3.5 AGN feedback

AGN feedback is a crucial process in galaxy evolution, involving energy and matter released by supermassive black holes. It can both stimulate and suppress star formation, shaping how galaxies grow and change over time.

This feedback comes in different forms, like radiation heating gas or jets pushing it out. Understanding AGN feedback helps explain galaxy properties we see today, like why massive galaxies stopped forming stars earlier than smaller ones.

Role of AGN feedback

• AGN feedback plays a crucial role in the evolution of galaxies and the intergalactic medium
• Involves the interaction between the energy and matter released by the central supermassive black hole and the surrounding gas and dust
• Can have both positive and negative effects on star formation and galaxy growth

Types of AGN feedback

Radiative vs kinetic feedback

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• Radiative feedback occurs when the intense radiation from the AGN heats and ionizes the surrounding gas
• Kinetic feedback involves the mechanical energy of jets and winds driven by the AGN
• The dominant mode of feedback depends on the AGN luminosity and the properties of the surrounding medium

Positive vs negative feedback

• Positive feedback can trigger star formation by compressing gas clouds (AGN-driven shocks)
• Negative feedback suppresses star formation by heating and expelling gas from the galaxy
• The balance between positive and negative feedback shapes the evolution of galaxies

Impact on galaxy evolution

Regulation of star formation

• AGN feedback can quench star formation in massive galaxies by heating and removing the cold gas reservoir
• Can explain the observed downsizing trend, where massive galaxies form stars earlier and more rapidly than lower-mass galaxies
• Helps maintain the quiescent state of early-type galaxies (ellipticals and lenticulars)

Influence on gas content

• AGN-driven outflows can expel significant amounts of gas from galaxies
• Can lead to the depletion of the interstellar medium and the quenching of star formation
• May also enrich the intergalactic medium with metals produced by stellar evolution

Effects on galaxy morphology

• AGN feedback can alter the morphology of galaxies by redistributing gas and stars
• Can contribute to the formation of bulges and the transformation of spiral galaxies into ellipticals
• May also affect the size and structure of the dark matter halo surrounding the galaxy

AGN feedback in clusters

Heating of intracluster medium

• AGN in the central galaxies of clusters can heat the surrounding intracluster medium (ICM)
• Prevents the ICM from cooling and forming stars, maintaining the hot gas reservoir
• Can explain the observed lack of cool cores in some galaxy clusters

Prevention of cooling flows

• AGN feedback can offset the radiative cooling of the ICM in cluster cores
• Injects energy into the ICM through jets and bubbles, balancing the cooling losses
• Helps solve the cooling flow problem, where the observed star formation rates in cluster cores are much lower than predicted by cooling models

Observational evidence

X-ray cavities in clusters

• Chandra X-ray observations have revealed large cavities in the ICM of some clusters
• These cavities are inflated by the jets and bubbles from the central AGN
• Provide direct evidence for the mechanical energy input from AGN into the ICM

Molecular outflows in galaxies

• Observations of molecular gas in galaxies have detected high-velocity outflows
• These outflows are likely driven by the AGN, as they are too energetic to be explained by stellar feedback alone
• Indicate that AGN can expel significant amounts of cold gas from galaxies

Suppression of star formation

• Studies have found a correlation between AGN activity and the suppression of star formation in galaxies
• Galaxies with active AGN tend to have lower star formation rates than similar galaxies without AGN
• Suggests that AGN feedback plays a role in quenching star formation, particularly in massive galaxies

Theoretical models

Hydrodynamical simulations

• Numerical simulations that include the hydrodynamics of gas and the effects of AGN feedback
• Can model the complex interactions between the AGN and the surrounding medium
• Have been successful in reproducing observed properties of galaxies and clusters (mass-size relation, color bimodality)

Semi-analytical models

• Simplified models that describe the evolution of galaxies using analytical prescriptions
• Incorporate AGN feedback as a key ingredient in regulating galaxy growth
• Can efficiently explore a wide range of parameters and scenarios

Challenges and uncertainties

Coupling of feedback energy

• The efficiency of AGN feedback depends on how well the energy couples to the surrounding gas
• Different feedback mechanisms (jets, winds, radiation) may have different coupling efficiencies
• Uncertainties in the coupling efficiency can affect the predicted impact of AGN feedback

Role of magnetic fields

• Magnetic fields can influence the propagation and dissipation of AGN-driven jets and bubbles
• Can affect the stability and morphology of the ICM in clusters
• The importance of magnetic fields in AGN feedback is still poorly understood

Relative importance of mechanisms

• The relative importance of different AGN feedback mechanisms (radiative vs kinetic, positive vs negative) is not well constrained
• Likely depends on the AGN properties (luminosity, accretion rate) and the surrounding environment
• Disentangling the effects of different mechanisms is a challenge for observations and simulations

Future research directions

Multi-wavelength observations

• Combining observations across different wavelengths (X-ray, optical, radio) can provide a more complete picture of AGN feedback
• Can probe different components of the feedback process (hot gas, cold gas, star formation)
• Upcoming facilities like JWST, SKA, and Athena will enable more detailed studies of AGN feedback

High-resolution simulations

• Increasing the resolution of hydrodynamical simulations can better resolve the small-scale physics of AGN feedback
• Can study the detailed interactions between the AGN and the interstellar medium
• Requires significant computational resources and improved numerical methods

Comparison of feedback modes

• Comparing the effects of different AGN feedback modes (radiative vs kinetic, positive vs negative) in simulations and observations
• Can help constrain the relative importance of different mechanisms in different environments
• May require targeted observations of specific types of AGN and galaxies