The emergence of the thick disk in a CDM universe. II. Colours and abundance patterns

The recently emerging conviction that thick disks are prevalent in disk galaxies, and their seemingly ubiquitous old ages, means that the formation of the thick disk, perhaps more than any other component, holds the key to unravelling the evolution of the Milky Way, and indeed all disk galaxies. In Paper I, we proposed that the thick disk was formed in an epoch of gas-rich mergers at high redshift. This hypothesis was based on comparing N-body SPH simulations to a variety of Galactic and extragalactic observations, including stellar kinematics, ages, and chemical properties. Here we examine our thick-disk formation scenario in light of the most recent observations of extragalactic thick disks. In agreement, our simulated thick disks are old and relatively metal rich, with V-I colors that do not vary significantly with distance from the plane. Furthermore, we show that our proposal results in an enhancement of α-elements in thick-disk stars as compared with thin-disk stars, consistent with observations of the relevant populations of the Milky Way. We also find that our scenario naturally leads to the formation of an old, metal-weak, stellar halo population with high α-element abundances.