Cool Core Dichotomy

Galaxy clusters are observed in two broadly distinct states: cool core clusters, in which the central ICM has a short cooling time (less than the Hubble time), exhibits a sharp central density and temperature peak, and hosts a brightest cluster galaxy (BCG) with active star formation and radio-mode AGN feedback; and non-cool core clusters, which have shallower entropy profiles, higher central temperatures, and BCGs with little current star formation. The dichotomy between these two populations — with roughly half of X-ray luminous clusters in each category — is not naturally explained by ΛCDM cluster formation models, which predict a continuous distribution of central cooling states rather than a bimodal one. The physical mechanism that creates and maintains this bimodality — particularly why some clusters maintain cool cores for many Gyr despite AGN heating while others never develop them — remains an open question.

Successive Collision Theory explains the cool core dichotomy through the angular momentum inheritance mechanism applied to the cluster formation geometry. In SCT, clusters formed from angular momentum-organized collision debris, and their formation geometry — whether the debris collapsed in a configuration with high or low net angular momentum — determined whether the resulting cluster developed a cool core. Clusters assembled from high-angular-momentum debris strata settled into more ordered, quasi-stable configurations with centrally concentrated cool gas maintained against disruption by the centrifugal support of the organized angular momentum field. The cool gas in these clusters is preferentially concentrated within the angular momentum barrier radius, which coincides with the BCG's gravitational sphere of influence — naturally producing the tight coupling between cool cores and BCG properties. Non-cool core clusters formed from lower-angular-momentum debris strata that underwent more violent, chaotic merging without sufficient angular momentum to stabilize a central cool reservoir.

The bimodal rather than continuous distribution of cool core properties reflects the underlying bimodality in the angular momentum distribution of the collision debris. The angular momentum field of the initial pocket collision was not smooth and continuous but had preferred strata set by the quantized geometry of the two colliding pocket boundaries — regions of high angular momentum near the collision edges and lower angular momentum near the center of the impact zone. Clusters that formed from these preferred high-angular-momentum strata systematically developed cool cores, while those from the low-angular-momentum strata did not. The fraction of clusters in each category — approximately equal — reflects the proportion of the debris volume in high versus low angular momentum strata, which is set by the geometry of the collision impact parameter.