The Pristine survey – X. A large population of low-metallicity stars permeates the Galactic disc

Abstract

The orbits of the least chemically enriched stars open a window on the formation of our Galaxy when it was still in its infancy. The common picture is that these lowmetallicity stars are distributed as an isotropic, pressure-supported component since these stars were either accreted from the early building blocks of the assembling Milky Way, or were later brought by the accretion of faint dwarf galaxies. Combining the metallicities and radial velocities from the Pristine and LAMOST surveys and Gaia DR2 parallaxes and proper motions for an unprecedented large and unbiased sample of very metal-poor stars at [Fe/H] ≤ −2.5 we show that this picture is incomplete. This sample shows strong statistical evidence (at the 5.0σ level) of asymmetry in their kinematics, favouring prograde motion. Moreover, we find that 31% of the stars that currently reside in the disk do not venture outside of the disk plane throughout their orbit. The discovery of this population implies that a significant fraction of stars with iron abundances [Fe/H] ≤ −2.5 formed within or concurrently with the Milky Way disk and that the history of the disk was quiet enough to allow them to retain their disk-like orbital properties.

Exploring the Galaxy’s halo and very metal-weak thick disc with SkyMapper and Gaia DR2

“…13 The azimuthal action corresponds to the vertical angular momentum Z for an axisymmetric potential as is used here. in agreement with Sestito et al (2019Sestito et al ( , 2020b and Di Matteo et al (2020). In particular, if we follow Sestito et al (2020b) and adopt Z / < 1.25×10 3 , shown as the dotted horizontal line in panel a), to characterize orbits that are confined to the disk, then ∼50% of our sample meets this definition.…”

“…The recent work of Sestito et al (2019Sestito et al ( , 2020b, Di Matteo et al (2020) and Venn et al (2020) has revealed an unexpected significant population of very low-metallicity stars residing in the plane of the Galaxy. We find a similar result in that ∼ 21 % of the stars in our sample have orbits that remain confined to within 3 kpc of the Galactic plane.…”

“…It has recently been shown (Sestito et al 2019(Sestito et al , 2020bDi Matteo et al 2020;Venn et al 2020) that, in contrast to the commonly accepted view, a significant fraction (∼ 20%) of very low-metallicity stars resides in the MW disk rather than in the halo. Here we find a similar result: for the stars in our sample 102 out of 475 (∼ 21 %) exhibit disk-like dynamics in having orbits that are confined to within 3 kpc of the plane of the Milky Way.…”

“…They suggest that this implies that a significant fraction of the MW's metal-poor stars formed with the Milky Way (thick) disk. Moreover, they note that as a consequence, the history of the disk must have been sufficiently quiescent that (presumably old) metal-poor stars were able to retain their disk-like orbits to the present-day (Sestito et al 2020b). Venn et al (2020) have also investigated the kinematics of metal-poor stars using a sample of 115 objects chosen from the Pristine survey (Starkenburg et al 2017) that have been observed at high dispersion.…”

“…A number of different possibilities have been discussed (e.g. Sestito et al 2019Sestito et al , 2020bDi Matteo et al 2020) including that the stars were accreted from small satellites once the MW disk had already formed, or that they represent low metallicity stars formed in the gas-rich building-blocks that came together to form the main body of the Galaxy's disk (see also the theoretical simulations presented in Sestito et al 2020a).…”