The Splash without a merger

TL;DR

This study shows that a metal-rich, low angular momentum stellar population in the Milky Way can form through clump scattering in a simulated galaxy, without requiring a major merger event.

Contribution

The paper demonstrates that clump scattering in an isolated galaxy simulation can produce a population similar to the Milky Way's low angular momentum stars, challenging the merger-based origin hypothesis.

Findings

Clump scattering creates a metal-rich, low angular momentum stellar population.

Simulated stars match the Milky Way's chemical and kinematic properties.

Major mergers are not necessary to explain this stellar population.

Abstract

The Milky Way's progenitor experienced several merger events which left their imprints on the stellar halo, including the Gaia-Sausage/Enceladus. Recently, it has been proposed that this event perturbed the proto-disk and gave rise to a metal rich ([Fe/H] $>-1$) low angular momentum ($v_{\phi} < 100$ km/s) stellar population. These stars have dynamical and chemical properties different from the accreted stellar halo, but are continuous with the canonical thick disk. In this letter, we use a hydrodynamical simulation of an isolated galaxy which develops clumps that produce a bimodal thin$+$thick disk chemistry to explore whether it forms such a population. We demonstrate clump scattering forms a metal-rich, low angular momentum population, without the need for a major merger. We show that, in the simulation, these stars have chemistry, kinematics and density distribution in good agreement with those in the Milky Way.