The Bullet Cluster is the type example of the behaviour of dark matter in a fast galaxy collision: the dark matter haloes of colliding galaxies do not interact but continue on their way, taking the visible galaxies with them, while the hot gases from each galaxy interact. Now data from a galactic merger in Abell 520 suggest that dark matter does not always behave in this way.
James Jee, of the University of California in Davis, and his team started from the unexpected distribution of gas and dark matter in Abell 520 detected with the Hubble Space Telescope in 2007. These data were considered poor quality and it was not until better quality HST data coupled with observations using NASA’s Chandra X-ray Observatory confirmed the gas distribution that the unusual pattern emerged clearly. Jee and his team then used the Canada–France–Hawaii Telescope and Subaru Telescope to infer the location of dark matter by gravitational lensing, and went back to the HST to the Wide Field Planetary Camera 2 to produce a more detailed map of the dark matter distribution.
This technique revealed that the dark matter in Abell 520 had collected into a “dark core,” containing far fewer galaxies than would be expected if the dark matter and galaxies were anchored together. Most of the galaxies have apparently sailed through the collision and moved far away.
It is not clear why this galactic collision is so different from others, notably the Bullet Cluster. Both are thought to be relatively recent encounters, but the team proposes that perhaps Abell 520 was a more complex collision, or that there are faint galaxies in the central region that are not bright enough to be visible. A third possibility is that dark matter, rather than not interacting with itself, can be “sticky” in some way, and some of it has clumped together. Jee et al. published the results in The Astrophysical Journal.