Annu. Rev. Astron. Astrophys. 1992. 30: 311-358
Copyright © 1992 by . All rights reserved


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2.2 Arcs

A rather different type of lens was first identified independently by Lynds & Petrosian (1986) and Soucail et al. (1987) who discovered a blue, luminous arc ~ 20" long in the rich cluster of galaxies, Abell 370 (Figure 3). Similar long features have been reported in Abell 963 (Lavery & Henry 1988, Ellis et al. 1991), Abell 1352, 1525, 1689, 2163, 2218 (Tyson, personal communication), Abell 2390 (Pello et al. 1991), Cl 0024+17 (Koo 1988), Cl 0302+17 (Fort 1992), Cl 0500-24 (Giraud 1988), Cl 1409+52 (Tyson et al. 1990), and Cl 2244-02 (Lynds & Petrosian 1986) (cf Table 1). Detailed theoretical models (Hammer 1987, Kovner 1988, Grossman & Narayan 1988, Lynds & Petrosian 1989) showed that arcs would be formed quite naturally by a deep cluster potential well producing a large tangential magnification in the image of a background galaxy (Paczynski 1987a).

Einstein cross Einstein cross
Figure 2.The ``Einstein cross'' gravitational lens Q2237+031. (Left) Spiral galaxy lens with redshift z = 0.039. (Right) Four quasar images surrounding the bright central bulge of the lens obtained using the Faint Object Camera on Hubble Space Telescope. (Images supplied courtesy C. Blades, D. Macchetto, and T. Tyson.)

The majority of the above clusters, plus about six other clusters in which long arcs have not yet been reported, exhibit numerous (up to 60) smaller scale arclets (Tyson et al. 1990, Fort 1992, Smail et al. 1991, Soucail 1991). These are again background galaxies, but they are much less magnified than the major arcs and are mostly singly imaged.

Abell 370
Figure 3. Giant arcs formed by the rich cluster of galaxies Abell 370. The source is a blue galaxy (z = 0.72) located close to a caustic. Also visible are many smaller arclets, elongated tangentially with respect to the cluster center. (Image supplied courtesy B. Fort and G. Soucail.)

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