Non-Thermal Pressure Fractions

The total pressure support in galaxy clusters and groups comes from both thermal pressure (from the random motions of hot gas particles) and non-thermal contributions including turbulence, bulk motions, cosmic ray pressure, and magnetic fields. X-ray observations measure the thermal pressure directly through the temperature and density of the hot intracluster medium, and hydrostatic mass estimates assume the total pressure equals the thermal pressure. In reality, non-thermal pressure fractions of 10–30 percent are expected from cosmological simulations of cluster formation, and observational constraints from pressure profile fitting, radial velocity dispersion, and comparison with weak lensing masses broadly support non-thermal fractions in this range. However, the radial profile of the non-thermal pressure fraction — how it varies from cluster center to outskirts — shows systematic differences between observations and simulations, and the total non-thermal pressure budget required to reconcile X-ray hydrostatic masses with weak lensing masses implies fractions that are sometimes larger than simulations naturally produce.

Successive Collision Theory provides a physically motivated source of non-thermal pressure in galaxy clusters through two mechanisms beyond those present in CDM simulations. First, the angular momentum inheritance mechanism deposits organized bulk and turbulent motion into the intracluster medium through the continuous infall of material along angular-momentum-organized orbits. The coherent orbital motions set by the inherited angular momentum field contribute a non-thermal pressure component that is spatially organized — stronger along the cluster's major axis and in the infalling filamentary streams — rather than isotropically distributed as thermal pressure is. This organized non-thermal component adds to the effective pressure support against gravitational collapse without being captured by hydrostatic equilibrium analyses that assume spherical symmetry, naturally producing hydrostatic mass biases consistent with those inferred from lensing comparisons.

Second, the gravitational superposition from overlapping nested frames contributes an effective pressure to the ICM that is distinct from both thermal and turbulent pressure. The superposition term in the modified stress-energy tensor acts as a distributed external gravitational contribution that partially supports the ICM against infall from the cluster outskirts, reducing the central pressure gradient required for hydrostatic equilibrium and effectively mimicking a non-thermal pressure fraction in radial pressure profile analyses. This superposition pressure contribution is largest in the cluster outskirts — where the infall of material from the parent frame introduces the most significant superposition contribution — and smallest in the cluster core, producing a radial non-thermal fraction profile that rises toward the cluster periphery, consistent with the observed tendency for non-thermal fractions to be highest in cluster outskirts.