Colliding galaxy clusters reveal dark matter
by W. Maria Wang
The line between reality and science fiction is becoming increasingly blurred in the seemingly vast emptiness of space, where a mysterious force prevents galaxies from flying apart. Sound like a Twilight Zone episode? According to Einstein's theory of gravity, most galaxies are missing visible mass required to account for their rotation speeds. Consequently, since 1933 astronomers have postulated a new form of matter -- dark matter -- to account for the gravitational potential holding these galaxies together. Proof for the existence of dark matter has recently been obtained by Marusa Bradac, Ph.D., of the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) at Stanford and her collaborators around the world.
The Hunt for Invisible Matter
Dark matter has been notoriously hard to detect; it doesn't emit or reflect radiation such as light or heat, and only interacts gravitationally with itself and normal matter. The composition of dark matter is also unknown. However, Bradac notes that we can study certain properties of dark matter. "We know [dark matter] doesn't scatter off each other and only interacts through gravity," says Bradac. "We also know how it forms structures, so when we run simulations with dark matter particles, it has to map out the galaxies the same way we observe them." These detailed maps of the galaxies are used to measure the mass distribution of dark matter inside them.
Just how much dark matter is there in the universe? The general consensus among astronomers is that 25% of the universe is composed of dark matter. "Whereas dark matter is all around us, it's a lot more smoothly distributed. Its density is not large enough on Earth for us to feel its gravity," says Bradac. "The concentration of regular matter, like the earth, is very high; that's why we experience its gravity directly."
Tracing Shadows in the Sky
Dark matter was finally caught in action through observations of a rare collision of two galaxy clusters three billion light years away. Bradac and her team were able to detect the dark matter through a phenomenon called gravitational lensing. "Gravitational lensing works in a similar way to how a regular magnifying glass works," Bradac explains. "When you have a background source emitting light and then you put a magnifying glass in between [the observer and the source].... you get a different, enlarged image. Except in this case it's not [made of] a glass type-material, but a big lump of dark and luminous matter. Due to its gravity, light will bend. That's why we see distortions and multiple images of the same background source." These characteristic increases in brightness from background galaxies allowed the team to map out the mass in the colliding clusters.
Using the Hubble Space Telescope, the Magellan Telescopes and the Very Large Telescope located in Chile, as well as NASA's Chandra X-ray Observatory, the team discovered that as theory predicted, there were two isolated regions of dark matter passing through the collision unaffected, and two smaller clumps of hot gas lagging behind. Bradac explains that in the clusters, "There are a couple thousand galaxies that are so far apart that they don't see each other, so the chance that two galaxies collide is very small. They basically just go through in such a collision. However, gas behaves like a fluid, so the gas did interact and we got ram pressure stripping."
Ram pressure stripping allowed the separation of dark matter on either side of the collision from the gas in the middle. Bradac describes this effect: "Imagine two gas clouds smashed together; they would heat up and create turbulence. That's why [the gas] slows down and stays in between." Previous observations of individual clusters with gas and dark matter yielded measurements of their combined effects, which made it difficult to say how much mass belonged to each component -- gravitational lensing only allows for determination of the total mass. But now that the gas and dark matter were separated, Bradac and her team could clearly see the mass that belonged to dark matter and to gas. These breakthroughs will be published in upcoming issues of the Astrophysical Journal and the Astrophysical Journal Letters.
Alternate Realities
Some scientists, however, believe that there is no dark matter, and that we need to modify our theories on gravity instead. "For this system in particular," Bradac says, "there was no way to modify the laws of gravity in order to explain the observations, although people are now trying hard to come up with alternative explanations. They might come up with modified theories [to] explain the system without dark matter. That's why it's important to continue finding new systems."
Beyond the Stars
Measuring the dark matter distribution in as many different systems as possible -- from clusters to individual galaxies -- and then comparing these measurements with simulations will help piece together the cosmological model: how the universe started and where it is going. Multiple systems are required for elucidating the behavior of dark matter and its profile. "We just got new data for the same cluster. Now we are trying to study the properties of dark matter in a bit more detail," Bradac says.
"Because these clusters act as lenses, they're magnifying the background sources. You can study sources further away than you would normally." In a sense, these massive clusters act as "gravitational telescopes." Researchers are hoping to use these clusters to see one of the first galaxies formed.
The Search Continues
Perhaps one of the most intriguing questions yet to be answered is: What is dark matter made of? Particle astrophysicists may soon be able to directly detect dark matter using the Large Hadron Collider (LHC) at CERN, the world's largest particle physics laboratory. "We don't know what the mass of dark matter is," states Bradac. "There are many different models, and depending on which is right, we might or might not detect it in 2007 [using the LHC, a particle accelerator]. It might be out of the mass range of detector sensitivity."
When asked about the arcane nature of her work, Bradac responded, "I had undergraduates working for me during the summer, and they were doing exactly the same type of research I was doing. Because it is such a collaborative effort, if students are interested they can join. It was so many different clues and sources of data that came together in order to create this story."
Factboxes:
Box #1: Most of the universe is dark. Only about 5% of it is ordinary matter that can be "seen"; the rest is 25% dark matter and 70% dark energy. Cosmologists have postulated the existence of dark energy -- a form of energy that exerts negative pressure -- to explain the expansion of the universe at an accelerating rate. However, even less is known of dark energy than of dark matter, which leads to the conclusion that we are still in the dark on 95% of the universe!
Box #2: The most likely candidate for dark matter is the hypothetical Weakly Interacting Massive Particle (WIMP). WIMPS are thought to be smaller than atoms and only interact with matter gravitationally. Other substances termed MAssive Compact Halo Objects (MACHOs) have also been considered to comprise dark matter. MACHOs include brown dwarfs, an intermediate between stars and planets, that are not luminous enough to be directly detected by telescopes. However, Bradac says, "If indeed dark matter is in the form of [MACHOs], it would cause an observable effect on the so-called micro-lensing event. When we look at stars in neighboring galaxies, we would have to see the occasional amplification due to these lumps of material, but not enough of these signatures have been seen in order to explain dark matter as being completely made of this normal matter."