tokens: left panel but for galaxies split into redshift-independent mass bins. Bottom left panel: evolution of the logarithmic derivative of the half-mass radius r 0.5 with respect to the mass ratio m m / M s ( m is the stellar mass gained between two consecutive outputs) for non-mergers or m the mass ratio of the merger, for all galaxies with M s 510 9 M and all time outputs between z 5.2 and 1.2. Filled blue symbols indicate resolved mergers, red and light green ones represent non-merger diffuse mass acquisition, with unresolved mergers (in red) or not (in green). Whilst the evolution is linear in each case, the dependence of radius growth on the mass ratio is found to be much steeper for mergers. Bottom right panel: same plot as the top left panel, except that we have split the merger sample according to different pre-merger morphologies: discs (light blue) and spheroids (red). The steepness of the radius versus m relation appears mainly caused by minor merger disruption of the discs. The top panels show the evolution of the average half-mass radius r 0.5 as a function of redshift when we split our galaxy sample in bins of final mass (i.e. galaxy masses at redshift z 1.2; top left panel) and in bins of constant stellar mass (i.e. independent of redshift; bottom right panel). These results are consistent with the overall evolution of the sizemass relationship from observations such as Huertas-Company etal. ( 2013 ) (though our simulated galaxies are a factor of 2 larger): galaxies of a given stellar mass display much larger radii at z 1.2 than their counterparts of similar stellar mass at z 5.2; most of the growth taking place between z 3 and 1.2. More specifically, at z 1.2, galaxies with M s 10 10.5 M display an average half-mass radius two to three times bigger than their counterparts at z 3 (see bottom right panel), while one can see on the bottom left panel that galaxies reaching M s 10 10.5 M at z 1.2 have also seen their half-mass radius grow by a factor of 2 to 3 since z 3 and by a factor of 4 since z 5.2. To better investigate the role of different stellar acquisition processes in growing the size of galaxies, we compute the logarithmic derivative of the half-mass radius r 0.5 with respect to M s as a function of the merger mass ratio m for post-merger structures (bottom left panel, blue curve) and the mass increase ratio m m / M s log 10 M s , where m is the stellar mass accreted between two consecutive outputs, for galaxies only subjected to diffuse stellar accretion (green curve for pure in situ formation and red curve when including possibly unresolved mergers). This is computed for all galaxies and all time outputs between z 5.2 and 1.2 with M s 510 9 M . Note that M s or M 0.5 are equivalent for the purpose of this comparison so we use them interchangeably. The result is shown in the top left panel of Fig. 8 . Note the similarity between results for the pure star formation sample (in green) and the full sample of galaxies
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