The collision velocity of the bullet cluster in conventional and modified dynamics

We consider the orbit of the bullet cluster 1E 0657−56 in both cold dark matter (CDM) and Modified Newtonian Dynamics (MOND) using accurate mass models appropriate to each case in order to ascertain the maximum plausible collision velocity. Impact velocities consistent with the shock velocity (∼ 4700 km s⁻¹) occur naturally in MOND. CDM can generate collision velocities of at most ∼3800 km s⁻¹, and is only consistent with the data, provided that the shock velocity has been substantially enhanced by hydrodynamical effects.