The Pristine survey XXIV: The Galactic underdogs -- Dynamic tales of a Milky Way metal-poor population

Abstract

Through the chemodynamical characterisation of metal-poor stars, one can efficiently probe the early history of the Milky Way. We aim at decontaminating a sample of $\sim$ 3M giant stars with Gaia DR3 XP-based \textit{Pristine-Gaia} metallicities, to investigate a subset of very metal-poor stars ([Fe/H] < -1.7) with disc-like orbits. We construct a statistically robust sample of $\sim$ 36 000 very metal-poor giants, using APOGEE and LAMOST to estimate and remove contamination from high $V_{\ddot{I}}•$ stars. We investigate the spatial and kinematic properties of the decontaminated sample, using $V_{\ddot{I}}•$ and the action space, both powerful to disentangle stellar populations. As in previous works, we find a pronounced asymmetry in $L_z$ and $V_{\ddot{I}}•$ in favour of prograde stars. This excess is mostly made of prograde-planar stars (10% of the very metal-poor population), and contains stars with $V_{\ddot{I}}•$ > 180 km s${}^{-1}$ and $Z_{\text{max}}$ < 1.5 kpc, down to [Fe/H] = -2.9 at a 2$\ddot{I}ƒ$ confidence level. While the overall orbital distributions of our sample match that of a halo, the highly prograde and planar subset (2% of the very metal-poor population) also bears characteristics classically associated with a thick disc, i.e., a spatial distribution compatible with a short-scaled thick disc, and a thick disc-like $Z_{\text{max}}$ - $R_{\text{max}}$ distribution. Additionally, assuming a stationary or prograde halo with $\overline{V_{\ddot{I}}•}$ $\sim$ 30-40 km.s${}^{-1}$ is not sufficient to suppress the kinematic signature of the highly prograde and planar subset. These results rule out any link with a thin disc, and instead, support a contribution from a \textit{metal-weak thick disc}.

Accepted for publication in A&A on June 24, 2024. The abstract has been modified to comply with arXiv's abstract requirements (word limit). The paper numbering has been modified from XXV to XXIV to remain consistent with the recent submissions of the Pristine collaboration