Were thick galactic disks made by levitation?

Abstract

The thick disk of our galaxy displays kinematic and chemical properties that are intermediate between those of the halo and the (thin) disk stellar populations. Not all disk galaxies have a thick disk. A theory of the origins of a thick disk can potentially provide insights into the physical state of our galaxy in its infancy. Levitation, a process that relies on adiabatic capture into resonance of stellar orbits in a growing disk, is presented as a plausible formation mechanism; a 22 resonance between vertical and epicyclic oscillations drifts to large vertical energies as the disk grows adiabatically. Resonant stars levitate several kiloparsecs above the plane, forming a thick disk whose spatial distributions, kinematics, and ages leave unique observational signatures on the sky. The same process can also produce the disk globular cluster system.