A snapshot of the oldest AGN feedback phases

TL;DR

This paper uses LOFAR observations to study the oldest phases of AGN-inflated bubbles in a galaxy group, revealing their evolution, buoyancy effects, and interaction with the surrounding medium over hundreds of millions of years.

Contribution

It provides the first detailed observational insights into the late evolution of ancient AGN bubbles using low-frequency radio data, highlighting their role in galaxy feedback.

Findings

Oldest AGN bubbles observed with unprecedented detail.

Bubbles can offset cooling of the intragroup medium.

Magnetic fields likely inhibit mixing with thermal gas.

Abstract

Active Galactic Nuclei (AGN) inject large amounts of energy into their host galaxies and surrounding environment, shaping their properties and evolution. In particular, AGN jets inflate cosmic-ray lobes, which can rise buoyantly as light `bubbles' in the surrounding medium, displacing and heating the encountered thermal gas and thus halting its spontaneous cooling. These bubbles have been identified in a wide range of systems. However, due to the short synchrotron lifetime of electrons, the most advanced phases of their evolution have remained observationally unconstrained, preventing us to fully understand their coupling with the external medium, and thus AGN feedback. Simple subsonic hydrodynamic models predict that the pressure gradients, naturally present around the buoyantly rising bubbles, transform them into toroidal structures, resembling mushroom clouds in a stratified atmosphere. The way and timescales on which these tori will eventually disrupt depend on various factors including magnetic fields and plasma viscosity. Here we report LOFAR observations below 200 MHz, sensitive to the oldest radio-emitting particles, showing the late evolution of multiple generations of cosmic-ray AGN bubbles in a galaxy group with unprecedented level of detail. The bubbles' buoyancy power can efficiently offset the radiative cooling of the intragroup medium. However, the bubbles have still not thoroughly mixed with the thermal gas, after hundreds of million years, likely under the action of magnetic fields.