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X-ray Bubbles in Galaxy Clusters and their Role in Generating Internal Gravity Waves and Shaping Ha Filaments

Presentation #101.04 in the session Galaxy Clusters/Large Scale Structures.

Published onJul 01, 2023
X-ray Bubbles in Galaxy Clusters and their Role in Generating Internal Gravity Waves and Shaping Ha Filaments

Radio-mode of AGN feedback has been widely accepted as a promising heating mechanism of the intracluster medium (ICM). High-resolution X-ray observations show that the mechanical energy released inside the bubbles of relativistic plasma is sufficient to balance the radiative cooling losses in cluster cores. However, through what specific way(s) the bubble energy is transferred to the ICM is still under debate. This is one of the biggest remaining questions in cluster studies.

In this talk, I will introduce internal gravity waves as an attractive way to solve the problem. These waves are generated through long-term interaction between intact X-ray bubbles and stratified ICM. They propagate horizontally and downwards from the rising bubbles, spreading energy over large volumes of the cluster core. Such a process was always missed in the previous studies due to the difficulty of modeling long-live bubbles in numerical simulations. We overcame this issue by developing a novel rigid-bubble model and found that terminal velocities of the flattened bubbles are small enough to drive efficiently internal waves. If our findings are scaled to the conditions of the Perseus cluster, the expected bubble’s terminal velocity is ∼100−200 km/s near the cluster inner region, broadly consistent with the direct measurement by the Hitomi satellite.

Meanwhile, our simulations showed that buoyantly rising bubbles play an important role in shaping the ~10-100 kpc long ionized/molecular filaments (e.g., Ha and CO) in cluster cores. The gas is dragged up by the eddies in the bubble wake and radially stretched during the propagation, which is expected to have high radial velocities, i.e., by a factor of ~2-3 higher than the bubble’s rise velocity. Our model explains both the shape and amplitude of the filaments’ velocity structure function measured in nearby clusters.

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