The origin of galactic metal-rich stellar halo components with highly eccentric orbits

TL;DR

This study uses cosmological simulations to explain the origin of the Milky Way's metal-rich, highly eccentric stellar halo component, linking it to ancient mergers with massive dwarf galaxies.

Contribution

It identifies the origin of high-metallicity, high-eccentricity halo stars as resulting from a specific ancient dwarf galaxy merger, providing a detailed formation scenario.

Findings

Most simulated galaxies have high radial velocity anisotropy in their halos.

Only a third of galaxies show significant high-beta, metal-rich halo components.

The dominant halo component originates from a merger with a massive dwarf galaxy 6-10 Gyr ago.

Abstract

Using the astrometry from the ESA's Gaia mission, previous works have shown that the Milky Way stellar halo is dominated by metal-rich stars on highly eccentric orbits. To shed light on the nature of this prominent halo component, we have analysed 28 Galaxy analogues in the Auriga suite of cosmological hydrodynamics zoom-in simulations. Some three quarters of the Auriga galaxies contain significant components with high radial velocity anisotropy, beta > 0.6. However, only in one third of the hosts do the high-beta stars contribute significantly to the accreted stellar halo overall, similar to what is observed in the Milky Way. For this particular subset we reveal the origin of the dominant stellar halo component with high metallicity, [Fe/H]~-1, and high orbital anisotropy, beta>0.8, by tracing their stars back to the epoch of accretion. It appears that, typically, these stars come from a single dwarf galaxy with a stellar mass of order of 10^9-10^10 Msol that merged around 6-10 Gyr ago, causing a sharp increase in the halo mass. Our study therefore establishes a firm link between the excess of radially anisotropic stellar debris in the Milky Way halo and an ancient head-on collision between the young Milky Way and a massive dwarf galaxy