Gravitational lensing by dark matter caustics.

摘要

There are compelling reasons to believe that the dark matter of the universe is constituted, in large part, by non-baryonic collisionless particles with very small primordial velocity dispersion. Such particles are called cold dark matter (CDM). The leading candidates are axions and weakly interacting massive particles (WIMPs). The word “collisionless” indicates that the particles are so weakly interacting that they move purely under the influence of gravity. Galaxies are surrounded by CDM and hence, because of gravity, CDM keeps falling onto galaxies from all directions. CDM infall produces a discrete number of flows and caustics in the halos of galaxies. There are two types of caustics in the halos of galaxies: inner and outer. An outer caustic is a simple fold (A₂) catastrophe located on a topological sphere enveloping the galaxy. An inner caustic is a closed tube whose cross-section is an elliptic umbilic (D₋₄₄) catastrophe, with three cusps.; In space, caustics are the boundaries that separate regions with differing numbers of flows. One side of a caustic surface has two more flows than the other. The density of CDM particles becomes very large as one approaches the caustic from the side with the extra flows. Dark matter caustics have specific density profiles and therefore precisely calculable gravitational lensing properties. This dissertation presents a formalism that simplifies the relevant calculations, and apply it to four specific cases. In the first three cases, the line of sight is tangent to a smooth caustic surface. The curvature of the surface at the tangent point is positive, negative, or zero. In the fourth case, the line of sight passes near a cusp, For each case we derive the map between the image and source planes. In some cases, a point source has multiple images and experiences infinite magnification when the images merge. A promising approach to reveal the locations of caustics is to observe the distortions in the images of extended sources (such as radio jets) passing by the caustics.