Unveiling the Dynamics of X-ray Bubbles in Galaxy Clusters

Congyao Zhang (The University of Chicago)

Galaxy clusters are colossal structures in the Universe, with masses that can range from a few hundred to several thousand trillion solar masses. They emit bright X-ray radiation due to the presence of hot gas filling their deep potential wells. Despite the efficient energy loss via radiation, most of the gas within the cluster cores does not cool below X-ray a few million degrees. This is where the role of supermassive black holes located at the centers of clusters comes into play. These black holes are widely accepted as the primary energy source that heats the gas and prevents it from overcooling catastrophically. Understanding these cooling and heating processes - their competition, balance, and role in shaping the cluster environment - is at the forefront of cluster studies.

Bubbles of relativistic plasma, seen in X-rays as cavities, are a crucial element in the story. Black holes located at the centers of galaxy clusters power jets that inflate giant bubbles and release most of their energy output there. From the energy point of view, we know observationally that bubbles have sufficient energy to balance the cooling losses. They buoyantly rise in the atmosphere and perturb the cluster core by uplifting gas from its innermost region and driving sound waves, gravity waves, and turbulence along their rising path (see Fig. 1). However, our knowledge of the bubbles’ behaviors and their dynamics is still limited. For example, we do not even know how long bubbles can survive in galaxy clusters! How they interact with and heat the cluster atmospheres, is another active question. The difficulties largely stem from the complexity of the bubble-cluster atmosphere system, which depends upon a number of physical properties including magnetic fields, gas microphysics, cluster environment, etc. 

Athena will be a key mission to address the above questions thanks to its unprecedented combination of large-area high-resolution imaging and spectroscopy. The current and past X-ray observatories provide only stationary views of bubbles in galaxy clusters. Hitomi and the upcoming XRISM mission can measure velocities, but their spatial resolution limits such studies to a relatively small number of nearby and bright galaxy clusters. Mapping gas velocity and metallicity of galaxy clusters with Athena will be a breakthrough in the field, providing a dynamic picture of the bubble interaction with the cluster atmosphere. We will directly see, in great detail, how buoyant bubbles drive gas motions and redistribute metals, which is vital to understanding the evolution of cluster cores.