Straight arcs in gravitational lens systems

Abstract

Faint straight arcs extending over ten to twenty arcseconds have been observed behind rich clusters of galaxies elongated by the lensing action of the foreground cluster. When a cluster of galaxies has its projected surface mass density barely above the minimum mass density required for multiple imaging of a background source, it is called a marginal lens for the specified redshifts of the lens and the source. A marginal lens produces images of large magnification and consequently, in a flux limited sample we expect ample number of such cases in spite of the rarity of such events. Rich clusters at redshift greater than around 0.2 are capable of multiple imaging of objects along their line of sight situated at redshift larger than about 1. The marginal lensing action of these clusters producing straight arcs will provide valuable diagnostic of the cluster as well as the large scale geometry of the Universe. Spectroscopy of the straight arcs is a valuable probe of the rotation curve and other characteristics of galaxies at high redshifts. Further, presence of mirror images of these galaxies provide observational constraints which would not be available for nearby galaxies.