Observational Evidence of AGN Feedback

TL;DR

This paper reviews observational evidence of AGN feedback mechanisms, highlighting how radiation, winds, and jets from active galactic nuclei influence galaxy evolution by heating or ejecting gas, thereby regulating star formation and black hole growth.

Contribution

It synthesizes current observational evidence for different AGN feedback modes, emphasizing the importance of radio and X-ray observations in understanding galaxy evolution.

Findings

Radio jets create bubbles in hot gas, heating the intracluster medium.

X-ray observations show feedback maintains a balance between heating and cooling.

Feedback mechanisms suppress star formation and regulate black hole growth.

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.