Velocity distribution of dark matter halos: A critical test for the Lambda cold dark matter model

The existence of a bullet cluster (such as 1E0657-56) poses a challenge to the concordance Λ cold dark matter model. Here we investigate the velocity distribution of dark matter halo pairs in large N-body simulations with differing box sizes and resolutions. We examine various basic statistics such as halo masses, relative halo velocities, collisional angles, and separation distances in our simulations. We then compare the results to the observational properties of 1E0657-56. Our results suggest that it is very difficult to produce such a halo pair with such a high relative velocity at a redshift of z=0. The relative velocities increase at higher redshift, and we find several candidate pairs in a volume of (2 h-1Gpc)3 at the same redshift as 1E0657-56, z=0.296. However, most of these candidate pairs are separated by too large of a distance.