Substructure in the stellar halo near the Sun. II. Characterisation of independent structures

TL;DR

This study characterizes stellar halo substructures near the Sun by clustering in integrals of motion space, analyzing their stellar populations, ages, and chemical compositions to understand the Milky Way's accretion history.

Contribution

It introduces a method combining clustering, metallicity distribution comparison, and isochrone fitting to identify and analyze halo substructures, including new insights into their origins.

Findings

Identified 12 extended substructures, including known streams.

Discovered distinct chemical and age properties among substructures.

Found that retrograde substructures are more metal-poor and older.

Abstract

In L\"ovdal et al, we presented a data-driven method for clustering in Integrals of Motion space and applied it to a large sample of nearby halo stars with 6D phase-space information. We identified a large number of clusters, many of which could tentatively be merged into larger groups. Our goal is to establish the reality of the clusters through a combined study of their stellar populations to gain more insights into the accretion history of the Milky Way. We develop a procedure that quantifies the similarity of clusters based on KS tests using their metallicity distribution functions, and an isochrone fitting method to determine their average age, which is also used to compare the distribution of stars in the Colour-Absolute magnitude diagram. This allows us to group clusters into substructures, and to compare substructures with one another. The clusters identified are merged into 12 extended substructures, while 8 small clusters remain as such. The large substructures include the previously known Gaia-Enceladus, Helmi streams, Sequoia, and Thamnos 1 and 2. We identify overdensities associated with the hot thick disc and hosting a metal-poor population. Especially notable is our largest substructure which, although peaking at the metallicity characteristic of the thick disk has a well populated metal-poor component, and dynamics in-between hot thick disc and halo. We identify additional debris in the region occupied by Sequoia with distinct kinematics, likely remnants of three different accretion events with progenitors of similar mass. We also identify different trends of [Mg/Fe] vs [Fe/H] for the various substructures confirming our dissection of the nearby halo. At least 20\% of the halo near the Sun is associated to substructures. When comparing their global properties, we note that those substructures on retrograde orbits are not only more metal-poor on average but also older.