Our Future: A simulation of what might happen when the Andromeda Galaxy hits ours shows tidal forces of gravity creating long plumes of material. The central regions will relatively quickly fall back together and merge into a single remnant galaxy.
When cars collide, it’s an accident. When galaxies collide, it’s Nature at work. Many astronomers believe such crashes are part of the natural evolution in the lives of galaxies and galaxy clusters, now scientists are blending scientific research and high-powered computer visual effects into vivid models of how they occur.
Astrophysicist Frank Summers, of the Space Telescope Science Institute (STScI), produced a simulated galaxy collision, using a combination of computer modeling research and the same special effects software used to make computer-generated movies.
The resulting animation shows a crash between two large spiral galaxies roughly the size of our own Milky Way and its larger neighbor Andromeda, themselves slated to collide in a few billion years.
"What makes me happy about this visualization is that it’s a presentation of accurate science to the public," Summers told. "It allows for a fluid process of education and the ability to create mental models of the universe."
Rip and tear
In his animation, Summers shows two galaxies at different planes of position, then documents their collision at a rate of about 10 million years per second. The entire sequence covers some 500 million years.
As the galaxies approach each other, they keep their spiral shapes up to the point of impact, where so-called "tidal forces" of gravity result in the formation of long plumes of stars, gas and dust called tidal tails. The centers of each galaxy then merge into one remnant core. The scenario is a likely preview for the expected interaction between the Milky Way and Andromeda galaxies, which some astronomers have already mapped out with computers models.
Summers used research data produced by astronomy professors and galactic modelers Chris Mihos, of Case Western Reserve University, and Lars Hernquist, of Harvard University. The researchers used a supercomputer to depict the collision in a project for the National Air and Space Museum's newly renovated Einstein Planetarium.
Dark matter
Astronomers who study galaxy structure spend most of their time modeling the effects of dark matter, that ubiquitous but unseen stuff that makes up most of a galaxy’s mass, Summers said. Only between 10 percent and 30 percent of a galaxy’s mass is visible, so astronomers study its rotation to determine the dark matter content, material that must be there based on known gravitational effects. They also look at how clusters of galaxies appear glued together by gravity.
By simulating collisions between galaxies, theorists can study the structures of galaxies and the architecture behind galactic clusters without having to wait the millennia it takes for such crashes to occur.
In the early days of the universe, the rate of collisions was about 10 to 100 times higher, simply because things were closer together. Although individual stars may not physically hit each other during a collision -- the space between them is still vast -- the gravitational effects of the encounter are enough to twist and distort galaxies beyond recognition.
Colliding spiral galaxies can become one elliptical galaxy, for example, which is the likely destiny for our Milky Way, astronomers say.
Galaxy building
"Mergers and interactions between galaxies are an essential part of their dynamical evolution," said John Dubinksi, an astronomy professor at University of Toronto who has modeled the eventual clash between the Milky Way and Andromeda galaxies. "The elliptical galaxies which represent around 10 percent of the galaxy population are most likely the product of a merger of two or more galaxies of nearly equal mass."
Galaxy collisions also contribute to star formation, as clouds of gas heat up and coalesce during the encounter. Observations from the Chandra X-ray Observatory suggest they may even contribute to the development of supermassive black holes.
In galaxy clusters, Dubinski said in an e-mail interview, elliptical galaxies outnumber spiral or irregular varieties. The clusters also often have a giant elliptical galaxy -- a product of many spiral galaxies merging together when the cluster first formed -- at their center.
Where we're headed
Most scientists agree that the Milky Way will cross paths with the Andromeda galaxy in about three billion years. Both galaxies are now spiral in shape, though Andromeda is about twice as large as the Milky Way.
The galaxies are separated by about 2.2 million light years (one light-year is about 6 trillion miles, or 10 trillion kilometers). That gap is closing at about 310,000 miles per hour (500,000 kph).
While a collision appears inevitable, astronomers admit that the sideways motion of Andromeda -- the galaxy’s speed perpendicular to its forward path toward the Milky Way -- could affect the encounter’s timing, but it has yet to be measured precisely. Dubinksi used an estimate of 12.4 miles per second (20 km per second) for his collision model.
"Even if the galaxies have a wider passage on the first pass, if they are on a bound orbit they are destined to merge eventually," Dubinski said. "If not on the first flyby, then within the second or third pass over the next 10 billion years, he added.
The clincher is gravity. Even if there’s enough space between the Milky Way and Andromeda to simply brush past each other at spiral arm’s length, their mutual gravity will ultimately win out, drawing the two galaxies together on successive flybys.Dubinski hopes to refine his model of the collision between the Milky Way and Andromeda galaxies in the future by modeling a system of about a trillion or so particles to match the number of stars in the two galaxies. But with the current growth in computer memory and speed, such computations won’t be possible for about 10 years, he said.
The Mice
Meanwhile, another pair of researchers has taken a pair of interacting galaxies called the Mice and worked backward to simulate what they figure has already taken place. The Mice, recently photographed by the Hubble Space Telescope's new camera, represent a collision in progress that could be very much like the pending crash of our own galaxy into Andromeda.
Joshua Barnes of the University of Hawaii worked with John Hibbard, now at the National Radio Astronomy Observatory, to animate a past that might have led to the present-day Mice.
Their computer animation shows two pinwheel galaxies falling together, swerving as they pass, and flinging out long tails of stars. At present the two galaxies have made one pass, and are coming back for a second and closer encounter. Eventually they will coalesce into a single galaxy, whose possibilities the simulation projects.
"Simulating colliding galaxies is a bit like investigating a car crash," Barnes says. "Suppose you had no witnesses, just a couple of wrecked cars. You might try different test crashes, varying things like speed and angle of impact, until you found a way to get the same damage as the original collision. That's basically what we did."
No comments:
Post a Comment