Distribution of halo-formation time (z_form) in NUM model. From left to right we show the distribution of haloes with log(M_vir/M_⊙) = 11.25, 11.75, 12.25, 12.75, 13.25, respectively. We show the mean z_form for each population as a vertical dashed line. Blue, orange, and red ones represent galaxies with 0 < d_fil/h⁻¹ Mpc < 4, 4 < d_fil/h⁻¹ Mpc < 8, 8 < d_fil/h⁻¹ Mpc < 12, respectively.