Dissecting simulated disc galaxies II: the age-velocity relation
  We study the relation between stellar ages and vertical velocity dispersion
(the age-velocity relation, or AVR) in a sample of seven simulated disc
galaxies. In our simulations, the shape of the AVR for stars younger than 9 Gyr
depends strongly on the merger history at low redshift, with even 1:10 - 1:15
mergers being able to create jumps in the AVR (although these jumps might not
be detectable if the errors on stellar ages are on the order of 30%). For
galaxies with a quiescent history at low redshift, we find that the vertical
velocity dispersion rises smoothly for ages up to 8-9 Gyr, following a power
law with a slope of ~0.5, similar to what is observed in the solar
neighbourhood by the Geneva-Copenhagen Survey. For these galaxies, we show that
the slope of the AVR is not imprinted at birth, but is the result of subsequent
heating. By contrast, in all our simulations, the oldest stars form a
significantly different population, with a high velocity dispersion. These
stars are usually born kinematically hot in a turbulent phase of intense
mergers at high redshift, and also include some stars accreted from satellites.
This maximum in velocity dispersion is strongly decreased when age errors are
included, suggesting that observations can easily miss such a jump with the
current accuracy of age measurements.
