This galaxy appears to have four bright objects in its nucleus, but in actuality, it is acting as a gravitational lens. The "imperfect" shape of the lens has produced multiple images a distant quasar. The cloverleaf appearance prompted astronomers to name it the Einstein Cross.
Quasars are "quasi-stellar" objects, in that they appear to be points of light like stars, but they have very large cosmological redshifts. This indicates that they must be very far away. To be this far away, and yet visible to us, they must be incredibly bright. They also have broad emission lines in their spectra, unlike stars.
The five bright white lights in this image are all images of the same quasar viewed through this gravitational lens created by a galaxy cluster. The strong gravitational field of the galaxy cluster warps space such that the light takes a curved path through the region. Recognizing that the bright spots are all the same object, and that some of the smeared out galaxies are also repeated images, allows us to infer the structure of the gravitational lens.
Some 200,000 quasars have been identified. Most quasars have very high redshifts, meaning that the light emitted from them has been traveling for a very long time, enough time for the expansion of the universe to stretch the light considerably. This leads us to believe that quasars were common in the early stage of galaxy evolution. With such high luminosities, we believe that these objects must be active galactic nuclei of early galaxies, supermassive black holes in the formation stage, with accretion disks emitting brilliant jets directly at us. These high energies could be the result of early galaxy mergers, and/or the formation of black holes directly from the gravitational collapse of dense regions of matter in the early universe.
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We saw that the rotation curves of spiral galaxies indicated that there must be a large halo of unseen matter present. Similarly, when we measure the orbital speeds of gravitationally bound galaxy pairs or galaxy clusters, we see that there must me more matter present than we otherwise observe. In this diagram, the Doppler-shifted light allows us to calculate how fast a binary pair of galaxies are orbiting their common center of mass. The orbital speeds we measure are too high. Moving at these speeds, the gravitational force between them would not be strong enough to hold them together, if the normal matter of the galaxies is taken into account. The galaxies should be flying apart. There must be more matter, dark matter, holding the system intact.
The image above shows a composite of three images. The galaxies are shown in visible light, with a couple of stars in the foreground. The red color indicates hydrogen gas and the blue color indicates the dark matter present.
In visible light, what we see is the collision of two galaxy clusters. In "recent" cosmological time, the smaller cluster, to the right, passed through the larger cluster to the left. The galaxies passed through relatively intact, while the hydrogen gas was much more strongly affected by the collision. The gas particles are charged particles, so they interacted with each other via the electromagnetic force. As the smaller cloud punched through the larger cloud, the smaller cloud was compressed into the "bullet" shape.
The blue part of the image, showing the distribution of the dark matter, was indirectly inferred, using gravitational lensing techniques. If you look closely, you can see some small, smeared out galaxies that have been magnified by the gravitational fields of the galaxy clusters. The geometric shapes of the gravity fields of the galaxy clusters was inferred by the smearing of the distant, magnified galaxies. Subtracting the gravitational effects of the visibly observed galaxies, astrophysicists could infer what dark matter must be present.
The most informative aspect of this mapping of dark matter is found by comparing it with the hydrogen gas clouds. Where the hydrogen gas clouds showed evidence of colliding, the dark matter haloes did not. The dark matter clouds associated with the galaxy clusters simply passed right through each other. This means that the dark matter must not be made of charged particles. It must be interacting via the gravitational force, but not the electromagnetic force.