Home Page Overview Site Map Index Appendix Illustration About Contact Update FAQ

Clusters of Galaxies

Dark Matter

By analyzing the distribution of luminous matter and the properties of the gravitational lensing due to total cluster mass in CL0025+1654, researchers have solved the problem of tracing the dark matter layout. Their resulting map shows the otherwise invisible dark matter in blue, and the positions of the cluster galaxies in yellow. The work, based on extensive Hubble Space Telescope observations, reveals that the cluster's dark matter is not evenly distributed, but follows the clumps of luminous matter closely as shown in Figure 04-10a. The finding bears not so much on whether the dark matter is self-interacting (in addition to gravity) but on the "bottom up" theory in which gravity is believed to have assembled increasingly large structures from small ones as the universe aged and expanded.

In Figure 04-10b the separation of dark matter (blue) from the X-ray emitting gas clouds (red) after the collision of a small and large clusters about 100 million years ago demonstrates that unlike the gas which feels electromagnetism as well as gravity, the dark matter clings to the clusters by gravity only; while the hot intra-cluster gas experiences an additional drag force that slows it down more. However a 2013 study of the collision in the Musket Ball cluster found that the galaxies are separated from its dark matter (Figure 04-10c in blue) by about 19,000 light years. Some dark matter was more in line with the gas (in red). The founding suggests that the dark matter does slow down by some sort of force
Dark Matter Dark Matter in Colliding Clusters Dark Matter in Colliding Clusters other than gravity, i.e., it is interacting with itself. The different scenario from the Bullet cluster could be related to the timing of the events. While the Musket Ball collision occurred much earlier about 700 million years ago, the Bullet collision happened later so that the galaxies and dark matter do not have enough time to divorce.

Figure 04-10a Dark Matter in CL0025+1654

Figure 04-10b Dark Matter in Bullet Cluster

Figure 04-10c Dark Matter in Musket Ball Cluster

Dark Matter Lensing Research in 2008 indicates that dark matter can produce a weak lensing effect to distort the image of distant galaxies. By noting the degree to which background galaxies appear unusually flat and unusually similar to neighbors, the dark matter distribution producing these weak gravitational lensing distortions can be estimated. Analysis of the shapes of 200,000 distant galaxies imaged does show the presence of a massive network of dark matter. Figure 04-10d is a computer-generated simulation of dark matter distribution. It shows the dark matter (in red) bending the light path from the apparent shape of distant galaxies (in blue) to a more flattened shape.

Figure 04-10d Dark Matter Lensing [view large image]

Figure 04-11 shows the computer simulations of the evolution of cold dark matter (upper sequence from left to right), in which ordinary gases cool, condense and fragment to make galaxies (in corresponding lower sequence). Each panel shows the projected cold dark matter or galaxy distribution in slices of thickness 15 h-1Mpc, where h denotes the Hubble constant in units of 100 km s-1Mpc-1. For the cold dark matter, the
Cold Dark Matter color hue from blue to red encodes the local velocity dispersion, and the brightness of each pixel is a measurement of the density. In the galaxy evolution panels, each galaxy is weighted by its stellar mass, and the color scale of the images is proportional to the total stellar mass. The cold dark matter evolves from a smooth, nearly uniform distribution into a highly clustered state, quite unlike the galaxies, which are strongly clustered from the beginning.

Figure 04-11 Cold Dark Matter
Evolution [view large image]

Go to Top of Page to Select
 or to Main Menu