Mohamad-Ziad Charif - Antares

Figure 2.1: A comparison between of the rotation speed of stars for typical spiralgalaxies (red curve) and what is expected from visible matter (blue curve).in the galactic halo. Eventually more evidence started to pile up (figure 2.2) thatsupported the idea of the existence of a dark halo existing in galaxies.2.1.2 Galaxy clustersGalaxy clusters are the largest bounded structures in the known universe, theyusually consists of a large number of galaxies (up to 1000) bound by their gravitationaleffect. The typical mass of these clusters is 10 14 M ⊙ ∼ 10 15 M ⊙ where M ⊙is the solar mass, and stretching from 2Mpc to 10Mpc (one parsec (pc) is about3.10 16 m or 3.2 light-years).The study of galaxy clusters provides us with more proof on the existence of darkmatter. Chandra X-ray Observatory study of the Abell 2029 cluster[6] showedthat about 10 14 M ⊙ of the mass of the cluster is contained in a form of invisiblematter exerting gravitational forces on the galaxies.2.1.3 Gravitational lensingGravitational lensing is a phenomena where the light from a very distant souce isbent around a massive object (such as a black hole or a galaxy) between the sourceand the observer (figure 2.3). While this effect was first predicted in 1930’s it wasfirst observed in 1979 by studying the twin quasars[7] SBS 0957+561 A & SBS0957+561 B.The first evidence from gravitational lensing hinting at invisible mass came fromthe study of the galaxy cluster Abell 1689[8]. With the development of new detectiontechniques in gravitational lensing[9], stronger evidence for dark matter hasbeen unveiled. In two studies[10, 11] of two galactic clusters merging while the7