How rare is the bullet cluster?

The galaxy cluster 1E 0657-56 has a bullet-like subcluster that is moving away from the centre of the main cluster at high speed. Markevitch et al. (2004) recently estimated a relative velocity of Vbullet= 4500+1100-800 km s-1, based on observations of the bow shock in front of the subcluster. The weak lensing analysis of Clowe, Gonzalez & Markevitch (2004) indicates that a substantial secondary mass peak is associated with this subcluster. We estimate the likelihood of such a configuration by examining the distribution of subhalo velocities for clusters in the Millennium Run, a large Λ cold dark matter (ΛCDM) cosmological simulation. We find that the most massive subhalo has a velocity as high as that of the bullet subcluster in only about one out of every 100 cluster-sized haloes. This estimate is strongly dependent on the precise velocity adopted for the bullet. One of the 10 most massive subhaloes has such a high velocity about 40 per cent of the time. We conclude that the velocity of the bullet subcluster is not exceptionally high for a cluster substructure, and can be accommodated within the currently favoured ΛCDM cosmogony.