Butcher & Oemler Cluster A2111: A Head-on Merger at z = 0.23

We present ROSAT PSPC and HRI observations of A2111, the richest galaxy cluster photometrically surveyed by Butcher & Oemler (1984). The cluster contains a distinct comet-shaped X-ray subcomponent, which appears hotter than the rest of the cluster. The galaxy and X-ray surface brightness distributions of the cluster show a similarly elongated morphology. These results suggest that the cluster is undergoing a head-on subcluster merger. This merger may also be responsible for the high fraction of gas-rich blue galaxies observed in the cluster. We have further detected a poor cluster along the merging axis and at a projected distance of about 5 Mpc from A2111.