The Low-$\alpha$ Splash Population in the Milky Way

Abstract

The Milky Way in-situ halo, also known as the Splash, consists of old (age $>$ 10 Gyr), metal-rich ([Fe/H] $> -0.7$), high-$\alpha$ stars, i.e., thick disk-like chemistry, on halo-like orbits (eccentricity $>$ 0.6). Its origin is linked to stars formed in the disk and dynamically heated by either internal or external agents. In this work, we investigate its low-$\alpha$ counterpart, the low-$\alpha$ Splash, motivated by recent findings of an old thin disk population. We conjecture that any mechanism capable of heating disk stars should affect both of present-day high- and low-$\alpha$ old populations. Using data from the APOGEE DR17 spectroscopic catalog, we identify metal-rich low-$\alpha$ stars with halo-like kinematics similar to those of the classical high-$\alpha$ Splash. We investigate their possible heating mechanisms using the GASTRO suite of simulations, which allows us to explore the effects of star-forming clumps as well as a major merger in the proto-disk of a Milky Way analog galaxy. Our main results show that only clumpy Milky Way models are able to produce Splash populations through scattering by clumps in the early Galaxy, including the low-$\alpha$ counterpart, whereas the model including only the merger and without an early clumpy phase fails to produce these populations. In the models, the low-$\alpha$ Splash corresponds to a subset of the old thin disk that was dynamically heated by the same mechanism responsible for the formation of the high-$\alpha$ Splash.