On the existence of a very metal-poor disc in the Milky Way

Abstract

ABSTRACT 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.