Chemo-dynamical properties of the Anticenter Stream: a surviving disc fossil from a past satellite interaction.

Using Gaia second data release (DR2), we trace the Anticentre Stream (ACS) in various stellar populations across the sky and find that it is kinematically and spatially decoupled from the Monoceros Ring. Using stars from LAMOST and SEGUE, we show that the ACS is systematically more metal-poor than Monoceros by 0.1 dex with indications of a narrower metallicity spread. Furthermore, the ACS is predominantly populated of old stars (similar to 10 Gyr), whereas Monoceros has a pronounced tail of younger stars (6 - 10 Gyr) as revealed by their cumulative age distributions. Put together, all of this evidence support predictions from simulations of the interaction of the Sagittarius dwarf with the Milky Way, which argue, that the ACS is the remains of a tidal tail of the Galaxy excited during Sgr's first pericentric passage after it crossed the virial radius, whereas Monoceros consists of the composite stellar populations excited during the more extended phases of the interaction. Importantly, the ACS can be viewed as a stand-alone fossil of the chemical enrichment history of the Galactic disc.