Observational Evidence of AGN Feedback (1204.4114v1)

Abstract: Radiation, winds and jets from the active nucleus of a massive galaxy can interact with its interstellar medium leading to ejection or heating of the gas. This can terminate star formation in the galaxy and stifle accretion onto the black hole. Such Active Galactic Nucleus (AGN) feedback can account for the observed proportionality between central black hole and host galaxy mass. Direct observational evidence for the radiative or quasar mode of feedback, which occurs when the AGN is very luminous, has been difficult to obtain but is accumulating from a few exceptional objects. Feedback from the kinetic or radio mode, which uses the mechanical energy of radio-emitting jets often seen when the AGN is operating at a lower level, is common in massive elliptical galaxies. This mode is well observed directly through X-ray observations of the central galaxies of cool core clusters in the form of bubbles in the hot surrounding medium. The energy flow, which is roughly continuous, heats the hot intracluster gas and reduces radiative cooling and subsequent star formation by an order of magnitude. Feedback appears to maintain a long-lived heating/cooling balance. Powerful, jetted radio outbursts may represent a further mode of energy feedback which affect the cores of groups and subclusters. New telescopes and instruments from the radio to X-ray bands will come into operation over the next few years and lead to a rapid expansion in observational data on all modes of AGN feedback.

• The paper demonstrates that both radiative and kinetic AGN feedback modes are crucial in regulating star formation and black hole growth.
• It uses X-ray observations of bubbles and high-velocity outflows as evidence to assess the impact of AGN jets and radiation on galaxy environments.
• The study highlights that even a small fraction of AGN energy output can surpass gravitational binding energy, influencing the co-evolution of galaxies and black holes.

Observational Evidence of AGN Feedback

The paper by A.C. Fabian provides a comprehensive review of the evidence and theoretical models associated with Active Galactic Nucleus (AGN) feedback processes in massive galaxies. AGN feedback encompasses the interactions between the radiation, winds, and jets emitted by a galaxy's central active black hole and its interstellar medium (ISM). This interaction is significant for understanding the regulation of star formation and the growth of supermassive black holes in galaxies.

Modes of AGN Feedback

The paper categorizes AGN feedback into two dominant modes based on the energy outflow's nature:

1. Radiative or Quasar Mode: Also termed as the wind mode, this occurs when the AGN is radiating near the Eddington limit, and the feedback primarily involves expelling cold gas from the galaxy. Collecting direct observational evidence for this mode has been challenging due to its transient nature, but it represents a critical phenomenon for explaining the correlation between black hole mass and stellar velocity dispersion.
2. Kinetic or Radio Mode: Often associated with radio jets, this mode dominates in massive elliptical galaxies, especially within the centers of galaxy clusters. It is responsible for heating the intracluster medium (ICM) and preventing excessive cooling, which would otherwise lead to heightened star formation rates.

Observational Evidence

For the kinetic mode, observational evidence is robust, particularly in the form of X-ray observations that reveal bubbles in the hot gaseous environments of galaxy clusters. These structures imply significant mechanical energy release from jets, supporting the maintenance of a heating/cooling balance that restricts radiative cooling. Such feedback is instrumental in averting unchecked star formation and regulating the ICM's temperature profile.

In contrast, evidence for the radiative or quasar mode is less direct, primarily inferred from high-velocity outflows and absorption features seen in exceptional cases. The radiative mode's impact is conjectured to have been most substantial during earlier cosmic epochs when quasars were more numerous.

Theoretical Considerations and Implications

Quantitative assessments illustrate that the energy output from AGN is sufficient to substantially impact galaxy evolution. The energy produced by the growth of the central black hole significantly exceeds the gravitational binding energy of the galaxy, allowing even a small fraction of this energy to influence the surrounding ISM.

Theoretical models posit that AGN feedback regulates the co-evolution of galaxies and black holes, yet alternative perspectives suggest that this process might not be the exclusive factor dictating galaxy properties. There remains debate over whether AGN feedback solely locks the proportionate growth of black holes and galaxies or merely constitutes one element amidst various evolutionary influences.

Future Prospects

As new observational instruments become operational across X-ray, radio, and other electromagnetic spectrums, a sharp increase in data concerning all modes of AGN feedback is anticipated. Such advances will likely refine existing models and address existing gaps in understanding regarding the coupling of AGN and galaxy evolution, the temporal behavior of feedback mechanisms, and the energy balance within galaxy clusters.

In summation, AGN feedback plays a pivotal role in shaping massive galaxies, with the paper presenting a detailed synthesis of both the prevailing observational data and theoretical models. While substantial progress has been achieved in identifying the mechanisms and impacts of AGN feedback, future insights are expected to emerge through technological advancements and deeper observational exploration.