Merger Dynamics of the Pair of Galaxy Clusters — A 399 and A 401 ∗

Convincing evidence of a past interaction between two rich clusters A399 and A401 was recently found by the X-ray imaging observations. In this paper we examine the structure and dynamics of this pair of galaxy clusters. A mixture-modeling algorithm has been applied to obtain a robust partition into two clusters, which allows us to discuss the virial mass and velocity distribution for each cluster. Assuming that these two clusters follow a linear orbit and they have once experienced a close encounter, we model the binary cluster as a twobody system. As a result, four gravitationally bound solutions are obtained. The recent X-ray observations seem to favor a scenario in which the two clusters with a true separation of 5.4hMpc are currently expanding at 583 km salong the direction with a projection angle of 67.5, and they will reach a maximum extent of 5.65 hMpc in about 1.0 h Gyr.