The currently accepted model of our universe includes a component called dark matter, a mysterious type of matter that cannot be seen but is believed to make up nearly a quarter of the matter in the universe. One form of evidence of the existence of dark matter is gravitational lensing. When light rays emitted by a source travel near a massive object, the gravitational field of that object causes the rays to bend. This can result in multiple images of the source (strong lensing) or in a distorted image (weak lensing). Analysis of gravitational lensing effects can reveal the distribution of dark matter and ordinary matter in the massive object.
It is precisely this type of analysis that was carried out on the newly discovered galaxy cluster amalgamation. An international team of astronomers led by Marusa Bradac, a postdoctoral researcher at the University of California in Santa Barbara‘s Department of Physics and Steve Allen of the Kavli Institute for Particle Astrophysics and Cosmology at Stanford, captured this merge and studied it. The discovery was made using the Hubble Space Telescope (HST) and the Chandra X-ray Observatory.
When clusters merge, the dark matter and ordinary matter can become temporarily separated. This makes merging clusters ideal for the detection and study of dark matter and its properties, as was demonstrated when the Bullet Cluster was observed two years ago. The Bullet Cluster provides strong support to the existence of dark matter, if not the strongest, to date.
The new cluster, MACSJ0025, is located 5.7 billion light years away, farther than the Bullet cluster, meaning the collision actually took place before that of the Bullet Cluster. Unlike the Bullet Cluster, the new cluster does not contain a “bullet”; a dense, x-ray bright core of gas that can be seen moving through the Bullet Cluster. Despite this, the energies present in this collision are nearly as extreme as those found in the Bullet Cluster.
The astronomers used HST images to study the gravitational lensing effects of the clusters and from them infer the dark matter and ordinary matter distribution in the newly formed cluster. The data from the Chandra X-Ray Observatory assisted in mapping more accurately the ordinary matter distribution, making use of the x-ray emissions from this matter, comprised primarily of hot gas.
When the two clusters that formed MACSJ0025 collided, the hot gas in the clusters slowed down but the dark matter didn’t. This led to the separation of the two types of matter in the form of two peaks of dark matter surrounding an ordinary matter component. In addition, most of the ordinary mass is located in the galaxy regions, supporting the theory that dark matter particles interact with each other only gravitationally and do not collide with ordinary matter particles.
Better gravitational lensing and x-ray data can further our understanding of the nature of dark matter and improve the current limits on dark and ordinary matter interactions. The discovery of more mergers will also aid in this quest and provide additional corroborating evidence regarding cluster formation and dark matter properties.
TFOT reported on simulations carried out to better understand the role of dark matter in the evolvement of our universe. TFOT also follows the development of various instruments that among other missions will aid in the study of dark matter: NASA’s approval of the High-Resolution Soft X-Ray Spectrometer, an instrument devised to study the extreme environments of the universe that will help researchers explore dark matter on a large scale as well as the evolution of large galactic structures; the launch of NASA’s new observatory telescope, the Gamma-ray Large Area Space Telescope that will help improve our understanding in a variety of subjects, from our solar system to dark matter, and even the most fundamental laws of physics.
Further information on the cluster merger can be found in the UCSB press release and in the Arxiv website. The results of the new study will appear in an upcoming issue of The Astrophysical Journal.