2020NatAs...4..900Q

Galaxy clusters are the most massive collapsed structures in the Universe, with potential wells filled with hot, X-ray-emitting intracluster medium (ICM). Observations, however, show that a substantial number of clusters (the so-called cool-core clusters) also contain large amounts of cold gas in their centres, some of which is in the form of spatially extended filaments spanning scales of tens of kiloparsecs1,2. These findings have raised questions about the origin of the cold gas, as well as its relationship with the central active galactic nucleus (AGN), whose feedback has been established as a ubiquitous feature in such galaxy clusters3-5. Here, we report a radiation-hydrodynamic simulation of AGN feedback in a galaxy cluster, in which cold filaments form from the warm, AGN-driven outflows with temperatures between 104 and 107 K as they rise in the cluster core. Our analysis reveals a new mechanism that, through the combination of radiative cooling and ram pressure, naturally promotes outflows whose cooling times are shorter than their rising times, giving birth to spatially extended cold gas filaments. Our results strongly suggest that the formation of cold gas and AGN feedback in galaxy clusters are inextricably linked and shed light on how AGN feedback couples to the ICM.