How stars formed in warps settle into (and contaminate) thick discs

Abstract

In recent years star formation has been discovered in the Milky Way’s warp. These stars formed in the warp (warp stars) must eventually settle into the plane of the disc. We use an N-body+smooth particle hydrodynamics model of a warped galaxy to study how warp stars settle into the disc. By following warp stars in angular momentum space, we show that they first tilt to partially align with the main disc in a time scale of ∼1Gyr⁠. Then, once differential precession halts this process, they phase mix into an axisymmetric distribution on a time scale of ∼6Gyr⁠. The warp stars end up contaminating the geometric thick disc. Because the warp in our fiducial simulation is growing, the warp stars settle to a distribution with a negative vertical age gradient as younger stars settle further from the mid-plane. While vertically extended, warp star orbits are still nearly circular and they are therefore subject to radial migration, with a net movement inwards. As a result warp stars can be found throughout the disc. The density distribution of a given population of warp stars evolves from a torus to an increasingly centrally filled-in density distribution. Therefore we argue that, in the Milky Way, warp stars should be found in the Solar Neighbourhood. Moreover, settled warp stars may constitute part of the young flaring population seen in the Milky Way’s outskirts.