Dark Matter – One Mystery Solved (Update)

Oct 07, 2005
Collusion of two spiral gallaxies

Until recently cosmologists could not explain why elliptical galaxies did not appear to have dark matter haloes surrounding them, which is known to exist in other types of galaxies. Analyzing data from a simulation performed on one of the most advanced supercomputers in the world, an Israeli cosmologist showed that the existing model of elliptical galaxies was wrong, proving that dark matter was there all along.

Image: Collusion of two spiral gallaxies.

The research that started the debate was published in 2003 in the journal Science by Aaron J. Romanowsky and his colleagues. In that article Romanowsky claimed that he found a surprising fact: dark matter, which is thought to be surrounding galaxies across the universe, is absent from one type of galaxy known as an “elliptical" (or rounded) galaxy. Romanowsky's research puzzled physicists since, until that time, it was commonly thought that dark matter surrounds every galaxy in the universe and causes the further stars in each galaxy to move much faster then they would have normally.

Avishai Dekel, Professor of Physics at the Hebrew University in Jerusalem, decided to address this cosmological puzzle. Drawing on highly advanced computer simulations he was able to calculate that, contrary to the 2003 paper, the stars in the outskirts of elliptical galaxies do not move slower than those in spiral galaxies. This suggests the existence of dark matter haloes surrounding elliptical galaxies similar to those surrounding spiral galaxies.

The reason why Romanowsky and his team believed elliptical galaxies do not posses dark matter haloes was because of observations which showed a slow movement of stars at the outskirts of elliptical galaxies contrary to the fast moving stars at the outskirts of spiral galaxies. The velocity of distant stars is measured using an effect called red shift (the reddening of light from a star that is moving away from us). The limitation of this method is that it is only possible to measure the component of velocity which is along the observer's line of sight.

Super-computer simulation of two spiral galaxies colliding

Image right: Super-computer simulation of two spiral galaxies colliding (Credit: arXiv.org)

This limitation would not be a problem if the orbits of the observed stars in the elliptical galaxies were randomly oriented with respect to the line of sight, as it was commonly assumed. However, the analysis performed by Professor Dekel and his team showed that the stars in elliptical galaxies that are farthest from the center are likely to be moving in elongated, eccentric orbits in such a way that most of their motion is perpendicular to the line of sight. Therefore, they could be moving at high velocities without exhibiting much motion toward or away from the observers.

This phenomenon is the result of the way elliptical galaxies are formed as a violent merger of two spiral galaxies. ''In the merger process that produces these galaxies, a lot of the stars get flung out to fairly large distances, and they end up in highly elongated orbits that take them far away and then back, close to the center,'' explains Dekel.

Professor Dekel's research was based on simulations carried out by fellow researchers
at the University of California, Santa Cruz. The simulations were conducted using the third fastest supercomputer in the world – the NASA Columbia supercomputer – which became operational in 2004. The Colombia supercomputer is built with 10,240 Intel Itanium® 2 processors running at 1.5 gigahertz each with 20 terabytes of shared memory and 10 petabytes (10 million gigabytes) of shared storage space. This behemoth was used to calculate the violent collision and merger of the spiral galaxies that lead to the formation of the elliptical galaxies.

The History of dark matter theory

The existence of dark matter was first suggested in the early 1930's by the Swiss physicist Fritz Zwicky who calculated that the radial velocities of eight galaxies was 400 times greater than that expected by the shared gravity of luminous matter in those galaxies. The explanation given by Zwicky to his extraordinary find was to suggest the existence of what he called "dark matter", or matter which cannot be directly observed but can be inferred indirectly by its gravitational influence on visible matter. Analogously, imagine a caveman, who never saw a modern city, looking at New York at night. Naturally he will assume that New York is just a collection of light sources since all he can see is a variety of bright dots. Just like New York, space has much more then meets the eye.

Dark Matter candidates

Image above: Dark Matter candidates. Click here to enlarge

Since the 1930's more and more evidence accumulated in support of the dark matter theory. Since the 1970's several different explanations were suggested by physicists in order to explain the nature of dark matter. One suggestion was that dark matter is basically just a large amount of ordinary matter also called Baryonic matter which consists of protons and neutrons. Various forms of Baryonic matter were suggested in order to account for the dark matter in the universe: black holes, neutron stars, brown dwarfs and even planets were all possible candidates. But a more intriguing option for the missing matter in the universe was also suggest by physicists – Non-Baryonic matter. Two distinct forms of Non-Baryonic matter were proposed – hot Non-Baryonic matter and cold Non-Baryonic matter. Hot Non-Baryonic matter is made of particles moving very fast, such as neutrinos, cold Non-Baryonic matter is possibly composed of what is known as WIMPS or weakly interacting massive particles.

In 2001 NASA launched the Wilkinson Microwave Anisotropy Probe (WMAP). Apart from calculating the age and size of the universe more accurately than ever before, WMAP also helped scientists to understand the composition of the universe more clearly and its findings only served to aggravate the already problematic dark matter question. WMAP discovered (with about 5% margin of error) that the universe is composed of 4% ordinary matter, 23% of an unknown type of dark matter, and 73% of an even more mysterious dark energy.

by Iddo Genuth - IsraCast

The original story can be found here.

For any comments about this story, please contact the writer of the original article.

© 2005 IsraCast. Published with permission of Iddo Genuth

Explore further: Venus Express spacecraft, low on fuel, does delicate dance above doom below

add to favorites email to friend print save as pdf

Related Stories

Team creates Milky Way structure simulations

Nov 18, 2014

If you took a photograph of the Milky Way galaxy today from a distance, the photo would show a spiral galaxy with a bright, central bar (sometimes called a bulge) of dense star populations. The Sun—very ...

Studying the physics of galaxies

Nov 03, 2014

Assistant Professor of Astronomy Evan Kirby arrived on campus in August. Born and raised in New Orleans, Kirby earned his BS in 2004 from Stanford University; his undergraduate thesis involved trips to Pasadena ...

It's filamentary: How galaxies evolve in the cosmic web

Nov 20, 2014

How do galaxies like our Milky Way form, and just how do they evolve? Are galaxies affected by their surrounding environment? An international team of researchers, led by astronomers at the University of ...

Physicists suggest new way to detect dark matter

Nov 18, 2014

For years physicists have been looking for the universe's elusive dark matter, but so far no one has seen any trace of it. Maybe we are looking in the wrong place? Now physicists from University of Southern ...

Elusive dark matter may be detected with GPS satellites

Nov 17, 2014

The everyday use of a GPS device might be to find your way around town or even navigate a hiking trail, but for two physicists, the Global Positioning System might be a tool in directly detecting and measuring ...

Recommended for you

Orion on track at T MINUS 1 Week to first blastoff

6 hours ago

At T MINUS 1 Week on this Thanksgiving Holiday, all launch processing events remain on track for the first blast off of NASA's new Orion crew vehicle on Dec. 4, 2014 which marks the first step on the long ...

Staying warm: The hot gas in clusters of galaxies

8 hours ago

Most galaxies lie in clusters, groupings of a few to many thousands of galaxies. Our Milky Way galaxy itself is a member of the "Local Group," a band of about fifty galaxies whose other large member is the ...

Bad weather delays Japan asteroid probe lift off

12 hours ago

Bad weather will delay the launch of a Japanese space probe on a six-year mission to mine a distant asteroid, just weeks after a European spacecraft's historic landing on a comet captivated the world.

User comments : 0

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.