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Since the chemical abundances of stars are the fossil records of the physical conditions in galaxies, they provide the key information for recovering the assembly history of galaxies. In this work, we explore the chemo-chrono-kinematics of…

Theory suggests that mergers play an important role in shaping galactic discs and stellar haloes, which was observationally confirmed in the MW thanks to Gaia data. In this work, aiming to probe the contribution of mergers to the in situ…

This paper explores the mapping between the observable properties of a stellar halo in phase- and abundance-space and the parent galaxy's accretion history in terms of the characteristic epoch of accretion and mass and orbits of progenitor…

Accreted stars in the Milky Way (MW) preserve information about the progenitor galaxies where they formed in their chemical and kinematic properties. In this study, we use the chemo-dynamical signatures in the merger debris to approximate…

Astrophysics of Galaxies · Physics 2026-03-16 Andrea Sante , Andreea S. Font , Daisuke Kawata , T. Lucas Makinen , Robert J. J. Grand

The hierarchical structure formation model predicts that stellar halos should form, at least partly, via mergers. If this was a predominant formation channel for the Milky Way's halo, imprints of this merger history in the form of moving…

Astrophysics of Galaxies · Physics 2017-02-01 Amina Helmi , Jovan Veljanoski , Maarten A. Breddels , Hao Tian , Laura V. Sales

Reconstructing the mass assembly history of the Milky Way relies on obtaining detailed measurements of the properties of many stars in the Galaxy, especially in the stellar halo. One of the most constraining quantities is stellar age, as it…

Astrophysics of Galaxies · Physics 2024-06-11 Danny Horta , Yuxi , Lu , Melissa K. Ness , Mariangela Lisanti , Adrian M. Price-Whelan

As galaxies form hierarchically, larger satellites may accrete alongside smaller companions in group infall events. This coordinated accretion is likely to have left signatures in the Milky Way's stellar halo at the present day. Our goal is…

Astrophysics of Galaxies · Physics 2025-08-20 Thomas M. Callingham , Amina Helmi

We model the formation of the Galactic stellar halo via the accretion of satellite galaxies onto a time-dependent semi-cosmological galactic potential. Our goal is to characterize the substructure left by these accretion events in a close…

Astrophysics of Galaxies · Physics 2015-05-18 Facundo A. Gómez , Amina Helmi , Anthony G. A. Brown , Yang-Shyang Li

The debris from past merger events is expected and, to some extent, known to populate the stellar halo near the Sun. We aim to identify and characterise such merger debris using Gaia DR3 data supplemented by metallicity and chemical…

In the $\Lambda$CDM model of structure formation galactic haloes build up by accretion of mass and mergers of smaller subunits. The most recent massive merger event experienced by the Milky Way (MW) halo was the accretion of the Large…

Astrophysics of Galaxies · Physics 2020-08-05 Tilly A. Evans , Azadeh Fattahi , Alis J. Deason , Carlos S. Frenk

We study the possibility that the Milky Way's cool stellar disc includes mergers with ancient stars. Galaxies are understood to form in a hierarchical manner, where smaller (proto-)galaxies merge into larger ones. Stars in galaxies, like…

Astrophysics of Galaxies · Physics 2022-07-27 Diane K. Feuillet , Sofia Feltzing , Christian Sahlholdt , Thomas Bensby

The Milky Way's (MW) inner stellar halo contains an [Fe/H]-rich component with highly eccentric orbits, often referred to as the "last major merger." Hypotheses for the origin of this component include Gaia-Sausage/Enceladus (GSE), where…

Astrophysics of Galaxies · Physics 2024-05-09 Thomas Donlon , Heidi Jo Newberg , Robyn Sanderson , Emily Bregou , Danny Horta , Arpit Arora , Nondh Panithanpaisal

It has been recently shown that the halo near the Sun contains several kinematic substructures associated to past accretion events. For the more distant halo, there is evidence of large-scale density variations -- in the form of stellar…

Astrophysics of Galaxies · Physics 2021-10-04 E. Balbinot , A. Helmi

We quantitatively examine the effects of accretion and environment on the HI content of galaxies within a cosmological hydrodynamic simulation that reproduces basic observed trends of HI in galaxies. We show that our model broadly…

The Milky Way stellar halo contains relics of ancient mergers that tell the story of our Galaxy's formation. Some of them are identified due to their similarity in energy, actions and chemistry, referred to as the "chemodynamical space",…

The Milky Way underwent its last significant merger ten billion years ago, when the Gaia-Enceladus-Sausage (GES) was accreted. Accreted GES stars and progenitor stars born prior to the merger make up the bulk of the inner halo. Even though…

Astrophysics of Galaxies · Physics 2020-04-17 Chris B. Brook , Daisuke Kawata , Brad K. Gibson , Carme Gallart , Andrés Vicente

Previous studies based on the analysis of Gaia DR2 data have revealed that accreted stars, possibly originating from a single progenitor satellite, are a significant component of the halo of our Galaxy, potentially constituting most of the…

Astrophysics of Galaxies · Physics 2019-11-27 P. Di Matteo , M. Haywood , M. D. Lehnert , D. Katz , S. Khoperskov , O. N. Snaith , A. Gómez , N. Robichon

Relying on the dramatic increase in the number of stars with full 6D phase-space information provided by the Gaia Data Release 3, we discover unambiguous signatures of phase-mixing in the stellar halo around the Sun. We show that for the…

Observations of the Milky Way's stellar halo find that it is predominantly comprised of a radially biased population of stars, dubbed the Gaia Sausage--Enceladus, or GSE. These stars are thought to be debris from dwarf galaxy accretion…

Astrophysics of Galaxies · Physics 2026-04-08 Dylan Folsom , Mariangela Lisanti , Lina Necib , Danny Horta , Mark Vogelsberger , Lars Hernquist

We use the Aquarius simulation series to study the imprint of assembly history on the structure of Galaxy-mass cold dark matter halos. Our results confirm earlier work regarding the influence of mergers on the mass density profile and the…

Cosmology and Nongalactic Astrophysics · Physics 2015-05-19 Jie Wang , J. F. Navarro , C. S. Frenk , S. D. M. White , V. Springel , A. Jenkins , A. Helmi , A. Ludlow , M. Vogelsberger
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