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

Abstract

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-α 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.

The chemical characterization of halo substructure in the Milky Way based on APOGEE

“…In a recent study Myeong et al (2022) state that the chemical properties of Heracles are consistent with the recently reported Aurora population (see also Rix et al 2022). Aurora was originally proposed by Belokurov & Kravtsov (2022) as an in situ component that is kinematically hot, with an approximately isotropic velocity ellipsoid, and very modest net rotation (see also Conroy et al 2022).…”

“…Aurora was originally proposed by Belokurov & Kravtsov (2022) as an in situ component that is kinematically hot, with an approximately isotropic velocity ellipsoid, and very modest net rotation (see also Conroy et al 2022). In a follow up study, Myeong et al (2022) ture Model (GMM) approach to describe the distribution of solar neighbourhood populations in chemo-dynamical space, including elemental abundances from APOGEE DR17 and GALAH DR3 (Buder et al 2021), as well as Gaia-based orbital energy. A key aspect of the Myeong et al (2022) methodology is that it refrains from adopting any selection criteria in chemical space.…”

“…In a follow up study, Myeong et al (2022) ture Model (GMM) approach to describe the distribution of solar neighbourhood populations in chemo-dynamical space, including elemental abundances from APOGEE DR17 and GALAH DR3 (Buder et al 2021), as well as Gaia-based orbital energy. A key aspect of the Myeong et al (2022) methodology is that it refrains from adopting any selection criteria in chemical space. As a result, one of the components emanating from their GMM fit is a highly eccentric (i.e., >0.85) population of APOGEE-Gaia stars deemed to be associated with Aurora, which occupy a locus in chemical abundance space that is qualitatively similar to that inhabited by the inner Galaxy Heracles population.…”

“…We examine this claim by applying our statistical method to compare the chemical compositions of Heracles and Aurora, adopting the Myeong et al (2022) sample for the latter population. To maximise the statistical robustness of the test, the comparison is performed at [Fe/H]=-1.2, where the samples for both substructures are the densest.…”

What is Missing from the Local Stellar Halo?

“…We assemble a composite Milky Way stellar halo from observationally motivated simulations representing nine accreted dwarf galaxies: Gaia-Sausage-Enceladus (GSE, e.g., Belokurov et al 2018;Haywood et al 2018;Helmi et al 2018;Bonaca et al 2020;Feuillet et al 2021;Buder et al 2022), Sagittarius (e.g., Ibata et al 1994;Law & Majewski 2010;Johnson et al 2020;Vasiliev et al 2021), Kraken (Kruijssen et al 2019(Kruijssen et al , 2020Massari et al 2019;Forbes 2020;Horta et al 2021;Pfeffer et al 2021;Naidu et al 2022b; though note that the stars and clusters associated with Kraken may belong to the proto-Milky Way recently characterized in Belokurov & Kravtsov 2022;Conroy et al 2022;Myeong et al 2022;Rix et al 2022), the Helmi Streams (e.g., Helmi et al 1999;Koppelman et al 2019;Limberg et al 2021;Matsuno et al 2022), Sequoia (e.g., Myeong et al 2019;Matsuno et al 2019Matsuno et al , 2021Monty et al 2020;Aguado et al 2021), Wukong/LMS-1 (hereafter Wukong, e.g., Naidu et al 2020;Yuan et al 2020;Malhan et al 2021Malhan et al , 2022Shank et al 2022), Cetus (e.g., Newberg et al 2009;Chang et al 2020;Thomas & Battaglia 2021;…”