Two-body collision model between the main cluster and the Bullet (group 6 in Table 2). The total mass of the system is shown on the y-axis as a function of the angle of the collision axis with respect to the plane of the sky. The estimated total mass range (1 σ) is illustrated by the two dashed lines. At the intersection of these lines with the model curve, we draw two vertical lines that identify the inferred allowed collision angles (in green on the x-axis), ~4°–10°.