Normalized density of the distribution of the difference between the initial (R_initial) and final (R_final) galactocentric distances of young (< 3.5 Gyr) stars in the two progenitor galaxies. Here, R_initial is the distance of each gas particle from the centre of its progenitor (main or secondary) at the beginning of the simulation (7.5 Gyr ago), whereas R_final is the galactocentric distance of the stellar particle that spawned from its gaseous progenitor and is situated in the remnant disc. Upper panel: Stars younger than 3.5 Gyr from the main progenitor. Lower panel: Stars younger than 3.5 Gyr from the secondary progenitor.