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 , \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 ] \sim - 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 } { M } _ { \odot } 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 .