最早的AGN反馈阶段的快照
A snapshot of the oldest AGN feedback phases
摘要：
活动星系核（AGN）向其宿主星系和周围环境注入大量能量，塑造了它们的性质和演化。特别是活动星系核喷流使宇宙射线瓣膨胀，它们可以在周围介质中以轻“气泡”的形式迅速上升，取代和加热遇到的热气体，从而停止其自发冷却。这些气泡已经在广泛的系统中被发现。然而，由于电子的同步加速器寿命很短，它们演化的最高级阶段在观测上仍然不受约束，这使得我们无法完全理解它们与外部介质的耦合，从而无法完全理解活动星系核的反馈。简单的亚音速流体动力学模型预测，压力梯度，自然存在于周围的螺旋上升的气泡，把它们变成环形结构，类似于蘑菇云在分层的大气。这些环面最终破裂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。在这里，我们报告LOFAR观测低于200兆赫，敏感的最古老的无线电发射粒子，显示了宇宙射线AGN气泡在星系群中的晚期演化，具有前所未有的细节水平。气泡的浮力可以有效地抵消群内介质的辐射冷却。然而，气泡仍然没有与热气体彻底混合，经过数亿年，可能是在磁场的作用下。
英文摘要：
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.
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