Investigating the merger origin of early-type galaxies using ultra-deep optical images by Duc, P, Cuillandre, J, Alatalo, K, Blitz, L, Bois, M, Bournaud, F, Bureau, M, Cappellari, M, Côté, P, Davies, R, Davis, T, De Zeeuw, P, Emsellem, E, Ferrarese, L, Ferriere, E, Gwyn, S, Khochfar, S, Krajnovic, D, Kuntschner, H, Lablanche, P, MacArthur, L, McDermid, R, Michel-Dansac, L, Morganti, R, Naab, T