The Low- α Splash Population in the Milky Way

Vollständiger Abstract

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Abstract The Milky Way metal-rich halo-like component, also known as the Splash, consists of old (age > 10 Gyr), metal-rich ([Fe/H] > −0.7), high- α 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- α counterpart, the low- α Splash , motivated by recent findings of an old thin-disk population. We conjecture that any mechanism capable of heating disk stars should affect both present-day high- and low- α old populations. Using data from the APOGEE DR17 spectroscopic catalog, we identify metal-rich low- α stars with halo-like kinematics similar to those of the classical high- α 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 protodisk of a Milky Way analog galaxy. Our main results show that only clumpy Milky Way models are able to produce Splash populations, including the low- α counterpart, whereas the model including only the merger and without an early clumpy phase fails to produce these populations. In the models, the low- α 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- α Splash.