I have been leading the effort to study in detail a newly discovered dissociative galaxy cluster merger (DLSCL J0916.2+2951* a.k.a the Musket Ball Cluster), were the collisionless dark matter (blue, in the figure below) and galaxies have become separated from the collisional cluster gas (red, in the figure below). To date only 5 dissociative mergers have been confirmed, the most famous being the Bullet Cluster. These dissections of the cosmos are proving to be some of the best laboratories for studying dark matter.
*This unattractive name refers to the fact that it was discovered in the Deep Lens Survey, is a cluster, and where it is located. Our team affectionately calls it the Musket Ball Cluster since it is similar to the Bullet Cluster yet older and slower.
Mapping the matter of the Musket Ball
The Musket Ball Cluster and its three matter components. Overlaid on the Hubble Space Telescope (HST) color image of the galaxies is the total mass distribution (blue) based on WL analysis of the HST images and the cluster gas distribution (red) based on Chandra X-ray observations. The bulk of the collisional gas is located between the two collisionless galaxy and mass concentrations, indicative of a dissociative merger. The image is 5x5 arcmin2 (~1.9x1.9 Mpc2 at z=0.53, or 6 million light-years on a side).
Galaxy clusters are predominantly comprised of dark matter (~85% total cluster mass), which as the name implies emits no light. The remainder of the mass is in the form of baryonic matter (i.e. the periodic table stuff) the majority of which is diffuse intracluster gas (~14% total cluster mass), and galaxies (~1%).
Galaxies of the Musket Ball
The galaxies of the Musket Ball are more easily seen in this color image from the Deep Lens Survey
. The contours represent the density of galaxies near the cluster. The two main subclusters are clearly visible. It is believed that the two subclusters are gravitationally bound and in the process of merging and in a few Gigayears (a.k.a US billion years not English billion) will combine to form a single large cluster.
Dark Matter of the Musket Ball
While dark matter does not emit light (so we cannot "see" it like we can see galaxies), we can detect its influence on the light of other galaxies, in particular galaxies whose light passes through the cluster's gravitational potential. According to Einstein's theory of general relativity, a massive object will deflect and distort light as it passes near the massive object. The method of weak gravitational lensing works by measuring these distortions and inferring what the mass of the object is and where it is located. Shown above in grayscale are the weak lensing maps (i.e. mass map) of the Musket Ball, as measured by the ground based Subaru telescope (left) and by Hubble Space Telescope.
When two clusters collide their galaxies rarely collide with one another, due to there being so much space between the galaxies. Thus galaxies can be considered collisonless particles. The cluster gas on the other hand is dispersed somewhat evenly throughout each subcluster and as a results the gas of one subcluster is much more likely to collide with the gas of the other. The result being that much of the gas will stop at the point of collision while the galaxies pass right on through. As can be seen in the images above, the mass of the cluster largely overlaps the galaxies (white contours). Using this observation and knowledge that there is ~6 times more gas mass than galaxy mass, we can infer that dark matter comprises ~85% of the total cluster mass and is (nearly?) collisionless. In a sense illuminating the dark matter.
For a more detailed account please see our journal article "Discovery of a Dissociative Galaxy Cluster Merger with Large Physical Separation
". Also check out the Chandra image release: DLSCL J0916.2+2951: Discovery of the Musket Ball Cluster