Active galactic nuclei inject large amounts of energy into their host galaxies and surrounding environment, shaping their properties and evolution^1,2. In particular, active-galactic-nuclei jets inflate cosmic-ray lobes, which can rise buoyantly as light ‘bubbles’ in the surrounding medium³, displacing and heating the encountered thermal gas and thus halting its spontaneous cooling. These bubbles have been identified in a wide range of systems^4,5. However, due to the short synchrotron lifetime of electrons, the most advanced phases of their evolution have remained observationally unconstrained, preventing us from fully understand their coupling with the external medium, and thus active galactic nuclei feedback. Simple subsonic hydrodynamic models^6,7 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^8,9. Here we report observations below 200 MHz, sensitive to the oldest radio-emitting particles, showing the late evolution of multiple generations of cosmic-ray active-galactic-nuclei 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 still have not thoroughly mixed with the thermal gas, after hundreds of million years, probably under the action of magnetic fields.

活跃的星系核向它们的宿主星系和周围环境注入大量能量，塑造它们的属性和演化^1,2。特别是，活动星系核喷流使宇宙射线叶膨胀，这些叶可以像周围介质³中的轻“气泡”一样浮力上升，置换和加热遇到的热气体，从而停止其自发冷却。这些气泡已在广泛的系统中被识别^4,5。然而，由于电子的同步加速器寿命很短，它们演化的最先进阶段在观察上仍然没有受到限制，这使我们无法完全理解它们与外部介质的耦合，从而了解活跃的星系核反馈。简单的亚音速流体动力学模型^6,7预测，在浮力上升的气泡周围自然存在的压力梯度将它们转变为环形结构，类似于分层大气中的蘑菇云。这些环面最终破坏的方式和时间尺度取决于各种因素，包括磁场和等离子体粘度^8,9. 在这里，我们报告了对最古老的无线电发射粒子敏感的 200 MHz 以下的观测结果，以前所未有的细节水平显示了星系群中多代宇宙射线活动星系核气泡的晚期演化。气泡的浮力可以有效抵消群内介质的辐射冷却。然而，经过亿万年之后，这些气泡仍然没有与热气彻底混合，可能是在磁场的作用下。