Feature - Zooming in on galaxy formation

This simulated cube of space about 2 billion light years across represents gas density logarithmically, thus highlighting the variation. The filaments indicate “warm-hot intergalactic medium”, or WHIM. WHIM constitutes about half of the universe's non-dark matter, yet we cannot see it very well. It emits and absorbs largely in the UV and soft X-ray portion of the electromagnetic spectrum, much of which is blocked by the earth's atmosphere. Simulations help complement the tentative detections made by orbiting telescopes. The knot-like structures at the intersections indicate large groups and clusters of galaxies—important objects to study for understanding the fundamental properties of our universe such as the amount of matter, dark energy, and the expansion rate. The largest knot, near the center, is a galaxy cluster with a mass roughly 2,000,000,000,000,000 times that of our sun! Image courtesy of Matthew Hall, NCSA.

It is easy to get lost in the beauty and awe of the universe, in its amazing structure of stars and galaxies. But how did the universe evolve into this structure?

The very early universe consisted of homogeneous gas with tiny perturbations.  As the gas cooled over time, it collapsed under gravity into clumps and then galaxies. To better understand the process of galaxy cluster formation, researchers at the University of California, San Diego and colleagues from other institutions have modeled a region of the universe and simulated its collapse. 

“We use cosmology simulations to better understand the things we actually see with our telescopes and to reduce statistical errors on estimates of cosmological parameters,” says collaborating astrophysicist Brian O’Shea of Michigan State University. “Simulations are a necessary part of understanding what we see.”

Using the San Diego Supercomputer Center’s DataStar system and TeraGrid, the researchers ran the largest detailed simulation of a cosmological structure to date. In the simulation, the region of study collapses from about 2 billion light years across to form a region of galaxy clusters only 25,000 light years across. 
 

A sample zoomed-in image (not real data) showing how the Adaptive Mesh Refinement technique is used. The yellow/red areas with high subdivision indicate high density gas (galaxy clusters); the green areas would correspond to filaments (WHIM); the bluer areas represent lower density gas.  Image courtesy of Greg Bryan, Columbia University.

To provide a closer view as the collapse occurs, the simulation employs a zooming technique called adaptive mesh refinement. First, a broadly spaced geometric grid of cells (a “mesh”) is placed over the region of study. As the gas collapses, the program subdivides high-density regions on the mesh into smaller cells, doubling the spatial resolution of those areas.  As the gas continues to collapse and its density increases, the process of subdivision continues, allowing researchers to follow the collapse with high resolution.

The researchers zoomed in to areas of the original region seven consecutive times, enhancing the resolution by a factor of 128 and achieving unprecedented levels of detail. The simulation has more than 400,000 subgrids (magnified areas), with a total of about one billion individual grid cells.

“The distribution of galaxy clusters in the universe can actually tell you a lot about the universe itself,” O’Shea says. “We can use these populations of galaxy clusters to learn things about dark energy, how much matter there is in the universe, and how fast the universe is expanding.”

—Amelia Williamson, iSGTW