Galactic ArchaeoLogIcaL ExcavatiOns (GALILEO) II. t-SNE Portrait of Local Fossil Relics and Structures

Abstract

Based on high-quality APOGEE DR17 and Gaia DR3 data for 1,742 red giants stars within 5 kpc of the Sun and not rotating with the Galactic disc ($V_{\varphi }<$ 100 km s${}^{-1}$), we use the nonlinear technique of unsupervised analysis t-SNE to detect coherent structures in the space of ten chemical-abundance ratios: [Fe/H], [O/Fe], [Mg/Fe], [Si/Fe], [Ca/Fe], [C/Fe], [N/Fe], [Al/Fe], [Mn/Fe], and [Ni/Fe]. Additionally, we obtain orbital parameters for each star using the non-axisymmetric gravitational potential {\tt GravPot16}. Seven structures are detected, including the Splash, Gaia-Sausage-Enceladus (GSE), the high-$\alpha$ heated-disc population, N-C-O peculiar stars, and inner disk-like stars, plus two other groups that did not match anything previously reported in the literature, here named Galileo 5 and Galileo 6 (G5 and G6). These two groups overlap with Splash in [Fe/H], G5 being lower metallicity than G6, both between GSE and Splash in the [Mg/Mn] versus [Al/Fe] plane, G5 in the $\alpha$-rich in-situ locus, and G6 on the border of the $\alpha$-poor in-situ one; nonetheless their low [Ni/Fe] hints to a possible ex-situ origin. Their orbital energy distributions are between the Splash and GSE, with G5 being slightly more energetic than G6. We verified the robustness of all the obtained groups by exploring a large range of t-SNE parameters, applying it to various subsets of data, and also measuring the effect of abundance errors through Monte Carlo tests.

Accepted by A&A. 19 pages, 15 figures, and 3 tables

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