Gravitational lenses

Quasars ("quasi-stellar objects") are objects with a star-like appearance. They are bright nuclei of active galaxies, which are so distant that they appear point-like to the observer. Quasars are among the most distant visible objects in the universe, which can be seen only because they are immensely luminous.

Individual galaxies can act as smaller-scale gravitational lenses on distant objects like the bright quasars. This way, multiple images) can be formed from single quasars. Four images of the quasar QSO 2237 surround in a cross-shaped pattern the centre of a spiral galaxy, whose spiral arms can be recognised. Such images allow the most exact mass determinations of individual galaxies, and offer an additional wealth of information on the gravitational lens and the source.

Individual galaxies can act as smaller-scale gravitational lenses on distant objects like the bright quasars. This way, ring-shaped images (Einstein rings) can be formed from single quasars, as shown in the image to the left. The ring is the image of the quasar QSO 1938. Such images allow the most exact mass determinations of individual galaxies, and offer an additional wealth of information on the gravitational lens and the source.

Galaxies are not randomly distributed on the sky. There are concentrations and voids. The image shows the colour-coded number density of galaxies on the sky in a cap of 50 degrees radius around the north pole of the Milky Way. The brighter the colour, the denser are the galaxies placed. Particularly pronounced are the galaxy clusters, which encompass some hundred up to a few thousand galaxies.

Some galaxy clusters like Cl 2244-02 exhibit extended, arc-like features, the so-called arcs. They appear because the mass in galaxy clusters deflects light (similar to a glass lens), and thereby distorts images of distant objects. The arcs are therefore strongly distorted images of galaxies, which are located far behind the galaxy clusters.

Some arcs reveal a surprisingly rich structure, for instance the arc in Abell 370 (left). This allows to reconstruct how the source must have looked before the distortion. Furthermore, very small structures can be observed only because of the magnifying effect of gravitational lensing. Gravitational lenses therefore act like telescopes on a cosmic scale, whose imaging systems are gigantic mass distributions rather than lenses or mirrors.

When galaxy clusters are observed in great detail, their mass distribution can be determined from the very many images in their surroundings, some of which are only weakly distorted. The shape of the lens can very accurately be reconstructed from these images. This allows the computation of mass maps of galaxy clusters.

Arcs in galaxy clusters can be modelled in simulations. In comparison with observed arcs, such simulations allow to deduce important properties of the universe. For example, it was found that the matter density in the universe is too small to ever hold or even reverse the expansion of the universe.

Light propagates along straight lines in empty space. An observer sees the image of a source in the direction from which the light is approaching. According to General Relativity, masses deflect light, similar to normal glass lenses. They are therefore called gravitational lenses. Lenses like the Sun bend light rays the stronger the closer they are to the light path. Therefore, the image appears pushed away from the lens. This effect is exaggerated here. Galaxies can bend the light path so strongly that light can travel on multiple paths. This way, multiple images of a distant quasar can appear. Such multiple images are indeed observed. The quasar 2237 for instance appears quadruply imaged. If a source is placed exactly on the optical axis, a ring-shaped image is formed, which is called an Einstein ring. Such Einstein rings are also observed. The quasar 1938 for example appears as a ring. Entire galaxy clusters can also act as gravitational lenses. They occasionally produce strongly distorted images of distant galaxies. A spectacular example is the large arc in the galaxy cluster Cl 2244. Galaxies further away from the cluster are only weakly distorted, but they are very numerous. These galaxies allow to properly map strength and orientation of the distortion around a galaxy cluster. Such distortion maps can be used to reconstruct the mass distribution in galaxy clusters. In such a map, the height displayed corresponds to the amount of reconstructed material.