Stripped in the Dark

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Science  03 Apr 2009:
Vol. 324, Issue 5923, pp. 15
DOI: 10.1126/science.324.5923.15a

Galaxies appear to reside inside dark-matter halos, which over time gather in large clusters held together by the force of gravity. These can be violent places. Dark-matter halos collide and interact with one another, and as they get closer to the cluster's core, they can have matter stripped off them and be ripped apart by strong tidal forces. Although it is not possible to detect dark matter directly, its presence can be inferred from the gravitational effects it has on luminous matter. One such effect is gravitational lensing, whereby light is deflected by the presence of massive objects, producing elongated images of the objects behind them. By analyzing the distorted shapes it is possible to derive the mass distribution of the objects acting as a lens, a technique Natarajan et al. used to study the dark-matter halos in the massive lensing cluster Cl 0024+16. They found that the halo masses decreased with decreasing distance to the center of the cluster, as predicted by theories of cosmic structure formation; this result is mimicked in numerically simulated clusters, strengthening the evidence for tidal stripping in clusters of galaxies. — MJC

Astrophys. J. 693, 970 (2009).

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