RINGS OF DARK MATTER IN COLLISIONS BETWEEN CLUSTERS OF GALAXIES

Several lines of evidence suggest that the galaxy cluster Cl0024+17, an apparently relaxed system, is actually a collision of two clusters, the interaction occurring along our line of sight. Recent lensing observations suggest the presence of a ringlike dark matter structure, which has been interpreted as the result of such a collision. In this paper, we present N-body simulations of cluster collisions along the line of sight to investigate the detectability of such features. We use realistic dark matter density profiles as determined from cosmological simulations. Our simulations show a shoulder in the dark matter distribution after the collision, but no ring feature even when the initial particle velocity distribution is highly tangentially anisotropic (sigma(theta)/sigma(r) >> 1). Only when the initial particle velocity distribution is circular do our simulations show such a feature. Even modestly anisotropic velocity distributions are inconsistent with the halo velocity distributions seen in cosmological simulations and would require highly fine-tuned initial conditions. Our investigation leaves us without an explanation for the dark matter ringlike feature in Cl0024+17 suggested by lensing observations.