Published in Vol. 5 of The Astronomy and Astrophysics Review, pp. 239:292 - 1994.

ABSTRACT. The discovery of giant gravitational arcs and arclets in rich clusters of galaxies is one of the major events of the last decade in observational cosmology. High resolution imaging in subarcsecond seeing conditions of giant arcs gives information on the cluster potential and the matter distribution within the inner regions of clusters. Ultra-deep photometry of the clusters reveals numerous arclets with an orthoradial orientation from which one can infer the projected mass profile at large distance and the redshift distribution of the faintest distant background galaxies which are unobservable with standard spectroscopic techniques. Thanks to the strong magnification factor, the spectroscopy of giant arcs is possible and we can therefore observe with great detail a few very distant galaxies. Individual redshifts of arcs give the total mass of the lens, whereas the spectroscopy of a large sample of arcs also gives information on the redshift distribution of distant galaxies. It is obvious that cluster lenses play an important role as large natural telescopes for probing the distant universe. Finally, observations of multiple-arc configurations due to different sources may even constrain the cosmological parameters. We are now confident that gravitational lensing will be an essential tool within the next decade for observing very high redshift galaxies and the weak shear generated by the largest structures of the universe.

In this review we summarize the present status of gravitational arc(let)s surveys with particular emphasis on the most important issues which have arisen during the last years and on the prospects for the future, regarding the rebirth of the Hubble Space Telescope, the coming of a new generation of Very Large Telescopes, and the development of large CCDs in the optical and the infrared.

KEY WORDS: Gravitational Lensing - Clusters of galaxies - Cosmology

Table of Contents

INTRODUCTION

BASIC CONCEPTS OF GRAVITATIONAL LENSING

The Thin Gravitational Lens

Caustics and Formation of Arcs

The Singular Isothermal Sphere (SIS)

The Non Singular Squeezed Isothermal Potential

OBSERVATION OF ARCS

Imaging and Photometry of Arcs

Spectroscopy of Arcs

ARC(LET)S AS A PROBE OF DARK MATTER

Large Arcs and Central Potential

The Dark Matter Distribution from Giant Arcs

The Gravitationally Lensed Pairs

The Weak Distortion Regime

Microstructures in Arcs and Minilensing by Small Dark Halos

Occurrence of Arcs and Evolution of Clusters

Optical Rings

Towards Probing the Mass Spectrum of Gravitational Structures With Weak Lensing

Mapping Large Scale Structures

Weak Shear and Amplification Bias Near Radiosources

GRAVITATIONALLY LENSED IMAGES AND THE EVOLUTION OF GALAXIES

Giant Arcs

Distance of Faint Arclets

COSMOGRAPHY

FUTURE OUTLOOK

REFERENCES