Substructure in the stellar halo near the Sun. I. Data-driven clustering in Integrals of Motion space

Abstract

Context. Merger debris is expected to populate the stellar halos of galaxies. In the case of the Milky Way, such debris should be apparent as clumps in a space defined by the stars' orbital Integrals of Motion. Aims. Our aim is to develop a data-driven and statistically based method for finding such clumps in Integrals of Motion space for nearby halo stars and evaluating their significance robustly. Methods. We use data from Gaia EDR3 extended with radial velocities from ground-based spectroscopic surveys to construct a sample of halo stars within 2.5 kpc from the Sun. We apply a hierarchical clustering method that makes exhaustive use of the single linkage algorithm in a 3D space defined by the commonly used integrals of motion energy E, together with two components of the angular momentum, L z and L ⊥ . To evaluate the statistical significance of the clusters found, we compare the density within an ellipsoidal region centered on the cluster to that of random sets with similar global dynamical properties. By picking out the signal at the location of their maximum statistical significance in the hierarchical tree, we extract a set of significant unique clusters. By describing these clusters with ellipsoids, we estimate the proximity of a star to the cluster center using the Mahalanobis distance. Additionally, we apply the HDBSCAN clustering algorithm in velocity space to each cluster to extract subgroups representing debris with different orbital phases. Results. Our procedure identifies 67 highly significant clusters (> 3σ), containing 12% of the sources in our halo set, and in total 232 subgroups or individual streams in velocity space. In total, 13.8% of the stars in our data set can be confidently associated to a significant cluster based on their Mahalanobis distance. Inspection of the hierarchical tree describing our data set reveals a complex web of relationships between the significant clusters, suggesting that they can be tentatively grouped into at least 6 main large structures, many of which can be associated to previously identified halo substructures, and a number of independent substructures. This preliminary conclusion is further explored in an accompanying paper by Ruiz-Lara et al. (2022), where we also characterize the substructures in terms of their stellar populations. Conclusions. Our method allows us to systematically detect kinematic substructures in the Galactic stellar halo with a data-driven and interpretable algorithm. The list of the clusters and the associated star catalogue are provided in two tables in electronic format.

High-precision chemical abundances of Galactic building blocks. II. Revisiting the chemical distinctness of the Helmi streams

“…This is especially prominent in Ca, Ti and neutron-capture elements (Fig- ures 4 and 6). This might indicate that the Helmi streams do not extend toward high E n , which is consistent with recent identification of Helmi streams' members based on clustering analysis of halo stars (Lövdal et al 2022;Ruiz-Lara et al 2022).…”

Live Fast, Die $α$-Enhanced: The Mass-Metallicity-$α$ Relation of the Milky Way's Disrupted Dwarf Galaxies

“…The sample of disrupted dwarf galaxies analyzed in this work is as defined in Naidu et al (2020a), and includes Sagittarius (Ibata et al 1994;Law & Majewski 2010;Johnson et al 2020), Gaia-Sausage-Enceladus (Belokurov et al 2018b;Haywood et al 2018;Myeong et al 2018;Helmi et al 2018;Feuillet et al 2021;Buder et al 2022), the Helmi Streams (Helmi et al 1999;Koppelman et al 2019b;Limberg et al 2021), Sequoia (Myeong et al 2019;Matsuno et al 2019Matsuno et al , 2021bMonty et al 2020;Aguado et al 2021), Wukong (independently discovered as LMS-1 in Yuan et al 2020, and studied in Malhan et al 2021Shank et al 2022), Thamnos (Koppelman et al 2019a;Sofie Lövdal et al 2022;Ruiz-Lara et al 2022), and I'itoi.…”

GASTRO library I: the simulated chemodynamical properties of several GSE-like stellar halos

“…There are several automated techniques used to search for substructures in the MW stellar halo (e.g. Koppelman et al 2019;Yuan et al 2020;Limberg et al 2021;Malhan et al 2022;Sofie Lövdal et al 2022), which are important to elucidate the accretion history of our Galaxy. However, several substructures identified in the MW have a separation in energy, angular momentum and [Fe/H] consistent with the single merger event models presented here.…”