Dark Halos Predicted from Hydrostatic Structure in Galaxy Clusters

Abstract

This paper tests whether the radial structure usually attributed to dark halos in galaxy clusters can be predicted directly from hydrostatic geometry. Using LoCuSS profiles, we define the residual mass as M_res = M_hydro - M_gas and show that hydrostatic compactness, C_hydro = M_hydro(r2500) / M_hydro(r500), predicts the residual halo slope with Spearman rho = 0.981 and p = 7.40e-33. The result survives null tests, mass controls, gas-compactness controls, and leave-one-cluster-out prediction. An external fixed-radius validation with X-COP reproduces the same ordering. This is Paper II of the “Dark Halos Predicted” series: Paper I tested baryonic radial transitions in galaxies; here the same residual-shape question is extended to galaxy clusters.