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ABSTRACT  We present a simplified dynamical model of the ‘Bullet’ system of two colliding clusters. The model constrains the masses of the system by requiring that the orbits of the main and the subcomponents satisfy the cosmological initial conditions of vanishing physical separation a Hubble time ago. This is also known as the timing argument. The model considers a system embedded in an overdense region. We argue that a relative speed of 4500 km s −1  between the two components is consistent with cosmological conditions if the system is of a total mass of  2.8 × 10 15   h  −1  M ⊙   and is embedded in a region of a (mild) overdensity of 10 times the cosmological background density. Combining this with the lensing measurements of the projected mass, the model yields a ratio of 3:1 for the mass of the main relative to that of the subcomponent. The effect of the background weakens as the relative speed between the two components is decreased. For relative speeds lower than ∼3700 km s −1 , the timing argument yields masses which are too low to be consistent with lensing.

Back‐in‐time dynamics of the cluster IE 0657–56 (the Bullet system)