The first direct measurement of cluster weather found dead calm where forecasts promised storms. Hitomi's micro-calorimeter, in its brief 2016 flight, resolved the velocity structure of the Perseus cluster's core and measured a line-of-sight velocity dispersion of just 164 +/- 10 km/s between 30 and 60 kiloparsecs: turbulent kinetic pressure below ten percent of thermal, in a core hosting an active nucleus visibly inflating cavities, ringed by cold fronts, and stirred by an ongoing sloshing spiral. Simulations of cool-core clusters with operating AGN feedback predicted substantially stronger motions; the measured atmosphere is barely disturbed by the violence it demonstrably contains. XRISM, the recovery mission, has generalized the surprise: velocity dispersions of 120 to 170 km/s in the cores of Centaurus, Abell 2029, and Hydra A, and quiescence even in the dynamically disturbed core of Ophiuchus, establishing calm as the rule rather than a Perseus peculiarity.
The quiet creates a paired crisis with the heating budget: AGN feedback must transport and dissipate enough energy to balance radiative cooling (the cooling-flow requirement), yet whatever distributes the heat leaves almost no kinetic trace, ruling out the strong-turbulence transport channels most simulations employ and forcing the modeling toward sound waves, internal gravity waves, cosmic-ray streaming, each capable in part, none established as sufficient. Pressure-profile turbulence fractions inferred at larger radii (the X-COP results) sit equally low, so the calm extends beyond cores: the cluster atmospheres are systematically less turbulent than hierarchical assembly plus feedback modeling produces.
The standing is a precision anomaly at the start of its instrument era: XRISM is converting single-number dispersions into velocity maps cluster by cluster, and every map so far deepens the question of how cluster gas can be simultaneously heated, stirred, sloshed, and still.