4.7 Optical Rings
Rings occur when the alignment of the observer, the lens and the source is almost perfect, and the source is covering the whole internal caustic, forming the so called ``Einstein ring''. The first rings were observed around galaxies in radio surveys (see Refsdal & Surdej 1994 for a recent review of radio rings). However, Pelló et al. (1992), and Tyson (private communication) announced the discovery of one optical ring in two rich clusters of galaxies. If the rings are actual gravitational lens events, they will give interesting information on the masses of individual galaxies in clusters, and also on the slope of the cluster mass profile (Kochanek & Blandford 1991). For a steep potential profile most of the possible ring events would be distorted, and more likely small arcs would be formed. If almost perfect circular rings were observed in rich clusters, this would favor cluster potentials with large core radii, in contradiction with most of the results found from the observations of giant arcs. This inspired Kassiola & Kovner (1993b) to try to find a consistent arc model for the circular ring in A2218. They found that they can reproduce the ring but with a very peculiar coincidental alignment of potentials. The first tentative spectra of these two ring candidates do not show evidence of their being distant sources, and even support the hypothesis that they are at the cluster redshift (Pelló, Le Borgne, Fort, & Bernstein 1993 private communications). In conclusion, the absence of rings in the centers of rich clusters is fully consistent with small core radii for the distribution of dark matter. Therefore, they should be more likely found at the outskirts of clusters if the profile is not too steep. The contribution of the cluster can aid the galaxy lens in enlarging the radius of the ring, but the ring will often be embedded in the halo of the galaxy. It is clear that such rings should be searched for with the HST even in blank fields (Miralda-Escudé & Lehár 1992). The density of the background galaxies is so high that rings should be numerous in the sky. They will be serendipitously found in some HST deep fields by looking carefully at B-I images. The rings can be as frequent as arcs in blank fields because a distant galaxy can easily cover the whole internal caustic (astroid) of a galaxy lens. The spatial resolution of the HST (0.1 arcsec) will allow the study of galaxy rings (arcs) with a radii of a few arcsec, in a similar way as cluster arcs are studied with ground based observations in subarcsecond seeing. This means that we could soon have a new tool to directly probe the mass distributions of deflecting galaxies, if we find them.