  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.
