Milky Way's Eccentric Constituents with Gaia, APOGEE, and GALAH

We report the results of an unsupervised decomposition of the local stellar halo in the chemodynamical 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 evidence of this process.

Automated summary

We present the results of an unsupervised decomposition of the local stellar halo using abundance measurements from APOGEE DR17 and GALAH DR3. Our analysis reveals that only four independent components dominate the halo in the solar neighborhood, including one newly identified component called Eos. We find that the GS/E is the main progenitor of the Galactic stellar halo, and Aurora is consistent with the chemical properties of the Heracles merger. The presence of Eos suggests that at least some of the outer thin disk of the Galaxy started its evolution in the gas polluted by the GS/E.