Measuring and modelling the Splash with APOGEE/Gaia and ARTEMIS

TL;DR

This study combines APOGEE and Gaia data with simulations to analyze the properties and origins of the Splash population in the Milky Way, revealing its connection to minor and retrograde mergers.

Contribution

It provides new insights into the chemo-dynamical properties of the Splash and demonstrates that Splash-like populations can arise from various merger scenarios, including minor and retrograde events.

Findings

Splash differs chemically from the high-alpha disk but varies smoothly with orbital properties.

Splash-like populations are common in simulations, often from minor or retrograde mergers.

A correlation exists between Splash star fraction and retrograde accreted stars in the disk.

Abstract

Using combined data from SDSS-IV/APOGEE and Gaia, we study the chemo-dynamical properties of the Splash population in comparison with those of the high-alpha disc. We investigate a wide range of abundance ratios, finding that the Splash differs from the high-alpha disc overall. However, these differences result from a smooth variation of chemical compositions as a function of orbital properties. The Splash occupies the high-alpha, high-[Al,K/Fe], and low-[Mn/Fe] end of the high-alpha disk population. In agreement with previous studies, we find that Splash stars are distributed over large heights from the Galactic mid-plane. To further elucidate the relation between the Splash and the high-alpha disk, we turn to simulations. Using a sample of Milky Way-like galaxies with and without major accretion events from the ARTEMIS simulations, we find that Splash-like populations are ubiquitous, though not always resulting from major mergers. Lower mass progenitors can also generate Splash-like features, as long as they are on retrograde orbits. Moreover, we find a strong correlation between the mass fraction of Splash stars and the fraction of retrograde accreted stars in the disk. Some galaxies with minor (retrograde) mergers contain more pronounced Splash populations than others with major, but prograde, mergers. For stars in the high-alpha disks, we also find a decrease in the [alpha/Fe] with increasing orbital angular momentum. This trend is found in hosts with both major or minor mergers. Our results suggest that a number of relatively low-mass mergers on retrograde orbits could result in populations that are qualitatively similar to the Splash.