Hubble Sets Dark Matter Record Straight
November 29, 2012

New Hubble Observation Sets Dark Matter Record Straight For Abell 520

Lee Rannals for — Your Universe Online

A new observation of Abell 520 using the Hubble Space Telescope has helped astronomers determine that the core of the galaxy cluster is not over-dense in dark matter after all.

Astronomers earlier this year spotted an overabundance of dark matter in the heart of the galaxy cluster. This observation was puzzling because dark matter and galaxies should be anchored together.

Scientists have evidence that dark matter is responsible for 90 percent of the gravity within galaxies and clusters of galaxies. Because it is detected through its gravity and not its light, they refer to it as "dark matter."

Astronomers reported about their latest in The Astrophysical Journal and said that the core is not over-dense in dark matter.

“The earlier result presented a mystery. In our observations we didn´t see anything surprising in the core,” study leader Douglas Clowe, an associate professor of physics and astronomy at Ohio University, said in a statement. “Our measurements are in complete agreement with how we would expect dark matter to behave.”

The announcement from earlier this year was from astronomers using Hubble's Wide Field Planetary Camera 2. It suggested that a clump of dark matter was left behind during a clash between massive galaxies in Abell 520. The dark matter collected into a "dark core" contained fewer galaxies than what would be expected if the dark and luminous matter were connected.

Because dark matter is not visible, it is found indirectly through its gravitational effects. The gravity from both dark and luminous matter warps space, bending and distorting light from galaxies and clusters behind it.

Astronomers use this effect, known as gravitational lensing, to infer the presence of dark matter in massive galaxy clusters. The teams from both studies this year used this technique to map out the dark matter in the cluster.

Clowe's team used Hubble's Advanced Camera for Surveys (ACS) to measure the amount of dark matter in the cluster. ACS observed the cluster in three colors, helping astronomers point out foreground and background galaxies from the galaxies in the cluster.

Astronomers estimated the amount of dark matter in the cluster by measuring the amount of gravitational "shear" in the Hubble images. Shear is the warping and stretching of galaxies by the gravity of dark matter.

“The WFPC2 observation could have introduced anomalous shear and not a measure of the dark matter distribution,” Clowe said in the statement.

By using a different Hubble camera, the team was able to measure less shear in the cluster's core than previously. In the study, the ratio of dark matter to normal matter is 2.5 to 1, which is what astronomers expected. The earlier observations showed a 6-to-1 ratio of dark matter to normal matter.

“This result also shows that as you improve Hubble´s capabilities with newer cameras, you can take a second look at an object,” Clowe said.