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In the preceding sections we attempted to present the very compelling evidence that arc(let)s can be powerful tools for cosmology. From a mere curiosity at the time of their discovery gravitational arcs are gaining a status of priority objects to be observed in large surveys. We summarize the scientific interest of arc(let)s in Fig. 22. Many programmes are now regularly submitted and accommodated on the largest telescopes. During the last five years arc(let)s have already proven to be the most direct and promising way to map the distribution of Dark Matter on large scales. They have revealed the possibility of using giant gravitational telescopes that Nature has put at our disposal to explore the deep universe. Ultimately, arclets signal peculiar lines of sight where the images of distant sources at cosmological distances and the lenses can be used as a huge optical bench to study the geometry of the universe itself. The new results obtained from the studies of arc(let)s are already significant but probably even more impressive is the extrapolation of what could be done with future observations.

We now have direct evidence that the centers of rich clusters of galaxies have a large amount of Dark Matter which can collapse within high density concentrations with a core radius less than 100 h50-1 Mpc. It is almost certain that the luminous component of central cDs traces the ellipticity and the orientation of this peaked component of Dark Matter. The first measurements of weak shear have allowed a detection of the mass component which extends as far as 5 h50-1 Mpc from the cluster center. This detection will probably be extended out to the largest distances and to the biggest structures observable in the universe. If the measurement of the amplitude of extremely weak shear is still delicate, it is quite certain that observers will make significant progress in near future. Preliminary results obtained for the shear pattern in a few cluster fields give evidence that the Dark Matter is condensed in substructures traced by the distribution of light. Similar measurements should be made for the determination of mass profiles around poor clusters, isolated bright elliptical galaxies or dense groups of galaxies. A massive dark halo which is apparently not related to any visible structure at observable redshifts has been reported in the field of the double QSO 2345+007. Therefore, how the distribution of light is related to the mass distribution is still an open question but the answer is within reach if enough telescope time is devoted to this observational programme. With the possible observation of very weak shear there is now a good hope of investigating the mass spectra of dark halos having an equivalent velocity dispersion of 600 km/s or lower and to infer their mass profiles. The results would be very useful, either to compare with recent simulations of dissipative collapse of clusters which predict that the mass density profile of dark matter resembles an isothermal sphere or a de Vaucouleurs law (see for instance Pearce, Thomas & Couchman 1993, Cen & Ostriker 1993, Flores & Primack 1994), or possibly to constrain these simulations. The perturbations of caustics also allow us to probe the mass of smaller interlopers on the line of sight of large multiple arcs down to 10-6 solar masses. In conclusion, observations of arc(let)s would be the best way to study the mass spectrum of Dark Matter on a wide range of mass scale from sub-dwarf galaxies to Large Scale Structures.

Redshift surveys of arcs, and redshift distributions of arclets seem to confirm that most faint galaxies with B < 26.5 are at redshifts below 1, with a magnitude-redshift diagram compatible with non-evolution models. This is in remarkable agreement with the deepest redshift surveys up to B = 24. However, these preliminary analyses of the distribution and evolution of distant galaxies are still in their infancy. None are based on a large and homogeneous catalog of arclets, and selection effects bias the samples towards low redshift systems both for the lenses and arclets. Finally, the spectral energy distribution of arc(let)s shows marginal evidence that they are distant galaxies with continuous star formation rather than systems undergoing a first or single burst of star formation. Due to the limited amount of telescope time the gravitational telescope effect has not really been used up to now to probe the deep universe. This observational field is still to be investigated. One of the best hopes is to detect for the first time very young, perhaps primeval galaxies, and to study their dynamics.

Figure 22
Figure 22. A short summary of the use of arc(let)s for cosmology.

From the results obtained up to now, we are convinced that the observations of arc(let)s will address within the next decade a large list of key questions in observational cosmology:

Although most of the observational strategies and theoretical tools are ready for the investigation of these crucial questions, a lot of effort still needs to be made on the observational side. For all of the programmes we have mentioned in this review it is necessary to use extremely long exposure times, to have sub-arcsecond seeing or better, and to explore wide fields of view. With very large telescopes it will be possible to start a systematic redshift survey of the giant arcs and brightest arclets. The large light collecting area of the new generation of telescopes will allow us to push the deep field photometry close to the confusion limit in the optical. We should detect more easily many faint arcs of very low surface brightness and gravitational pairs that can help the potential modeling. It is almost certain that with such CCD images it will be possible to give a strong constraint on the galaxy number counts at large redshifts and to start a detailed analysis of the distribution and evolution of arclets with redshifts. Very large telescopes and the HST equipped with large IR arrays are undeniably the best tools to extend such surveys to the near IR (even to the thermal infrared for ground based telescopes) and to search for young galaxies in formation. High resolution spectroscopy of emission lines (R = 5000) will also be possible to study the kinematics along the magnification axis of giant arcs. Furthermore, the strongly magnified arcs of distant galaxies should also be observed at radio wavelengths in order to detect molecules or radicals in galaxies at high redshift, and to analyze the photometric and chemical evolution in young galaxies as a whole (since it is not easy to obtain good spatial resolution at submillimeter wavelengths for the moment; see Blain & Longair 1993). But the strongest hopes concern the Hubble Space Telescope which is now refurbished. The imagery of arcs with a resolution of 0.01 arcsecond will give an unprecedented view of the gravitational images which can be unambiguously selected from the parity changes between each of the multiple images. There is even a serious possibility that we can find more gravitational pairs that are not resolved with ground based telescopes. Many arclets have already revealed substructures and we suspect in some cases that we are actually observing merging objects (Cl0024+1654) or HII regions magnified near the critical line (MS2137). The HST images will really open the way for the fine-tuning of lens potentials and for the detection of caustic perturbations. However, for the detection of the very weak shear of Large Scale Structures in an all sky survey, it will probably be necessary to dedicate a substantial amount of observing time on a 4 meter class telescope (or a larger telescope) on a good site with sub-arcsecond seeing. Even in this case the telescope will be efficient only if it is equipped with a large CCD mosaic with about a one square degree field of view. There is a proposal from a French-American-Canadian team to study and implement in the near future such a wide field camera at the prime focus of the CFHT just behind the large field corrector.

In conclusion, with very large telescopes like the Keck Telescopes and the VLT, the Hubble Space Telescope and a wide field imaging telescope, we are at the point of beginning to exploit the full potential of gravitational arc surveys. In fact, with a 50 year delay, we are now ready to start an observational programme which was within the fantastic vision of Zwicky (1937).


We first want to thank L. Woljer for proposing that we write this review and for his advice and patience during the delayed redaction of the paper. We thank all the enthusiastic participants in the arc(let)s program, in particular I. Kovner and L. Nottale for numerous fruitful discussions and encouragment at the early stages of the arcs discovery, and also R. Blandford, S. D'Odorico, R. Ellis, O. Le Févre, M. Fitchett, F. Hammer, A. Kassiola, C. Kochanek, J.-F. Leborgne, G. Mathez, J. Miralda-Escudé, R. Pelló, J.-P. Picat, Y. Rio, P. Schneider, G. Soucail, T. Tyson, L. Vigroux, and also H. Bonnet, F. Casoli, J.-C. Cuillandre, P. Encrenaz. J.-P. Kneib, A. J. Maeland, I. Smail, and L. van Waerbeke. We especially thank G. Luppino for allowing us to publish a picture of MS0440+02 in this review and sending us updated data prior to publication. We thank M. Dantel-Fort for her assistance in doing the figures of this review, and for her strong participation in the data reduction and observations at all stages of the Toulouse arc survey; and T. Bridges, P.-Y. Longaretti, and the referee for careful reading and corrections of the manuscript. The observational results and the modelings were done from observations at CFHT, ESO, Calar Alto, WHT through long term Key Programs in collaboration with Barcelona, Durham-Cambridge, T. Tyson, and J. Miralda-Escudé. We are particularly grateful to the CFHT and ESO staffs that help us and share our enthusiasm during the observations. This work was supported by the Centre National de la Recherche Scientifique, l'Université Toulouse III Paul Sabatier, and the Groupe de Recherche Cosmologie et Grandes Structures.

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