Galaxy cluster ‘dark core’ not so dark after all

When NASA’s Hubble Space Telescope observed an overabundance of dark matter in the core of the merging dark galaxy cluster Abell 520 earlier this year, astronomers were surprised. However, new evidence suggests that the core is not over-dense in dark matter after all.

The initial observation that indicated an overabundance of dark matter in Abell 520’s core, which was imaged using the Wide Field and Planetary Camera 2 (WFPC2), was unexpected because dark matter and galaxy clusters should be anchored together, even during a collision. So another team of astronomers later repeated the observations of Abell 520, with a different, more sensitive Hubble camera.

Study leader Douglas Clowe, Associate Professor of Physics and Astronomy at Ohio University said: “The earliest result presented a mystery. In our observations we didn’t see anything surprising in the core. Our measurements are in complete agreement with how we would expect dark matter to behave.”

Clowe’s team used Hubble’s Advanced Camera for Surveys (ACS) to determine the amount of dark matter in the cluster. ACS observed the cluster in three colours which allowed the astronomers to distinguish the foreground and background galaxies from the galaxies involved in the cluster. From these images, the team were able to produce an extremely accurate map of the cluster’s dark matter.

“With the colours, we got a more precise selection of galaxies,” Clowe explained.

To estimate the amount of dark matter in the cluster, the team measured the amount of gravitational “shear” in the Hubble images. Shear is the warping and stretching of galaxies by the gravity of dark matter. If more warping is present, this indicates the presence of more gravity than can be deduced from the presence of luminous matter, therefore requiring the presence of dark matter to explain the observation. Clowe believes the previous observations could have introduced anomalous shear and led to an inaccurate measure of dark matter distributions.

With the new images, Clowe’s team measured less shear in the cluster’s core than was previously found. In the study, the ratio of dark matter to normal matter (stars, gases etc.) is 2.5 to 1, which is what the astronomers expected. This is compared to the earlier value of a 6 to 1 ratio, which challenged theories about how dark matter behaves.

“The result also shows that as you improve Hubble’s capabilities with newer camera, you can take a second look at an object,” Clowe added.

The study findings are published in The Astrophysical Journal.