The velocity dispersions of stars near the Sun are known to increase with stellar age , but age can be difficult to determine so a proxy like the abundance of \alpha elements ( e.g. , Mg ) with respect to iron , [ \alpha /Fe ] , is used .
Here we report an unexpected behavior found in the velocity dispersion of a sample of giant stars from the RAdial Velocity Experiment ( RAVE ) survey with high quality chemical and kinematical information , in that it decreases strongly for stars with [ Mg/Fe ] > 0.4 dex ( i.e. , those that formed in the first Gyr of the Galaxy ’ s life ) .
These findings can be explained by perturbations from massive mergers in the early Universe , which have affected more strongly the outer parts of the disc , and the subsequent radial migration of stars with cooler kinematics from the inner disc .
Similar reversed trends in velocity dispersion are also found for different metallicity subpopulations .
Our results suggest that the Milky Way disc merger history can be recovered by relating the observed chemo-kinematic relations to the properties of past merger events .