Multiple retrograde substructures in the Galactic halo: A shattered view of Galactic history

TL;DR

This study investigates multiple retrograde substructures in the Milky Way halo using Gaia DR2 and spectroscopic surveys, revealing their properties, possible origins, and the complexity of Galactic history.

Contribution

It identifies and characterizes several retrograde halo substructures, clarifying their properties and potential links, and highlights the challenges in reconstructing Galactic merger history.

Findings

Retrograde stars are more common at [Fe/H] < -1.

Sequoia is confined to high-energy orbits.

Evidence for a distinct structure called Thamnos.

Abstract

Aims. Several kinematic and chemical substructures have been recently found amongst Milky Way halo stars with retrograde motions. It is currently unclear how these various structures are related to each other. This Letter aims to shed light on this issue. Methods. We explore the retrograde halo with an augmented version of the Gaia DR2 RVS sample, extended with data from three large spectroscopic surveys, namely RAVE, APOGEE and LAMOST. In this dataset, we identify several structures using the HDBSCAN clustering algorithm. We discuss their properties and possible links using all the available chemical and dynamical information. Results. In concordance with previous work, we find that stars with [Fe/H] $<-1$ have more retrograde motions than those with [Fe/H] $>-1$. The retrograde halo contains a mixture of debris from objects like Gaia-Enceladus, Sequoia, and even the chemically defined thick-disc. We find that the Sequoia has a smaller range in orbital energies than previously suggested and is confined to high-energy. Sequoia could be a small galaxy in itself, but since it overlaps both in integrals-of-motion space and chemical abundance space with the less bound debris of Gaia-Enceladus, its nature cannot be fully settled yet. In the low-energy part of the halo we find evidence for at least one more distinct structure: Thamnos. Stars in Thamnos are on low inclination, mildly eccentric retrograde orbits, moving at $v_{\phi}\approx-150$ km/s, and are chemically distinct from the other structures. Conclusions. Even with the excellent Gaia DR2 data it remains challenging to piece together all the fragments found in the retrograde halo. At this point, we are very much in need of large datasets with high-quality high-resolution spectra and tailored high-resolution hydrodynamical simulations of galaxy mergers.