Halo formation redshifts as a function of the halo mass. Top panel: Results for z_form, defined as the redshift when half of the maximum halo mass was accreted. Bottom panel: Results for z_lastmm, the redshift at which the last major merger occurred. The blue lines represent the complete z = 0 snapshot and the black line the z = 0 void halo samples. The error bars were estimated via the Jackknife resampling method and using 50 subsamples.