Evidence for an On-going Cluster/Group Merger in Abell 2255

We report on new X-ray, radio, and optical observations of the rich cluster Abell 2255. These new observations have revealed interesting and unusual characteristics at each wavelength band that are inconsistent with a simple, relaxed cluster. ROSAT all-sky survey (RASS) and CCD r-band images show that the cluster X-ray centroid does not coincide with any bright cluster galaxies or subgroup of galaxies. Furthermore, RASS spectra indicate cool (2 keV) gas within 5' of the X-ray centroid whereas Einstein MPC spectra indicate a temperature of 7.3 keV for the cluster as a whole. At 1240 km s⁻¹, A2255 has one of the highest known Abell cluster velocity dispersions; also, the two brightest cluster galaxies are separated by an unusually large 2635 km s⁻¹. Our new VLA 20 cm imaging has revealed a distorted, off-center, steep-spectrum diffuse radio source, previously labeled a radio halo, and a peculiar ridge of radio emission ≍0.6 Mpc north of the X-ray centroid. We believe that these cluster properties can be understood if A2255 is in the process of merging with a galaxy group. We present a numerical Hydro/N-body simulation of such a merger which nicely reproduces the X-ray and galaxy velocity properties, and the merger has sufficient energy to power the diffuse cluster radio emission.