Surface density distributions of the stellar component viewed edge-on before the first buckling (t = 4 Gyr), at the time of first buckling (t = 4.5, 4.65 and 5 Gyr), at the time of second buckling (t = 7.1 Gyr), and at the end of evolution (t = 10 Gyr). The surface density was normalized to the central maximum value in each case and the contours are equally spaced in logΣ with ΔlogΣ = 0.05.