic structures.

Massive black hole feedback refers to the processes by which energy and momentum released by accreting supermassive black holes (SMBHs) at the centers of galaxies are transferred to their local and large-scale environments, affecting the thermal, dynamical, and structural properties of galaxies, groups, and clusters. This feedback links SMBH activity to both the suppression of star formation and the regulation of baryonic and dark matter distributions across cosmic history. The following sections synthesize the principal mechanisms, observational diagnostics, theoretical frameworks, and broader implications of massive black hole feedback.

1. Physical Mechanisms of Feedback

Massive black hole feedback is generally categorized into two primary channels—radiative and mechanical (jet/wind) feedback—distinguished by the mode and rate of accretion onto the SMBH (0903.4424, Ruszkowski et al., 2019).

Radiative Feedback: At high accretion rates, most notably during quasar phases (approaching the Eddington limit), SMBHs emit intense electromagnetic radiation. This radiation can:
- Heat ambient gas via photoionization and Compton processes (e.g., X-ray and optical/UV photons).
- Exert radiation pressure, especially on cold, dusty gas. The combination of direct pressure and trapping of IR radiation (via dust) can drive shell-like outflows with velocities of 0.1–0.4c, as observed in luminous quasars and dusty high-redshift systems (Nardini et al., 2015, Ishibashi, 2019).
- Regulate star formation by increasing the entropy and temperature of the interstellar medium (ISM), reducing the capacity for collapse and molecular cloud formation (Kim et al., 2011).
Mechanical Feedback: At lower accretion rates (below Eddington), energy deposition is dominated by:
- Collimated relativistic jets, which drive kinetic outflows and inflate large cavities ("bubbles") in hot gas, launching shocks and turbulence on scales from kiloparsecs to hundreds of kiloparsecs.
- Sub-relativistic disk winds, broader in opening angle but capable of distributing energy to the ISM and circumgalactic medium (CGM).
- The mechanical energy input is quantified as $E_{\mathrm{mech}} \simeq pV$ , where $p$ is the pressure of the ambient medium and $V$ the cavity volume (0903.4424).

Both feedback channels may alternate or coexist, with their efficiency and observable signatures modulated by the properties of the host galaxy, local gas content, dust fraction, and the SMBH’s instantaneous accretion state.

2. Observational Diagnostics

Massive black hole feedback is diagnosed through a combination of multiwavelength observations, providing direct evidence of its energy injection signatures:

X-ray Cavities, Bubbles & Shock Fronts: Chandra and XMM-Newton imaging reveals large cavities and ripples in the X-ray-emitting hot gas in systems such as MS0735.6+7421, Perseus, and M87. Energetics inferred from these features are consistent with AGN jet outbursts that have injected $10^{60}$ – $10^{62}$ erg (0903.4424, Zinger et al., 2020).
Fast Outflows in Quasars: UV and X-ray spectroscopy identifies high-velocity absorption lines (e.g., Fe XXVI in PDS 456) consistent with wide-angle or quasi-spherical winds driven from the accretion disk, with kinetic power sometimes exceeding 10% of quasar bolometric luminosity (Nardini et al., 2015).
Thermodynamics of the ICM and CGM: Entropy ( $K = k_B T n_e^{-2/3}$ ), temperature, and cooling time profiles, derived from X-ray observations, often show elevated values or sharp rises outside compact cores, indicating sustained non-gravitational heating (Eckert et al., 16 Jun 2025).
Sunyaev–Zel’dovich (SZ) and Inverse Compton Effects: High-resolution microwave measurements are sensitive to the pressure and composition of AGN-inflated bubbles—disentangling thermal versus relativistic plasma content (Ruszkowski et al., 2019).
Radio & Multi-phase Gas Morphologies: Radio jets and associated structures serve as tracers of mechanical feedback, while molecular (CO, HCN), atomic ([C I]), and ionized (H36α) line emissions, especially when resolved on subparsec scales via ALMA, directly reveal the inflow and feedback-driven outflows near SMBHs (Izumi et al., 2023).

3. Theoretical Frameworks and Scaling Laws

Feedback is modeled in both idealized and cosmological simulations with subgrid prescriptions tied to physically motivated criteria:

Energy and Momentum Input: Total feedback energy imparted by an SMBH during growth is frequently compared to the binding energy of the host system. The feedback is considered "ejective" if the integrated kinetic (or radiative) energy exceeds the gas binding energy within a characteristic radius, i.e., $\int \dot E_{\mathrm{kin}}\, dt > E_{\mathrm{bind,gal}}$ (Terrazas et al., 2019).
Duty Cycles and Self-regulation: Simulations indicate that AGN feedback operates in highly intermittent bursts (duty cycles $\sim$ few percent) in massive ellipticals, with flaring triggered by central cooling catastrophes and resulting in rapid cessation of both accretion and star formation (Ciotti et al., 2011).
Precise Triggering Conditions: In cosmological models such as IllustrisTNG and TNG50, the transition to a vigorous mechanical/kinetic feedback ("wind mode") typically coincides with the SMBH mass exceeding $\sim 10^8\, M_\odot$ , with feedback activation thresholds tied to Eddington-scaled accretion rates and properties of the local gas reservoir (Zinger et al., 2020, Frosst et al., 10 Sep 2024).
Mass Scaling and Quenching: Models show that the impact of feedback scales steeply with SMBH-to-stellar mass ratio. Kinetic energy (from jets) may become the leading source of non-gravitational heating in massive halos, overtaking stellar feedback in systems where $M_{\mathrm{BH}}/M_\star \gtrsim 0.003$ (Heckman et al., 2023).
Environments and Episodicity: The effectiveness and self-regulation of feedback are found to depend on halo mass, potential depth, and CGM pressure. In more massive systems, feedback is more continuous; in lower-mass or higher-pressure environments, AGN outbursts become highly episodic or, if unregulated, explosive, possibly expelling significant gas (Prasad et al., 2020).

4. Impact on Galaxy, Group, and Cluster Evolution

Massive black hole feedback is central to several key aspects of cosmic structure formation:

Regulation of Star Formation: By heating, ejecting, and preventing the cooling of interstellar and circumgalactic gas, feedback acts as the chief quenching agent for star formation in massive galaxies, explaining the observed transformation from blue, star-forming to red, passive systems (Choi et al., 2018, Ruszkowski et al., 2019).
Morphological and Structural Transformation: AGN-driven outflows cause "puffing up" of central stellar distributions, reducing core stellar densities and facilitating size growth through subsequent dry minor mergers (Choi et al., 2018). Gas holes in discs and stabilization against bar formation in massive spirals are also direct consequences of kinetic feedback (Frosst et al., 10 Sep 2024).
Hot Atmospheres and Quiescent Populations: In groups and clusters, feedback increases gas entropy and cooling times, sustaining long-lived hot atmospheres, quenching both central and satellite galaxies, and creating "fossil" groups with old stellar populations (Martin-Navarro et al., 2019, Eckert et al., 16 Jun 2025).
Limits on Galaxy Mass: Extreme feedback episodes can inject energy comparable to or exceeding the total binding energy of halo gas, ejecting baryons from the group potential, disrupting cool cores, and suppressing the formation of ultra-massive galaxies ( $M_\star \gg 10^{12} M_\odot$ ) (Eckert et al., 16 Jun 2025).
Scaling Relations: Observed and simulated relationships between SMBH mass and bulge stellar mass, as well as the abrupt transitions in specific star formation rate versus SMBH mass, are direct consequences of efficient feedback regulation (Terrazas et al., 2019).

5. Global Energy and Momentum Budgets

Quantitative inventories clarify the relative roles of AGN and stellar feedback over cosmic time (Heckman et al., 2023):

Channel	Kinetic Energy Density (erg Mpc $^{-3}$ )	Momentum Fraction (relative to stars)
Jets (AGN)	$2.6 \times 10^{59}$	0.1
Winds (AGN)	$4.3 \times 10^{58}$	0.04
Stellar (SNe, winds)	$\sim (0.65-6.5) \times 10^{59}$	1.0

AGN-jet feedback dominates the non-stellar kinetic energy input, especially at higher galaxy masses and later times ( $z\sim1$ –0).
Stellar feedback is the primary source of momentum, but SMBH feedback becomes increasingly dominant in the most massive galaxies, contributing substantially to quenching and the heating of hot halos.

6. Future Prospects and Open Challenges

Several frontier questions remain regarding the mechanics and consequences of massive black hole feedback:

Energy Coupling and Dissipation: The detailed physical processes—whether via jets, bubbles, sound waves, shocks, or cosmic rays—by which SMBH-injected energy is distributed and thermalized across ~9 orders of magnitude in scale remain uncertain. New spectral and spatial X-ray capabilities (e.g., resolving Fe K α profiles to velocities $\lesssim 10$ km/s) are expected to directly map and quantify these processes (0903.4424, Ruszkowski et al., 2019).
Feedback Efficiency and Environmental Dependence: The sensitivity of feedback effectiveness to environmental parameters, such as CGM pressure and potential depth, requires further high-resolution numerical modeling with varied initial and boundary conditions (Prasad et al., 2020).
Multiphase Outflow and Accretion Physics: Direct spatially resolved observations on subparsec to kiloparsec scales using ALMA and JWST are providing constraints on how much inflowing gas is processed through the SMBH accretion disk versus expelled as outflows or recycled as turbulence, emphasizing the role of gravitational instabilities in fueling SMBHs and the importance of multiphase outflow feedback (Izumi et al., 2023).
Unified Feedback Models: There is presently no consensus on a unified feedback prescription; models differ substantially in how they distribute SMBH energy, handle momentum transfer, and represent accretion/outflow coupling (Eckert et al., 16 Jun 2025).

7. Broader Implications and Synthesis

Massive black hole feedback emerges as the principal agent shaping the baryon cycle in massive galaxies, groups, and clusters. It regulates cold gas content, influences structural and morphological transformation, quenching, and the assembly of stellar and dark matter profiles. AGN feedback energizes the hot CGM and ICM, ensures persistent divergence between blue and red galaxy populations, and prevents the overproduction of massive galaxies. Observational evidence, supported by multiwavelength diagnostics, numerical simulations (e.g., IllustrisTNG, FIRE, MACER), and theoretical modeling, converges on a picture where feedback couples SMBH energy output to the fate of baryonic structure formation over cosmic time, but significant questions remain regarding the microphysics and efficiency of the coupling, as well as the environmental modulation of feedback processes. Future observational and simulation advances will further clarify the quantitative role and mechanisms of feedback in the cosmic evolution of structure.