Stellar orbital properties as diagnostics of the origin of the stellar halo

TL;DR

This study uses cosmological simulations to analyze the metallicities, ages, and orbital properties of Milky Way-like halo stars, revealing that both accreted and in situ stars share similar orbital characteristics, especially their rotation sense.

Contribution

It demonstrates that halo stars formed in situ or accreted exhibit surprisingly similar orbital properties, highlighting the role of angular momentum alignment in galaxy formation.

Findings

Most halo stars are on short-axis tubes with disk-like rotation.

Accreted satellites often share the disk's angular momentum direction.

In situ and kicked-up stars have comparable orbital characteristics.

Abstract

We examine metallicities, ages and orbital properties of halo stars in a Milky-Way like disk galaxy formed in the cosmological hydrodynamical MaGICC simulations. Halo stars were either accreted from satellites or they formed in situ in the disk or bulge of the galaxy and were then kicked up into the halo ("in situ/ kicked-up" stars). Regardless of where they formed both types show surprisingly similar orbital properties: the majority of stars of both types are on short-axis tubes with the same sense of rotation as the disk -- implying that a large fraction of satellites are accreted onto the halo with the same sense of angular momentum as the disk.