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 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.Comment: 5 pages, 3 figures, accepted for publication in MNRA

“Treasures” from the Canadian War Museum’s Backlog

In the 1970s Canadian War Museum curatorial staff decided they needed to bolster the scope of the museum’s national military collection. Consequently, they placed ads in Legion magazine and enclosed flyers in veterans’ pension cheques inviting veterans to send in military or military related items they had in their possession that they thought would be of interest to the museum. The response was overwhelming. In the end, a grand total of 24,400 objects poured in; many more than was anticipated and much too large a number for the museum’s small staff to properly register and catalogue. The only option was to carry out a basic inventory and then pack the objects away in boxes (which occupied a total of 197 pallets) pending the day when sufficient resources would become available to process them adequately, so that they would become properly identified and usable museum artifacts