Dark matter (DM) is one of the pillars of our cosmological understanding yet its nature is still elusive. It makes up about 85% of the Universe’ matter but its presence has been revealed only through gravity. The most interesting probes of its presence and nature is the merger of two clusters (or groups) of galaxies. Mergers mix the contents of galaxy groups and clusters, but each component behaves differently. While the galaxies and DM from both colliding parties take part in merger, they are almost unaffected by this event and remain confined in the original sub-structures. However, particles in the two colliding clouds of hot gas (which is the main baryonic, i.e. ordinary matter, component of clusters of
galaxies) interact with one another via the electromagnetic force, and the resulting friction causes the gas from the two merging parties to be slowed down and separated from the dark matter. The presence of DM and constraints on its self-interaction properties can therefore be inferred by measuring a spatial offset between the X-ray emission of the hot gas and the total mass distribution as revealed by gravitational lensing.
This separation has been found only in a handful of very massive clusters, such as in the famous Bullet Cluster. However a team led by Dr. Fabio Gastaldello (INAF-IASF Milan) and including Dr. Stefano Ettori and Dr. Massimo Meneghetti from the Observatory of Bologna have found a much smaller group in the process of merging that also show separating dark matter (see figure on the side). As part of the X-ray follow-up with XMM-Newton of strong lensing systems at the mass scale of group of galaxies found in the lensing survey of the Canada-France-Hawaii Telescope (CFHT) they discovered that in the object SL2S J08544-0121 the group hot gas is concentrated in one large cloud whereas DM and galaxies are split in two distinct parts. The strong lensing arcs located in the component in the West mark one of the DM clump (and their asymmetric configuration indicates the need of the other mass component, as also traced by the bimodal galaxy distribution) which is clearly separated by the peak of the X-ray emission. In the image on the side, the gas and the dark matter distributions recovered from the X-ray and gravitational lensing analysis are shown in red and in blue, respectively. The total mass of this object, nicknamed the Bullet Group, is seven times less than in the Bullet Cluster. Galaxy groups like this are more common than large clusters, so there should be many more waiting to be discovered in upcoming surveys.
Related publication: Gastaldello et al. (2014)