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 . ( ) recently estimated a relative velocity of V _ { bullet } = 4500 ^ { +1100 } _ { -800 } ~ { } \ > { km } { s } ^ { -1 } , based on observations of the bow shock in front of the subcluster .
The weak lensing analysis of ( ) 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 \Lambda CDM cosmological simulation .
We find that the most massive subhalo has a velocity as high as that of the bullet subcluster in only about 1 out of every 100 cluster-sized halos .
This estimate is strongly dependent on the precise velocity adopted for the bullet .
One of the ten most massive subhalos has such a high velocity about 40 \% 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 \Lambda CDM comogony .