These simulations were made by our group, principally Ayana Holloway. They show the cluster A1689 as viewed over the next few billion years. As it moves through the Universe its alignment with the background faint blue galaxies constantly changes. The data for the cluster (the orange bright galaxies) are from the Hubble Space Telescope, and the mass model is one which we have developed for this particular cluster, based on very deep multi-color HST and ground-based imaging of A1689, via strong lens parametric techniques. [For a description of parametric strong lens mass determination, see Tyson, Kochanski, & Dell'Antonio 1998, ApJ 498, L107. ]
These first two movies are based on two extreme mass models: LENSMAX.mpg has much of the mass in the halos of the cluster galaxies, while LENS0.mpg has very little mass in the cluster galaxies (most mass residing in a single dark mass).
The background galaxies are placed in six redshift sheets, out to z = 3. Fainter and bluer galaxies are at higher redshift. Some galaxies just behind the cluster (coded brighter and less blue) are only slightly distorted by the concentration of dark matter. The highest redshift faint galaxies easily form large arcs (perturbed Einstein rings) at larger radii.
Our simulated mass distribution is composed of dark matter halos for individual cluster galaxies as well as the dominant diffuse mass of the cluster. This largest mass has a soft core. A non-singular lens mass produces an odd number of images of a background galaxy. When the galaxy is at large projected radius (weak lens regime) it is slightly distorted orthogonal to the radius vector. As the galaxy moves into the critical region it splits into an odd number of images, usually two arcs on opposite sides of the lens and a third, demagnified, image near the center. The details of these images and their parity and magnification are sensitive to the gradient of the lens mass. For regions near the core where the lens mass density exceeds the critical mass density, the magnification determinant is negative and the lens-focussed light bundle comes to a focus before reaching Earth. Images inside this radius (one of the odd images) appear demagnified and with opposite parity. These faint radial spokes may be seen for many of the background galaxies over a finite range of impact parameter, and are a sensitive indicator of the size of the soft mass core. Another observed feature of our lens model for this cluster is the unusually thin arcs (images if high-z galaxies); this is due to a mass profile which is steeper than isothermal.