Volume 631, November 2019
|Number of page(s)||6|
|Section||Letters to the Editor|
|Published online||13 November 2019|
Letter to the Editor
Multiple retrograde substructures in the Galactic halo: A shattered view of Galactic history
Kapteyn Astronomical Institute, University of Groningen, Landleven 12, 9747 AD Groningen, The Netherlands
2 Dipartimento di Fisica e Astronomia, Università degli Studi di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy
3 INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
4 Center for Computational Astrophysics, Flatiron Institute, 162 5th Avenue, New York, NY, 10010, USA
Accepted: 24 October 2019
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 yet be fully settled. 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 vϕ ≈ −150 km s−1, and are chemically distinct from the other structures.
Conclusions. Even with the excellent Gaia DR2 data, piecing together all the fragments found in the retrograde halo remains challenging. 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.
Key words: Galaxy: halo / solar neighborhood / Galaxy: kinematics and dynamics / Galaxy: formation / Galaxy: evolution
© ESO 2019
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