Multiple retrograde substructures in the Galactic halo: A shattered view of Galactic history. (arXiv:1909.08924v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Koppelman_H/0/1/0/all/0/1">Helmer H. Koppelman</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Helmi_A/0/1/0/all/0/1">Amina Helmi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Massari_D/0/1/0/all/0/1">Davide Massari</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Price_Whelan_A/0/1/0/all/0/1">Adrian M. Price-Whelan</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Starkenburg_T/0/1/0/all/0/1">Tjitske K. Starkenburg</a>
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.
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.
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