The Stellar Disk–Halo Connection

Abstract

The thick disk is the stellar disk-halo connection. At least near the solar circle, this component is on average as old as the system of disk globular clusters, or ~ 12 Gyr. This implies that it most probably formed early in the process of Galaxy formation, so that its properties – chemical abundances, stellar kinematics and spatial distribution – contain clues to the physics of these stages of Galaxy evolution. Its present-day importance for the interstellar disk-halo connection lies in the evolution of its constituent stars – gas loss through winds on the red giant and asymptotic giant branches, through planetary nebulae prior to white dwarf formation, and through supernovae. This gas loss results in mass injection, momentum injection and energy injection into the interstellar medium from a stellar population with a scale height of ~ 1kpc.