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The velocity dispersions of stars near the Sun are known to increase withstellar age, but age can be difficult to determine so a proxy like theabundance of alpha elements (e.g., Mg) with respect to iron, [alpha/Fe], isused. Here we report an unexpected behavior found in the velocity dispersion ofa sample of giant stars from the RAdial Velocity Experiment (RAVE) survey withhigh quality chemical and kinematical information, in that it decreasesstrongly for stars with [Mg/Fe] > 0.4 dex (i.e., those that formed in the firstGyr of the Galaxy's life). These findings can be explained by perturbationsfrom massive mergers in the early Universe, which have affected more stronglythe outer parts of the disc, and the subsequent radial migration of stars withcooler kinematics from the inner disc. Similar reversed trends in velocitydispersion are also found for different metallicity subpopulations. Our resultssuggest that the Milky Way disc merger history can be recovered by relating theobserved chemo-kinematic relations to the properties of past merger events.

A new stellar chemo-kinematic relation reveals the merger history of the Milky Way disc