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

TL;DR

This study uses simulations to show how stars formed in the Milky Way's warp settle into the disc, contaminate the thick disc, and migrate radially, affecting the galaxy's stellar distribution and age gradients.

Contribution

It provides a detailed dynamical model of warp star settling, phase mixing, and radial migration, revealing their impact on the thick disc and stellar populations.

Findings

Warp stars tilt and align with the disc within ~1 Gyr.

They phase mix into an axisymmetric distribution over ~6 Gyr.

Warp stars are present throughout the disc, including the Solar Neighbourhood.

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 $\sim1$ Gyr. Then, once differential precession halts this process, they phase mix into an axisymmetric distribution on a time scale of $\sim 6$ Gyr. The warp stars end up contaminating the geometric thick disc. Because the warp in our fiducial simulation is growing, the {\it 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.