Solving the Cooling Flow Problem through Mechanical AGN Feedback

TL;DR

This paper demonstrates that mechanical AGN feedback via subrelativistic outflows can effectively solve the cooling flow problem in galaxy clusters, groups, and galaxies, maintaining cool-core structures over billions of years.

Contribution

It introduces a self-regulated, mechanically driven AGN feedback model that reproduces key observed features and prevents catastrophic cooling without destroying cool cores.

Findings

Mechanical feedback quenches cooling flows for several Gyr.

Bipolar outflows produce observed features like bubbles and shocks.

Turbulence drives cold gas formation, fueling feedback.

Abstract

Unopposed radiative cooling of plasma would lead to the cooling catastrophe, a massive inflow of condensing gas, manifest in the core of galaxies, groups and clusters. The last generation X-ray telescopes, Chandra and XMM, have radically changed our view on baryons, indicating AGN heating as the balancing counterpart of cooling. This work reviews our extensive investigation on self-regulated heating. We argue that the mechanical feedback, based on massive subrelativistic outflows, is the key to solving the cooling flow problem, i.e. dramatically quenching the cooling rates for several Gyr without destroying the cool-core structure. Using a modified version of the 3D hydrocode FLASH, we show that bipolar AGN outflows can further reproduce fundamental observed features, such as buoyant bubbles, weak shocks, metals dredge- up, and turbulence. The latter is an essential ingredient to drive nonlinear thermal instabilities, which cause the formation of extended cold gas, a residual of the quenched cooling flow and, later, fuel for the feedback engine. Compared to clusters, groups and galaxies require a gentler mechanical feedback, in order to avoid catastrophic overheating. We highlight the essential characteristics for a realistic AGN feedback, with emphasis on observational consistency.