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 ( 12 ; 13 ) 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 .