On the straight arc in A2390

Abstract

We investigate the long “straight arc” observed in the cluster of galaxies Abell 2390. The arc consists of three segments of comparable optical brightness, and its optical spectrum shows a non-monotonic 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. Analytical investigation of the formation of straight arcs shows that single cusps always produce curved images, while fold catastrophes and “beak-to-beak” and “lips” “calamities” can form straight images. On the basis of these results we derive detailed models for the observed features. We argue that it is not possible to reproduce the straight arc without postulating some unseen matter not traced by the cluster. A fold model is consistent with the optical data, but it requires a large and very elongated unseen component, and it is not consistent with the infrared observations. A beak-to-beak model agrees with both infrared and optical data and requires only a small unseen galaxy. Future observations may distinguish between these possibilities.