Milky Way's eccentric constituents with $Gaia$, APOGEE $\&$ GALAH

TL;DR

This study decomposes the local stellar halo using chemo-dynamical data from Gaia, APOGEE, and GALAH, identifying four main components with distinct origins and chemical properties, including a new in situ component called Eos.

Contribution

The paper introduces a novel chemo-dynamical decomposition of the stellar halo revealing a new in situ component, Eos, and clarifies the origins of known halo structures.

Findings

GS/E is the main progenitor of the stellar halo.

Eos is a new in situ halo component with high [Al/Fe].

Aurora is consistent with the Heracles merger.

Abstract

We report the results of an unsupervised decomposition of the local stellar halo in the chemo-dynamical space spanned by the abundance measurements from APOGEE DR17 and GALAH DR3. In our Gaussian Mixture Model, only four independent components dominate the halo in the Solar neighborhood, three previously known $Aurora$, $Splash$ and Gaia-Sausage/Enceladus (GS/E) and one new, $Eos$. Only one of these four is of accreted origin, namely the GS/E, thus supporting the earlier claims that the GS/E is the main progenitor of the Galactic stellar halo. We show that $Aurora$ is entirely consistent with the chemical properties of the so-called Heracles merger. In our analysis in which no predefined chemical selection cuts are applied, $Aurora$ spans a wide range of [Al/Fe] with a metallicity correlation indicative of a fast chemical enrichment in a massive galaxy, the young Milky Way. The new halo component dubbed $Eos$ is classified as $in situ$ given its high mean [Al/Fe]. $Eos$ shows strong evolution as a function of [Fe/H], where it changes from being the closest to GS/E at its lowest [Fe/H] to being indistinguishable from the Galactic low-$\alpha$ population at its highest [Fe/H]. We surmise that at least some of the outer thin disk of the Galaxy started its evolution in the gas polluted by the GS/E, and $Eos$ is an evidence of this process.