Co-formation of the Galactic disc and the stellar halo

TL;DR

This study combines Gaia and SDSS data with cosmological simulations to reveal that the Milky Way's stellar halo formed mainly from a major accretion event, leading to highly radial stellar orbits and minimal halo spin.

Contribution

It provides new evidence that the inner stellar halo resulted from a significant early merger, challenging models of continuous dwarf satellite accretion.

Findings

Halo velocity ellipsoid varies strongly with metallicity.

Extreme orbital anisotropy (beta~0.9) observed for stars with [Fe/H]>-1.7.

Inner halo formed from a major merger with a massive satellite 8-11 Gyr ago.

Abstract

Using a large sample of Main Sequence stars with 7-D measurements supplied by Gaia and SDSS, we study the kinematic properties of the local (within ~10 kpc from the Sun) stellar halo. We demonstrate that the halo's velocity ellipsoid evolves strongly with metallicity. At the low [Fe/H] end, the orbital anisotropy (the amount of motion in the radial direction compared to the tangential one) is mildly radial with 0.2<beta<0.4. However, for stars with [Fe/H]>-1.7 we measure extreme values of beta~0.9. Across the metallicity range considered, i.e. -3<[Fe/H]<-1, the stellar halo's spin is minimal, at the level of 20<v_theta (km/s) <30. Using a suite of cosmological zoom-in simulations of halo formation, we deduce that the observed acute anisotropy is inconsistent with the continuous accretion of dwarf satellites. Instead, we argue, the stellar debris in the inner halo were deposited in a major accretion event by a satellite with Mvir>10^10 Msun around the epoch of the Galactic disc formation, i.e. between 8 and 11 Gyr ago. The radical halo anisotropy is the result of the dramatic radialisation of the massive progenitor's orbit, amplified by the action of the growing disc.