  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 V_bullet = 4500 +1100/-800
km/s, based on observations of the bow shock in front of the subcluster. The
weak lensing analysis of Clowe et al. (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 LCDM 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 LCDM comogony.

Astrophysics