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

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 ~ 3M giant stars with Gaia DR3 XP-based 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 ~ 36 000 very metal-poor giants, using APOGEE and LAMOST to estimate and remove contamination from high V_φ stars. We investigate the spatial and kinematic properties of the decontaminated sample, using V_φ and the action space, both powerful to disentangle stellar populations. As in previous works, we find a pronounced asymmetry in Lz and V_φ 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_φ > 180 km s⁻¹ and Zₘₐₓ < 1.5 kpc, down to [Fe/H] = -2.9 at a 2σ 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 Zₘₐₓ - Rₘₐₓ distribution. Additionally, assuming a stationary or prograde halo with V_φ̄ ~ 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 metal-weak thick disc.