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October 16, 2012
Dr. Harald Ebeling
Ms. Louise Good
Combining observations from Mauna Kea with data taken by telescopes in space, astronomers at the Institute for Astronomy (University of Hawaii at Manoa) and their collaborators have developed a technique that allows them to map collisions of giant galaxy clusters in three dimensions.
Astronomers studying the solar system are fortunate. Their targets move, rotate, obscure, and deflect each other on timescales of hours, months, or years, allowing researchers to see them from different angles. Scientists exploring the distant universe are at a disadvantage in this regard. Most of their targets, such as black holes, galaxies, or clusters of galaxies, are so huge that it takes tens or hundreds of millions of years for an object to present us with a noticeably changed view.
“Being unable to see these large-scale structures from different angles makes it very difficult to figure out their three-dimensional shapes, let alone their relative motions and interactions,” explains Harald Ebeling, IfA astronomer and an expert on galaxy clusters. “All we see in our images is a 2-D projection of a 3-D structure onto the plane of the sky.”
Image by ESA. Art by Karen Teramura, UH Institute for Astronomy.
Luckily, when two galaxy clusters collide, astronomers can make use of a clever combination of observations to make the invisible visible. In three recent studies, Ebeling and an international team of collaborators created 3-D models of merging galaxy clusters. Creating these models requires mapping all the components of a cluster: the galaxies that we see in visible light, the hot gas permeating the cluster that emits X-rays, and the invisible dark matter that can be detected only because its gravity distorts the images of objects behind the cluster. To collect all these data, Ebeling’s team used three world-class observatories: the Mauna Kea Observatories (specifically, the Keck I telescope of the W. M. Keck Observatory and the Canada-France-Hawaii Telescope), the Chandra X-ray Observatory, and the Hubble Space Telescope.
Combining the data to create a credible 3-D model of a complicated system like a merging cluster still involves a lot of physical interpretation. Admits Li-Yen Hsu, IfA graduate student and lead author of one of the three studies, “Our understanding of the shape and motion of the cluster kept evolving as we added more and more observational evidence. It’s a little like solving a jigsaw puzzle with half of the pieces missing.”
Eventually, enough pieces of the puzzle were collected to unravel, for instance, the geometry of MACSJ0717.5+3745, a giant triple merger of clusters fed by a filament of dark matter that extends 60 million light-years into space. The team was also able to measure the mass of the entire structure and found that filaments may contain more than half of the mass of the entire universe. Two other cluster mergers, examined in studies led by Hsu and fellow IfA graduate student I-Ting Ho, turned out to proceed along trajectories that are much more complex than suggested by the systems’ appearance in projection on the sky. By revealing these objects’ 3-D geometry, scientists can now correct for projection effects and determine the true properties of merging clusters.
The results of these 3-D reconstructions of some of the most massive structures in the universe will appear in three articles to be published by the Monthly Notices of the Royal Astronomical Society.
Dark matter: A subtle effect called gravitational lensing, predicted by Einstein’s theory of General Relativity, causes light rays originating from objects behind the cluster to be bent by the distribution of mass in the cluster “lens.” From the resulting distortions in the images of background galaxies, astronomers can derive highly accurate models of the dark matter distribution, but only in projection, not in 3-D. The extraordinary resolution of the Hubble Space Telescope is critical to detecting the effects of gravitational lensing.
X-ray gas: Gas that is present at very low density throughout the entire universe becomes concentrated in galaxy clusters where it is heated to temperatures of millions of degrees, enough to cause it to emit X-rays. Observations at high spatial resolution are made possible by the Chandra X-ray Observatory, an orbiting X-ray telescope that can measure the distribution and temperature of this hot gas. During cluster collisions, shock fronts are created. Their strength and orientation yield crucial clues as to the directional motion of the merging clusters, as well as to how close they are to each other in 3-D.
Galaxies: Although galaxies contribute only a small fraction of the total mass of a galaxy cluster, they represent crucial test particles that allow us to measure the motion of clusters along our line of sight. The final, critical ingredient that permits the reconstruction of the three-dimensional geometry of these cosmic collisions is thus the measurement of the speed and location of galaxies along the line of sight, or in more physical terms, the measurement of their radial velocities. These measurements were performed using the Keck I telescope, the largest of its kind in the world.
The three articles:
I-T. Ho, H. Ebeling, J. Richard, An X-ray/optical study of the geometry and dynamics of MACS J0140.0-0555, a massive post-collision cluster merger arXiv:1207.6235
M. Jauzac, E. Jullo, J.-P. Kneib, H. Ebeling et al., A Weak-Lensing Mass Reconstruction of the Large-Scale Filament Feeding the Massive Galaxy Cluster MACSJ0717.5+3745 arXiv:1208.4323
L.-Y. Hsu, H. Ebeling, J. Richard, The three-dimensional geometry and merger history of the massive galaxy cluster MACS J0358.8-2955 arXiv:1209.2492
ESA press release: Dark Matter Filament Studied in 3D for the First Time
Founded in 1967, the Institute for Astronomy at the University of Hawaii at Manoa conducts research into galaxies, cosmology, stars, planets, and the sun. Its faculty and staff are also involved in astronomy education, deep space missions, and in the development and management of the observatories on Haleakala and Mauna Kea. The Institute operates facilities on the islands of Oahu, Maui, and Hawaii.