Flavours in the box of chocolates: chemical abundances of kinematic substructures in the nearby stellar halo

TL;DR

This study investigates the chemical compositions of stellar substructures in the nearby halo, revealing their potential origins and diversity through iron and alpha-element abundances using combined TGAS and RAVE data.

Contribution

It provides the first detailed chemical analysis of halo substructures identified in integrals of motion space, highlighting their possible origins and chemical homogeneity.

Findings

Over half of the substructures have low metallicity dispersion.

Two substructures may be remnants of disrupted globular clusters.

Most substructures are alpha-enhanced, with one showing a unique abundance trend.

Abstract

Stellar halos contain tracers of the assembly history of massive galaxies like our own. Exploiting the synergy between the TGAS and the spectroscopic RAVE surveys, Helmi et al. (2017) recently discovered several distinct substructures in the Solar neighbourhood, defined in integrals of motion space. Some of these substructures may be examples of the building blocks that built up the stellar halo. We analyse the chemical properties of stars in these substructures, with focus on their iron and $\alpha$-element abundances as provided by the RAVE survey chemical pipeline. We perform comparisons of the \feh and \mgfe distributions of the substructures to that of the entire halo sample defined in the TGAS$\times$RAVE dataset.We find that over half of the nine substructures have $\sigma_{\text{[Fe/H]}} \leq 0.3$~dex. Two of the substructures have $\sigma_{\text{[Fe/H]}} \leq 0.1$~dex, which makes them possible remnants of disrupted globular clusters. As expected most substructures and the vast majority of our stellar halo sample are $\alpha$-enhanced. Only one substructure shows a distinct [Mg/Fe] vs [Fe/H] abundance trend distinct from the rest of the halo stars in our sample.