The Biggest Splash

TL;DR

This study uses Gaia data to identify a metal-rich, eccentric, retrograde stellar component called the Splash, linked to the Milky Way's ancient merger, revealing insights into galaxy formation and evolution.

Contribution

It introduces the Splash as a new stellar component associated with the Milky Way's last major accretion event, based on detailed kinematic and chemical analysis.

Findings

Identification of a metal-rich, eccentric, retrograde stellar population.

The Splash is linked to the thick disc and formed before the last major merger.

Constraints on the timing of the last massive accretion event to about 9.5 Gyr ago.

Abstract

Using a large sample of bright nearby stars with accurate Gaia Data Release 2 astrometry and auxiliary spectroscopy we map out the properties of the principle Galactic components such as the "thin" and "thick" discs and the halo. We show that in the Solar neighborhood, there exists a large population of metal-rich ([Fe/H]>-0.7) stars on highly eccentric orbits. By studying the evolution of elemental abundances, kinematics and stellar ages in the plane of azimuthal velocity v_phi and metallicity [Fe/H], we demonstrate that this metal-rich halo-like component, which we dub the Splash, is linked to the alpha-rich (or "thick") disc. Splash stars have little to no angular momentum and many are on retrograde orbits. They are predominantly old, but not as old as the stars deposited into the Milky Way in the last major merger. We argue, in agreement with several recent studies, that the Splash stars may have been born in the Milky Way's proto-disc prior to the massive ancient accretion event which drastically altered their orbits. We can not, however, rule out other (alternative) formation channels. Taking advantage of the causal connection between the merger and the Splash, we put constraints of the epoch of the last massive accretion event to have finished 9.5 Gyr ago. The link between the local metal-rich and metal-poor retrograde stars is confirmed using a large suite of cutting-edge numerical simulations of the Milky Way's formation.