The self-regulated AGN feedback loop: the role of chaotic cold accretion

TL;DR

This paper reviews how chaotic cold accretion (CCA) regulates supermassive black hole growth and feedback, creating a self-regulating cycle that maintains galaxy and cluster gas in thermal equilibrium.

Contribution

It introduces the concept of chaotic cold accretion as a key mechanism linking black hole activity with galaxy evolution, emphasizing its role in self-regulated feedback loops.

Findings

CCA boosts accretion rates up to 100 times the Bondi rate.

AGN outflows quench cooling flows and star formation.

The feedback cycle maintains the hot halo's thermal balance.

Abstract

Supermassive black hole accretion and feedback play central role in the evolution of galaxies, groups, and clusters. I review how AGN feedback is tightly coupled with the formation of multiphase gas and the newly probed chaotic cold accretion (CCA). In a turbulent and heated atmosphere, cold clouds and kpc-scale filaments condense out of the plasma via thermal instability and rain toward the black hole. In the nucleus, the recurrent chaotic collisions between the cold clouds, filaments, and central torus promote angular momentum cancellation or mixing, boosting the accretion rate up to 100 times the Bondi rate. The rapid variability triggers powerful AGN outflows, which quench the cooling flow and star formation without destroying the cool core. The AGN heating stifles the formation of multiphase gas and accretion, the feedback subsides and the hot halo is allowed to cool again, restarting a new cycle. Ultimately, CCA creates a symbiotic link between the black hole and the whole host via a tight self-regulated feedback which preserves the gaseous halo in global thermal equilibrium throughout cosmic time.