TOPoS

Context. The goal of the Turn-Off Primordial Stars survey (TOPoS) project is to find and analyse turn-off (TO) stars extremely low metallicity. To select targets for spectroscopic follow-up at high spectral resolution, we relied on low-resolution spectra from Sloan Digital Sky Survey (SDSS). Aims. In this paper, use metallicity estimates obtained our analysis SDSS construct distribution function (MDF) Milky Way, with special emphasis its metal-weak tail. provide underlying out which TOPoS sample was extracted. Methods. We made photometry, Gaia distance derived parallaxes derive a estimate large over 24 million TO stars. This used bias are available. Results. determined that strongly biased in favour metal-poor stars, as intended. A comparison unbiased photometric allows us correct selection bias. selected sub-sample reliable combined radial velocities proper motions compute actions orbital parameters Galactic potential. allowed characterise dynamically, particular belongs -Sausage-Enceladus (GSE) accretion event. thus also able MDF GSE. Conclusions. tail study very similar H3 Hamburg/ESO Survey. average three MDFs an error bar each bin. Inasmuch GSE structure representative progenitor galaxy collided appears be deficient compared suggesting latter has been largely formed by low-mass galaxies rather than massive galaxies, such progenitor.