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Using the astrometry from the ESAs 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 significant components with high radial velocity anisotropy, beta > 0.6. However, only in one third of the hosts do the high-beta 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, beta>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 10^9-10^10 Msol 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 Milky Way halo and an ancient head-on collision between the young Milky Way and a massive dwarf galaxy
Based on the second Gaia data release (DR2), combined with the LAMOST and APOGEE spectroscopic surveys, we study the kinematics and metallicity distribution of the high-velocity stars that have a relative speed of at least 220 ${rm km s^{-1}}$ with r
The nature of the Sombrero galaxy (M 104 = NGC 4594) has remained elusive despite many observational studies at a variety of wavelengths. Here we present Hubble Space Telescope imaging of two fields at $sim$16 and 33 kpc along the minor axis to exami
The contribution of dissolved globular clusters (GCs) to the stellar content of the Galactic halo is a key constraint on models for GC formation and destruction, and the mass assembly history of the Milky Way. Earlier results from APOGEE pointed to a
We present new limits on the ejection of metal-rich old-population hypervelocity stars from the Galactic center (GC) as probed by the SEGUE-2 survey. Our limits are a factor of 3-10 more stringent than previously reported, depending on stellar type.
Based on the second Gaia data release (DR2) and APOGEE (DR16) spectroscopic surveys, wedefined two kinds of star sample: high-velocity thick disk (HVTD) with $v{phi}>90km/s$ and metal-richstellar halo (MRSH) with $v{phi}<90km/s$. Due to high resoluti