  An XMM-Newton imaging spectroscopy analysis of the galaxy cluster A1644 is
presented. A1644 is a complex merging system consisting of a main and a sub
cluster. A trail of cool, metal-rich gas has been discovered close to the sub
cluster. The combination of results from X-ray, optical, and radio data, and a
comparison to a hydrodynamical simulation suggest that the sub cluster has
passed by the main cluster off-axis and a fraction of its gas has been stripped
off during this process. Furthermore, for this merging system, simple effects
are illustrated which can affect the use of clusters as cosmological probes.
Specifically, double clusters may affect estimates of the cluster number
density when treated as a single system. Mergers, as well as cool cores, can
alter the X-ray luminosity and temperature measured for clusters, causing these
values to differ from those expected in equilibrium.
