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Hidden deep in the halo: selection of a reduced proper motion halo catalogue and mining retrograde streams in the velocity space
Akshara Viswanathan, Else Starkenburg, Helmer H. Koppelman +3
The Milky Way halo is one of the few galactic haloes that provides a unique insight into galaxy formation by resolved stellar populations. Here, we present a catalogue of 47…
High-precision chemical abundances of Galactic building blocks. II. Revisiting the chemical distinctness of the Helmi streams
Tadafumi Matsuno, Emma Dodd, Helmer H. Koppelman +6
Context: The Helmi streams are a kinematic substructure whose progenitor is likely a dwarf galaxy. Although 20 years have passed since their discovery, it is still unclear whether…
Substructure in the stellar halo near the Sun. II. Characterisation of independent structures
Tomás Ruiz-Lara, Tadafumi Matsuno, S. Sofie Lövdal +3
In Lövdal et al, we presented a data-driven method for clustering in Integrals of Motion space and applied it to a large sample of nearby halo stars with 6D phase-space information…
Substructure in the stellar halo near the Sun. I. Data-driven clustering in Integrals of Motion space
S. Sofie Lövdal, Tomás Ruiz-Lara, Helmer H. Koppelman +3
Aims: Develop a data-driven and statistically based method for finding such clumps in Integrals of Motion space for nearby halo stars and evaluating their significance robustly. Me…
Age Determination of Galaxy Merger Remnant Stars using Asteroseismology
Camilla C. Borre, Víctor Aguirre Børsen-Koch, Amina Helmi +12
The Milky Way was shaped by the mergers with several galaxies in the past. We search for remnant stars that were born in these foreign galaxies and assess their ages in an effort t…
High-precision chemical abundances of Galactic building blocks. The distinct chemical abundance sequence of Sequoia
Tadafumi Matsuno, Helmer H. Koppelman, Amina Helmi +5
Context: Sequoia is a retrograde kinematic substructure in the nearby Galactic halo, whose properties are a matter of debate. For example, previous studies do not necessarily agree…
Substructures, Resonances and debris streams
Emma Dodd, Amina Helmi, Helmer H. Koppelman
The local stellar halo of the Milky Way contains the debris from several past accretion events. Here we study in detail the structure and properties of nearby debris associated wit…
Time evolution of gaps in stellar streams in axisymmetric Stäckel potentials
Helmer H. Koppelman, Amina Helmi
When a subhalo interacts with a cold stellar stream it perturbs its otherwise nearly smooth distribution of stars, and this leads to the creation of a gap. The properties of such g…
Frequencies, chaos and resonances: a study of orbital parameters of nearby thick disc and halo stars
Helmer H. Koppelman, Jorrit H. J. Hagen, Amina Helmi
We study the distribution of nearby thick disc and halo stars in subspaces defined by their characteristic orbital parameters. Our aim is to establish the origin of the structure r…
Galactic potential constraints from clustering in action space of combined stellar stream data
Stella Reino, Elena M. Rossi, Robyn E. Sanderson +4
Stream stars removed by tides from their progenitor satellite galaxy or globular cluster act as a group of test particles on neighboring orbits, probing the gravitational field of…
A massive mess: When a large dwarf and a Milky Way-like galaxy merge
Helmer H. Koppelman, Roy O. Y. Bos, Amina Helmi
Circa 10 billion years ago the Milky Way merged with a massive satellite, Gaia-Enceladus. To gain insight into the properties of its debris we analyse in detail the suite of simula…
Determination of the escape velocity of the Milky Way using a proper motion selected halo sample
Helmer H. Koppelman, Amina Helmi
The {\it Gaia} mission has provided the largest catalogue ever of sources with tangential velocity information. However, using this catalogue for dynamical studies is difficult bec…
The Reduced Proper Motion selected halo: methods and description of the catalogue
Helmer H. Koppelman, Amina Helmi
The Gaia mission has provided the largest ever astrometric chart of the Milky Way. Using it to map the Galactic halo is helpful for disentangling its merger history. The identifica…
Multiple retrograde substructures in the Galactic halo: A shattered view of Galactic history
Helmer H. Koppelman, Amina Helmi, Davide Massari +2
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 structur…
Origin of the system of globular clusters in the Milky Way
Davide Massari, Helmer H. Koppelman, Amina Helmi
. The assembly history experienced by the Milky Way is currently being unveiled thanks to the data provided by the mission. It is likely that the globular cluster s…
Characterization and history of the Helmi streams with Gaia DR2
Helmer H. Koppelman, Amina Helmi, Davide Massari +2
The halo of the Milky Way has long been hypothesized to harbour significant amounts of merger debris. This view has been supported over more than a decade by wide-field photometric…
The merger that led to the formation of the Milky Way's inner stellar halo and thick disk
Amina Helmi, Carine Babusiaux, Helmer H. Koppelman +3
The assembly process of our Galaxy can be retrieved using the motions and chemistry of individual stars. Chemo-dynamical studies of the nearby halo have long hinted at the presence…
Scratching the surface with TGAS and RAVE: disk moving groups in the Solar neighborhood
Helmer H. Koppelman, Titania Virginiflosia, Lorenzo Posti +2
There is long tradition extending more than a century on the identification of moving groups in the Solar neighborhood. However, with the advent of large kinematic surveys, and esp…
One large blob and many streams frosting the nearby stellar halo in Gaia DR2
Helmer H. Koppelman, Amina Helmi, Jovan Veljanoski
We explore the phase-space structure of nearby halo stars identified kinematically from Gaia DR2 data. We focus on their distribution in velocity and in "integrals of motion" space…
The time evolution of gaps in tidal streams
Amina Helmi, Helmer H. Koppelman
We model the time evolution of gaps in tidal streams caused by the impact of a dark matter subhalo, while both orbit a spherical gravitational potential. To this end, we make use o…