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Mapping the Universe in 3-D

Image by European Space Agency (ESA). Art by Karen Teramura, IfA.

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 IfA astronomer Harald Ebeling, 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.”

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 Keck I telescope of the W. M. Keck Observatory, the Chandra X-ray Observatory, and the Hubble Space Telescope.

Although galaxies contribute only a small fraction of the total mass of a galaxy cluster, they constitute crucial test particles that allow scientists 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 the measurement of the speed and location of galaxies along the line of sight. These measurements were performed using the Keck I telescope, the largest of its kind in the world.

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. The orbiting Chandra X-ray Observatory can measure the distribution and temperature of this hot gas. When clusters collide, scientists can detect crucial clues about the directional motion of the merging cluster, as well as to how close they are to each other in 3-D.

Gravitational lensing causes light rays originating from objects behind a cluster to be bent by the distribution of mass in the cluster “lens.” From the resulting distortions of background galaxies, astronomers can derive highly accurate models of the distribution of dark matter, matter whose presence cannot be seen but can be inferred by the effect it has on visible matter. The extraordinary resolution of the Hubble Space Telescope is critical to detecting the effects of gravitational lensing.

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. As IfA graduate student Li-Yen Hsu, the lead author of one of the three studies, put it, “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.