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Strong gravitational lensing around galaxy cluster CL0024+17, demonstrating at least three layers projected onto a single 2D image. The + shaped objects are nearby stars in our own galaxy (the + created by optical effects in the telescope). The yellow, elliptical galaxies are members of the cluster, all at a similar redshift and gravitationally bound. Also amongst this group of galaxies is a halo of invisible dark matter. The elongated blue objects are much more distant galaxies, physically unassociated with, and lying behind, the cluster. Gravitational lensing has distorted their apparent images into a series of tangential arcs. Figure credit: NASA/ESA/M.J. Jee (John Hopkins University).
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We review progress in understanding dark matter by astrophysics, and particularly via the effect of gravitational lensing. Evidence from many different directions now all imply that five sixths of the material content of the universe is in this mysterious form, separate from and beyond the ordinary "baryonic" particles in the standard model of part...
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... "radial arcs" are generally difficult to see because they are usually less magnified and appear inside the Einstein radius, behind any light emitted by the lens object itself. An example of strong gravitational lensing around a massive galaxy cluster is shown in figure 1. The first strong gravitational lens was discovered with the Jodrell Bank MkIA radio telescope in 1979 [27]. ...
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... optimism was well founded, for weak lensing really burst onto the Figure 2. The statistical signals sought by measurements of weak gravitational lensing are slight but coherent distortions in the shapes of distant galaxies. (Left): A tangential, circular pattern of background galaxies is produced around a foreground mass overdensity, reminiscent of the tangential arcs of strong lensing seen in figure 1. On much larger scales, an opposite, radial pattern is produced by foreground voids. ...
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... that the astrophysics literature normally parameterises this in terms of the 'concentration' or ratio of the radius containing most of the mass to the scale radius (individual galaxies are expected to have concentrations of around 7-13, and massive clusters of 5-6). A sample of 98 galaxies acting as strong lenses was ingeniously found Large-scale structure Figure 10. The observed radial distribution of mass around elliptical galaxies in the Hubble Space Telescope COSMOS survey, decomposed into its various components [177]. ...
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... the stacked gravitational lensing signal behind them, in agreement with dynamical analysis, shows an apparent conspiracy between the dark matter and baryonic components to produce an overall "isothermal" ρ ∝ r −2 density profile out to very large (∼ 140 kpc) radii [173,174] (or even further [175]). Figure 10 demonstrates the conspiracy of central baryons, the galaxy's own dark matter halo, and the haloes of neighbouring galaxies; none of them is individually isothermal [161,74,176,177]. In addition, the location of the transition from the host halo to large-scale structure marks the typical size of dark matter structures and its occurrence at the scale expected from simulations itself provides strong support for the CDM paradigm [72]. ...
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... has been raging for several years about the inner profile of dark matter haloes within the scale radius. Examples of elliptical galaxies shown in figure 11 have been found with a cuspy β ≈ 1 inner dark matter component, as expected from the original NFW simulations. These include the "jackpot" double Einstein ring, which provides the best lensing-only galaxy mass profile, and also shows that a dark halo is required. ...
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... that can be further studied because they act as strong gravitataional lenses appear to be a representative sample of all galaxies -chosen solely by the chance of another source lying directly behind them. Two independent studies confirm that Figure 11. Left: The "cosmic eye" elliptical galaxy lens [225]. ...
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... all of the material in subhaloes is stripped from merging subhaloes and smoothly redistributed amongst the cluster. Speculation still continues [228,229,156,157] on the expected fraction of mass in substructure, with estimates typically ranging from ∼ 5-65% and options for the relatives sizes of the substructure shown in the left panel of figure 12. ...
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... of 10 4 − 10 7 M in elliptical galaxies provides the only explanation for observations of anomalous flux ratios in multiply imaged quasars [241,245,246]. Indeed, very deep near-infrared imaging in one of these cases, shown in the right panel of figure 12, did eventually find emission from one of the large pieces of substructure. Curiously, the amount of substructure implied by millilensing seems to be greater than that predicted by simulations, turning the missing satellites problem on its head [247,248]. ...
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... morphologies of the infalling galaxies appear to change dramatically only once they fall within ∼ 1 Mpc of the cluster core [269]. However, as shown in figure 13, the weak lensing signal around elliptical galaxies within 3 Mpc indicates truncated haloes with a mean total mass of 1.3 ± 0.8 × 10 12 M , compared to 3.7 ± 1.4 × 10 12 M for similarly bright galaxies in the outskirts [270]. Indeed, different physical effects seem to dominate in three distinct zones around a cluster [269,270]. ...
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... many attempts have been made to circumvent the need for dark matter by modifying general relativity. Some of these theories of gravity can also predict the accelerating expansion of the Figure 13. The removal of mass from the dark matter haloes around elliptical galaxies of a fixed luminosity by tidal gravitational forces, as they fall into a large cluster [270]. ...
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... the differences between baryons and dark matter are also highlighted by their different reactions to these processes. The most striking example of this, and the cluster that has provided the most direct empirical evidence for dark matter, is undoubtedly the "bullet cluster" 1E 0657-56 [285,286,287], shown in figure 14. ...
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... this is not always the case. The bullet cluster is strictly two clusters that collided about 150 million years Figure 14. The "bullet cluster" 1E0657-56 and "baby bullet" MACSJ0025.4-1222. ...
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... such objects are rare. One very similar "baby bullet" object has been detected [290] (see right panel of figure 14), as well as a "cosmic train wreck" counter-example [291], which shows separated dark matter and gas components, but in a complex distribution that probably implies a collision between three clusters. Collisions along the line of sight would provide complementary information, but the one possible example [292] is probably an artefact of spurious instrumental effects [293] and substructure within the cluster [294]. ...
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... facilities are being designed or converted for dedicated measurements of gravitational lensing. The Japanese 8m Subaru telescope on Mauna Kea, Hawaii, shown in the left panel of figure 15, was built with weak gravitational lensing measurements Figure 15. The investigation of dark matter via gravitational lensing has a bright near-term future. ...
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... facilities are being designed or converted for dedicated measurements of gravitational lensing. The Japanese 8m Subaru telescope on Mauna Kea, Hawaii, shown in the left panel of figure 15, was built with weak gravitational lensing measurements Figure 15. The investigation of dark matter via gravitational lensing has a bright near-term future. ...
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... Hubble Space Telescope already offers imaging with more than ten times higher resolution -and crucially stability -than even the Subaru telescope, although over a narrow field of view [358]. The proposed High Altitude Lensing Observatory (HALO), illustrated in the right panel of figure 15, will fly on a long- duration balloon above 99% of the atmosphere. If its pointing accuracy can be successfully stabilized, it will offer wide-field imaging of almost space-based resolution for about the same cost as most cameras currently under construction for ground-based telescopes. ...
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