Dissecting simulated disc galaxies I: the structure of mono-age
  populations
  We study seven simulated disc galaxies, three with a quiescent merger
history, and four with mergers in their last 9 Gyr of evolution. We compare
their structure at z=0 by decomposing them into "mono-age populations" (MAPs)
of stars within 500 Myr age bins. All studied galaxies undergo a phase of
merging activity at high redshift, so that stars older than 9 Gyr are found in
a centrally concentrated component, while younger stars are mostly found in
discs. We find that most MAPs have simple exponential radial and vertical
density profiles, with a scale-height that typically increases with age.
Because a large range of merger histories can create populations with simple
structures, this suggests that the simplicity of the structure of
mono-abundance populations observed in the Milky Way by Bovy et al. (2012b,c)
is not necessarily a direct indicator of a quiescent history for the Milky Way.
Similarly, the anti-correlation between scale-length and scale-height does not
necessarily imply a merger-free history. However, mergers produce
discontinuities between thin and thick disc components, and jumps in the
age-velocity relation. The absence of a structural discontinuity between thin
and thick disc observed in the Milky Way would seem to be a good indicator that
no merger with a mass ratio larger than 1:15-1:10 occurred in the last 9 Gyr.
Mergers at higher redshift might nevertheless be necessary to produce the
thickest, hottest components of the Milky Way's disc.
