Pressure Turbulence Profiles
The turbulent pressure in the intracluster medium of galaxy clusters — arising from random bulk motions, internal waves, and chaotic flows — contributes to the total pressure support and introduces scatter in X-ray scaling relations. Cosmological simulations of cluster formation predict specific profiles of turbulent pressure as a function of cluster radius, with turbulence typically contributing 5–20 percent of the total pressure in the bulk of the cluster and rising toward the cluster outskirts where ongoing accretion drives strong infall motions. Observations from X-ray spectroscopy (measuring velocity broadening and centroid shifts of emission lines with Hitomi/XRISM) and from Sunyaev-Zeldovich pressure profile fitting find turbulent contributions that are broadly consistent with simulations in some cases but show systematic differences in both amplitude and radial profile shape in others, with certain clusters exhibiting more centrally concentrated turbulence or different radial slopes than simulations produce.
Successive Collision Theory produces turbulent pressure profiles that differ systematically from CDM simulation predictions through the angular momentum inheritance mechanism. In SCT, galaxy clusters formed from angular-momentum-organized collision debris, and the ICM retains organized bulk motions that are coherent on scales set by the cluster's inherited angular momentum — larger-scale and more organized than the turbulence generated by random small-scale mergers in CDM simulations. These large-scale coherent motions contribute to the non-thermal pressure budget in a way that appears as turbulent broadening in X-ray spectral lines but with velocity structure functions that differ from Kolmogorov turbulence: the power is more concentrated at large scales (low wavenumber) than standard turbulence cascade models predict. XRISM measurements of the Perseus cluster's velocity field, which show surprisingly laminar large-scale motions alongside moderate small-scale turbulence, are consistent with this SCT picture of predominantly angular-momentum-organized rather than randomly stirred ICM dynamics.
The gravitational superposition from overlapping nested frames introduces a spatially varying effective pressure contribution to the ICM that modifies the turbulent pressure profile without itself being turbulent. In the cluster core, the superposition contribution is large and relatively uniform, suppressing the need for turbulent pressure support there and producing lower measured turbulence than simulations predict for the core region. In the cluster outskirts, where the superposition contribution falls off more steeply, the ICM requires more turbulent and bulk motion pressure to maintain equilibrium with the infalling material from the surrounding filamentary large-scale structure, producing higher turbulent fractions in the outskirts. This radial gradient in superposition pressure support — decreasing from center to outskirts — naturally produces the observed trend for turbulent fractions to increase with cluster radius, but with a steeper radial gradient than CDM simulations generate from merger-driven turbulence alone.