Yuan Li - Turbulence in the intra-cluster medium

Turbulence is ubiquitous in astrophysics and strongly impacts many processes related to galaxy evolution, including particle acceleration, magnetic field amplification, and the energy deposition of feedback from stars and black holes. In galaxy clusters, the majority of the volume is occupied by the hot, X-ray-emitting plasma known as the intra-cluster medium (ICM). Turbulence in the ICM can be driven by structure formation and supermassive black hole feedback, but directly observing it remains a challenge due to the limited spatial and spectral resolution of current X-ray telescopes. To address this limitation, my group has developed an innovative method to measure ICM turbulence using cool clouds as kinematic tracers. These clouds are found in both cluster centers and the outskirts in the tails of jellyfish galaxies. They can be observed with ground-based optical and radio telescopes that provide far superior spatial and spectral resolution compared to X-ray telescopes. We study the clouds’ kinematics by measuring their velocity structure functions (VSFs) across a wide range of scales. In cluster centers, the motions of the clouds are directly impacted by supermassive black hole feedback. Intriguingly, the VSFs we observe are significantly steeper than the classical Kolmogorov expectations, and the underlying physical reasons are still a subject of debate. In cluster outskirts, we have used jellyfish galaxy tails to probe turbulence driven by Kelvin-Helmholtz instability, finding evidence of turbulent motions on scales below the plasma mean free path. These measurements place unprecedented constraints on the effective isotropic viscosity of the ICM.