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In This Issue...
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Science
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A Closer Look at the Milky Way Halo
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A Closer Look at the Milky Way Halo
UCSC Computer Simulation Study Discovers New Details about Dark Matter Halo
By Judith Wellner
Some stars in the Milky Way are nearly as old as the universe, and it seems like scientists have been wondering about the heavens almost as long. However, as technology has exploded, so too has the knowledge about the galaxies, at least the parts that can be seen. Now, more effort is being given to understanding the parts of the Milky Way that are dark.
Since telescopes can only access the luminous components of the galaxy, scientists have been experimenting with simulations that mirror the structure and behavior of the more massive dark matter component.
A UCSC research team, led by Juerg Diemand, a postdoctoral fellow at UCSC and first author of the paper, has recently completed a study that takes us closer to a better understanding of the structure of the dark matter halo that envelops the Milky Way. The experiment used NASA’s most powerful supercomputer and created the largest ever simulation of the formation and evolution of the dark matter halo.
The team found that the structure of the halo is more complex than previous simulations have determined, especially in the inner halo where the Milky Way and our solar system are located. The study has been accepted for publication in the Astrophysical Journal.
Light and Darkness
The Milky Way is a spiral galaxy. The term “milky” originates from the faint white light which appears in a band across the sky where most of the stars in our galaxy lie.
The main disk of the galaxy is composed of 200 to 400 billion stars. While the diameter of the galaxy is about 80,000 to 100,000 light-years, the diameter of the dark halo extends out to about 2.5 million light-years.
Our own solar system is but a small part of the overall galaxy. One way to think about this is by perspective. If we could shrink the whole galactic disk to be 80 miles wide, our solar system would take up a mere 0.08 inches.
Dark Matter
Every galaxy is surrounded by a halo of dark matter. Nobody knows what this mysterious matter is, but according to scientists, it accounts for about 82 percent of the matter in the universe.
“It’s one of the biggest questions of science currently,” said Diemand. “We know that it’s present, and we know that it cannot be one of the currently known particles, like for example those in atoms. But we don’t know what it actually is.”
Many experiments in space and on Earth are currently trying to detect this mysterious dark matter.
Recent studies have shown that the halo is not homogenous, as was previously thought, but rather clumpy. The dark matter is concentrated in sections of the halo called subhalos by scientists. Diemand’s research shows the structure and abundance of these subhalos in more detail.
A Universe in a Cube
Diemand had done similar experiments before, but never on this scale.
“When I was doing my Ph.D. at the University of Zurich, Switzerland, I did a couple of simulations in this direction,” he said. “Previous results showed that it was very important how many particles we have in the simulations. Here at UCSC we had the chance to use a very fast computer. So we could do something similar to what I had done before, but on a larger scale.”
The real dark matter halo consists of a much larger number of microscopic particles. Diemand explained that it wasn’t easy to set up a computer simulation that accurately mirrors the structure and behavior of this matter.
The experiment attempts to recreate the early state of the universe and then simulate its development over time to explain how galaxies form with the dark halos around them.
“First, we need to set up the initial conditions,” he said. “We set it up so it reflects the same type of mass fluctuation as happens in reality.”
The experiment takes some guess work.
“Basically we have a cube that contains a large volume of the universe,” he continued. “Then we decide on how many particles we can put into this cube.”
Before this study, the biggest simulation used up to about 20 million particles per halo. Diemand and his team used ten times more â€" 200 million.
“The approximation is much better when we use more particles,” said Diemand. “The simulation code has to deal with all the gravitational interactions between these particles,” Diemand continued. “That is a lot of work, as in principle each particle attracts every other.”
NASA Computer Used
After the Big Bang, the effect of gravity enhanced small differences in the density of dark matter to form the first clumps of the stuff. Then normal matter, the kind that forms gases and stars, fell into the “gravitational wells” (areas of strong gravity) created by the dark matter, and the normal matter created galaxies in the centers of the dark matter halos.
Describing the earlier, smaller fluctuations does not require computer simulation.
“You can use linear methods for the earlier, smaller fluctuations,” said Diemand. “But later, in the expanding universe, fluctuation became bigger and bigger over time. It is much more complex, and we need to use simulations to model it.”
The study took a couple of months to complete, and it involved running 300 to 400 computer processors at a time for 320,000 CPU-hours (hours of time that a processor spends working). The team used the Columbia supercomputer at the NASA Ames Research Center, one of the fastest computers in the world.
The team included coauthor Michael Kuhlen, who is now at the Institute for Advanced Study in Princeton, and the other coauthor, Piero Madau, professor of astronomy and astrophysics at UCSC.
What Comes Next
The team is planning to run an even more extensive simulation.
“We would like to push it further, but it will take a lot more computer time,” said Diemand.
Another direction is to try to link the dark structures to the stellar structures that we can observe in the halo of the Milky Way.
“Some of the stars and globular clusters around the Milky Way are thought to be very old, almost as old as the universe,” said Diemand. “When people look at them, it’s like a fossil record from the time when the universe was very young. … It’s like doing archeology. You find the fossils and try to understand where they come from. For this, our simulation is quite useful. We basically use it as a time machine.”
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