The partial compactification of the universal centralizer

The universal centralizer of a semisimple algebraic group is the family of centralizers of regular elements, parametrized by their conjugacy classes. When the group is of adjoint type, we construct a smooth, log-symplectic fiberwise compactification of the universal centralizer by taking the closure of each fiber in the wonderful compactification. We use the geometry of the wonderful compactification to give an explicit description of its symplectic leaves. We also show that the compactified centralizer fibers are isomorphic to certain Hessenberg varieties -- we apply this connection to compute the singular cohomology of the partial compactification, and to study the geometry of the corresponding universal Hessenberg family.