Turbulence Traced with Multiphase Filaments in the Centers of Galaxy Clusters

The intra-cluster medium (ICM) in the centers of galaxy clusters is heavily influenced by the "feedback" from supermassive black holes (SMBHs), which prevents catastrophic cooling and suppresses star formation. SMBH feedback can potentially drive turbulence in the ICM, as well as other processes such as type Ia supernovae and "sloshing" as a result of merging substructures. Due to the limited spatial and spectral resolutions of X-ray telescopes, it has been rather challenging to observe turbulence in the hot ICM directly. Recently, we developed a new method to measure turbulence in the hot ICM using multiphase filaments as tracers. These filaments are ubiquitous in cluster centers and can be observed at very high resolution using optical and radio telescopes. We study the kinematics of the filaments by measuring their velocity structure functions (VSF) over a wide range of scales in the centers of ten well-observed galaxy clusters. The motions of the filaments are turbulent in all clusters. There is a clear correlation between features of the VSFs and the activities of the SMBHs, suggesting that SMBHs are the main driver of turbulent gas motions in the centers of galaxy clusters. In all systems, the VSF is steeper than the classical Kolmogorov expectation and the slopes vary from system to system, similar to our previous findings. Several theoretical explanations have been proposed and one of them is that the VSFs we have measured so far mostly reflect the motion of the driver (SMBH jets) rather than the cascade of turbulence. We show that in a small subset of clusters, the VSF of the outer filaments far from the SMBH flattens on small scales to a slope consistent with classical Kolmogorov. This flattening is the most prominent in Abell 1795 with the most extended filaments. Our new results suggest that in the central tens of kpc, the ICM is dominated by bulk motions induced by SMBH feedback rather than turbulence cascade. Outside the SMBH-dominated region, the ICM turbulence is mainly driven by structure formation and shows a classical Kolmogorov scaling. The inferred level of turbulent heating is low, consistent with numerical simulations.