ON THE STRAIGHT ARC IN ABELL-2390

We investigate the long "straight arc" observed in the cluster of galaxies Abell 2390. The arc consists of three segments of comparable brightness, and its optical spectrum shows a nonmonotonic variation of velocity along its length. In addition to the optical data, recent infrared observations indicate that the longest segment of the arc is much brighter in infrared than the other two. We consider the formation of straight arcs analytically in terms of fold and cusp catastrophes and two singular sections of a cusp, the "beak-to-beak" and "lips" "calamities." We show that a single cusp always produces curved images, while straight images can be formed by fold catastrophes and these two types of calamities. We then use these results to derive detailed models for the observed features. It is possible to derive a fold model by introducing a sheetlike dark mass distribution. However, this appears to be incompatible with the infrared observations. Beak-to-beak models must postulate either an unseen cluster to balance Abell 2390 or an unseen galaxy (with mass-to-light ratio in excess of 100 in solar units) to offset the closest cluster galaxy. The latter model is more structurally stable and consistent with the optical and infrared data. Future observations may distinguish between these possibilities and strengthen the case for localized subluminous mass in rich clusters.