The Splash without a Merger

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 φ < 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.

Low-α metal-rich stars with sausage kinematics in the LAMOST survey: Are they from the Gaia-Sausage-Enceladus galaxy?

“…The splash process of the GSE merger event , as proposed to explain the high-α MRSK group, fails to account for the low-α group because the ancient GSE merger event happened before its formation. Instead, a clumpy Milky-Way-like analogue in the hydrodynamical simulation by Amarante et al [21] can produce a bimodal disk chemistry, similar to our data, without the need for a splash process. Based on this scenario, both high-α and low-α MRSK stars in the LAM-OST survey belong to the GSE galaxy.…”

“…Without a splash process, Amarante et al [21] developed a hydrodynamical simulation, in which a clumpy Milky-Waylike analogue succeeds to produce a bimodal disk chemistry with Sausage-kinematics, similar to our data. In their Figure 1, stars with −100 < V ϕ < 50 km/s have metallicity range from [Fe/H] = −1.5 to solar metallicity.…”

“…In support for this scenario, a low angular momentum (V ϕ < 100 km/s) stellar population with [Fe/H] > −1 was found by Bonaca et al [19], and was classified as a "heated thick disk", rather than an "in-situ halo". Later, more studies [5,20,21] reported the existence of Sausage-kinematic stars with thick-disk chemistry. They were named "splash" stars and become important imprints of the GSE merger event on the Galactic disk.…”

The COMBS Survey - II. Distinguishing the metal-poor bulge from the halo interlopers

“…However, they could also be the result of secular evolution, but are not produced in the simulation because of missing physics. For example, the simulation does not include clump formation, which can result in counter-rotating stars (Amarante et al 2020). In the next installment of this survey we will present the elemental abundances for these stars, which will provide further insights into the origins of these interesting counter-rotating stars.…”

LAMOST’s view on the Gaia-Sausage-Enceladus merger event

“…Gang Zhao and Yuqin Chen (National Astronomical Observatories, Chinese Academy of Sciences) recently explored the chemical and orbital properties of metal-rich Sausage-kinematic (MRSK) stars [3], and delivered a new picture of the GSE merger. That is, the GSE has a metal-rich component with both highalpha and low-alpha ratios, which are well reproducible in a hydro-dynamical simulation [4], where the GSE galaxy is a clumpy Milky-Way analogue that develops a bimodal disk chemistry. They also revealed an interesting response of our Galaxy to this ancient accretion event by discovering a pileup of MRSK stars at Z max =3-5 kpc [3], which led to the formation of a disk-halo transition at |Z|=4 kpc.…”