Further Evidence of a Merger Origin for the Thick Disk: Galactic Stars along Lines of Sight to Dwarf Spheroidal Galaxies

Abstract

The history of the Milky Way is written in the properties of its stellar populations. Here we analyze stars observed as part of surveys of local dwarf spheroidal galaxies, but which from their kinematics are highly likely to be nonmembers. The selection function-designed to target metal-poor giants in the dwarf galaxies, at distances of ∼100 kpc - includes F-M dwarfs in the Milky Way, at distances of up to several kiloparsecs. The stars whose motions are analyzed here lie in the cardinal directions of Galactic longitude l ∼ 270° and l ∼ 90°, where the radial velocity is sensitive to the orbital rotational velocity. We demonstrate that the faint F and G stars contain a significant population with V Φ ∼ 100 km s -1, similar to that found by a targeted, but limited in areal coverage, survey of thick disk and halo stars by Gilmore et al. This value of mean orbital rotation does not match either the canonical thick disk or the stellar halo. We argue that this population, detected at both l ∼ 270° and l ∼ 90°, has the expected properties of "satellite debris" in the thick disk-halo interface, which we interpret as remnants of the merger that heated a preexisting thin disk to form the thick disk.