Seminar: Tracing the accretion history of the Milky Way with halo stars

The standard cosmological scenario predicts a hierarchical formation for galaxies. Many accreted substructures have been found in the Galactic halo, identified as clumps in the energy-angular momentum space (E − Lz), assuming that these quantities are conserved during the merger. If these clumps also feature different chemical properties, such as the metallicity distribution function (MDF), they are associated with distinct merger debris.

The aim of this study is to explore to what extent we can couple kinematic characteristics and metallicities of stars in the Galactic halo to reconstruct the accretion history of the Milky Way (MW). In particular, we want to understand whether different clumps in the E − Lz space with different MDFs should be associated with distinct merger debris. Moreover, we want to test whether different chemical elements can help disentangling between different merger events.

By means of high-resolution N-body simulations of a MW-type galaxy accreting a satellite with a mass ratio of 1:10, we find that accreted stars from such a massive merger event redistribute in a wide range of E and Lz, due to the dynamical friction, and are thus not associated with a single clump. Because satellite stars with different metallicities can be deposited in different regions of the E − Lz space, this implies that a single accretion of ∼1:10 can manifest with different MDFs, in different regions of the E − Lz space.

Analysing the clumps' distributions in other abundance ratios with Gaussian Mixture Models, it is possible to identify distinct clumps that could be chemically compatible one with each other and thus associated to the same accretion event.