The outer low- disc of the Milky Way - I: evidence for the first pericentric passage of Sagittarius?

Phase-space data, chemistry, and ages together reveal a complex structure in the outer low-alpha disc of the Milky Way. The age-vertical velocity dispersion profiles beyond the Solar Neighbourhood show a jump at 6 Gyr for stars beyond the Galactic plane. Stars older than 6 Gyr are significantly hotter than younger stars. The chemistry and age histograms reveal a bump at [Fe/H] = -0.5, [alpha/Fe] = 0.1, and an age of 7.2 Gyr in the outer disc. Finally, viewing the stars beyond 13.5 kpc in the age-metallicity plane reveals a faint streak just below this bump, towards lower metallicities at the same age. Given the uncertainty in age, we believe these features are linked and suggest a pericentric passage of a massive satellite similar to 6 Gyr ago that heated pre-existing stars, and led to a starburst in existing gas. New stars also formed from the metal-poorer infalling gas. The impulse approximation was used to characterize the interaction with a satellite, finding a mass of similar to 10(11) M-circle dot, and a pericentric position between 12 and 16 kpc. The evidence points to an interaction with the Sagittarius dwarf galaxy, likely its first pericentric passage.