Hubble astronomers have made a map of dark matter within the galaxy cluster Abell 1689 using the cosmic magnifying glass effect.
Abell 1689 is made of about 1,000 galaxies and lies 2.2 billion light-years away. For years, astronomers have been attempting to discover how the dark matter was distributed within the galaxies of the Abell system. They knew how much dark matter there was, but not where it was hiding.
Dark matter is a substance that cannot be seen. It does not emit its own radiation nor does it absorb or reflect radiation from outside sources. However, astronomers have inferred dark matter’s existence by observing its effects; for example, when scientists compare the actual mass of the galaxy to the mass of all the visible matter in the galaxy, they find a large discrepancy. Also, dark matter, while it does not interact with light, still exerts a gravitational force. As a result, scientists can observe gravitational aberrations caused by dark matter.
In the past, astronomers have tried to identify the distribution of dark matter in galaxy clusters through tricky processes such as gravitational lensing, which relies on dark matter’s ability to distort incoming light. As a result of the distortion, light emitting objects appear magnified.
“Other methods [were] based on making a series of guesses as to what the mass map is, and then astronomers find the one that best fits the data,” said astronomer Dan Coe of NASA’s Jet Propulsion Laboratory in a press release. Basically, astronomers could create maps with only guesses.
However, in 2002, when the Advanced Camera for Surveys (ACS) was installed into Hubble, astronomers were able to snap a wide-range and high resolution picture of Abell 1689. This image allowed them to assemble smaller images of individual galaxies into their correct positions, and therefore map the distribution of matter, both visible and dark, throughout the galaxy cluster.
“Here we have figured out, for the first time, a way to arrange the mass of Abell 1689 such that it lenses all of these background galaxies to their observed positions,” said Coe in his press release. What they found was that most of the cluster’s dark matter was gathered in the center, and packed together more tightly than they expected.
Dark energy is an even less understood force than dark matter and acts to counter gravitational pull: it pushes matter apart. Astronomers believe this energy is more abundant and stronger than gravity and is responsible for the accelerating expansion of the universe. Because of this energy, galaxy clusters should be less dense and more loosely packed than if there was no force to counter gravity at all.
The matter map of Abell 1689, however, showed a density that seemed to be unaffected by dark energy’s repelling force. “Galaxy clusters, therefore, would had to have started forming billions of years earlier in order to build up to the numbers we see today,” Coe said in his press release. “Abell 1689 appears to have been well fed at birth by the dense matter surrounding it in the early universe.”
Abell’s matter gluttony is not enough evidence for astronomers to conclude that galaxy clusters were formed earlier on in the history of the universe than previously expected. But they hope to find out with Hubble’s new project CLASH (Cluster Lensing and Supernova survey with Hubble), which will snap more high-resolution pictures of 25 different galaxy clusters over the next three years.
