2024MNRAS.533..889Z

The question of whether the Milky Way's disc extends to low metallicity has been the subject of debate for many years. We aim to address the question by employing a large sample of giant stars with radial velocities and homogeneous metallicities based on the Gaia Data Release 3 BP/RP(XP) spectra. We study the 3D velocity distribution of stars in various metallicity ranges, including the very metal-poor (VMP) regime ($\mathrm{[M/H]} < -2.0$). We find that a clear, stand-alone disc population, i.e. that with a ratio of rotational velocity to velocity dispersion $v/\sigma > 1$, starts to emerge only around $\mathrm{[M/H]} \sim -1.3$, and is not visible for $\mathrm{[M/H]} < -1.6$. Using Gaussian mixture modelling, we show that there are two halo populations in the VMP regime: one stationary and one with a net prograde rotation of $\sim\!\! 80\ \mathrm{km\, s}^{-1}$. In this low-metallicity range, we are able to place constraints on the contribution of a rotation-supported thick disc sub-population to a maximum of $\sim\!\! 3$ per cent in our sample. We compare our results to previous claims of discy VMP stars in both observations and simulations and find that having a prograde halo component could explain most of these.