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The origin of galactic metal-rich stellar halo components with highly eccentric orbits.

Fattahi, Azadeh and Belokurov, Vasily and Deason, Alis J and Frenk, Carlos S. and Gómez, Facundo A. and Grand, Robert J. J. and Marinacci, Federico and Pakmor, Rüdiger and Springel, Volker (2019) 'The origin of galactic metal-rich stellar halo components with highly eccentric orbits.', Monthly notices of the Royal Astronomical Society., 484 (4). pp. 4471-4483.


Using the astrometry from the ESA’s Gaia mission, previous works have shown that the Milky Way stellar halo is dominated by metal-rich stars on highly eccentric orbits. To shed light on the nature of this prominent halo component, we have analysed 28 Galaxy analogues in the Auriga suite of cosmological hydrodynamics zoom-in simulations. Some three quarters of the Auriga galaxies contain prominent components with high radial velocity anisotropy, β > 0.6. However, only in one third of the hosts do the high-β stars contribute significantly to the accreted stellar halo overall, similar to what is observed in the Milky Way. For this particular subset we reveal the origin of the dominant stellar halo component with high metallicity, [Fe/H]∼−1, and high orbital anisotropy, β > 0.8, by tracing their stars back to the epoch of accretion. It appears that, typically, these stars come from a single dwarf galaxy with a stellar mass of order of 109 − 1010 M⊙ that merged around 6 − 10 Gyr ago, causing a sharp increase in the halo mass. Our study therefore establishes a firm link between the excess of radially anisotropic stellar debris in the halo and an ancient head-on collision between the young Milky Way and a massive dwarf galaxy.

Item Type:Article
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Publisher statement:© 2019 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.
Date accepted:14 January 2019
Date deposited:06 February 2019
Date of first online publication:16 January 2019
Date first made open access:No date available

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