The Bullet Cluster is a much-studied pair of galaxy clusters, which have collided head on. One has passed through the other, like a bullet travelling through an apple. In the Bullet Cluster, this is happening across our line of sight, so we can clearly see the two clusters.

The optical image from the Magellan and the Hubble Space Telescope shows galaxies in orange and white in the background. Hot gas, which contains the bulk of the normal matter in the cluster, is shown by the Chandra X-ray image, which showst the hot intracluster gas (pink). Gravitational lensing, the distortion of background images by mass in the cluster, reveals the mass of the cluster is dominated by dark matter (blue), an exotic form of matter abundant in the Universe, with very different properties compared to normal matter.

This was the first clear separation seen between normal and dark matter.

Credits: X-ray: NASA/CXC/CfA/M.Markevitch, Optical and lensing map: NASA/STScI, Magellan/U.Arizona/D.Clowe, Lensing map: ESO WFI

The system Abell 576 actually consists of two galaxy clusters that have been caught by XMM while in the process of merging into one another.

The two giants are expected to eventually settle into a single, combined galaxy cluster. The discovery adds to existing evidence that galaxy clusters can collide faster than previously thought.

An incoming cluster has collided with the main cluster and the cores of the two clusters cores are deflected. The clusters have collided nearly head on, and one has passed through the other, like a bullet travelling through an apple. Abell 576 is seen head-on, so one cluster is now almost directly behind the other.

Credits: University of Michigan (R. Dupke)

From here on Earth, we are able to see the Bullet Cluster across our line of sight, and hence the process of merger can be seen clearly. In Abell 576, where two galaxy clusters are merging head on, in our line of sight, it is harder to disentangle.

During the collision the hot gas (shown in red) in each cluster is slowed and distorted by a drag force, similar to air resistance. A bullet-shaped cloud of gas forms in one of the clusters.

Credits: NASA/CXC/M.Weiss

Abell 576 would have looked similar if the observer were in the plane of the sky (marked with an eye). The rectangular slice corresponds to the area viewed during the analysis of Abell 576, projected onto the Bullet Cluster.

The whitish bands that cross in the upper left are an artefact of the detector and are not real.

Credits: University of Michigan (R. Dupke)

The system Abell 576 actually consists of two galaxy clusters that have been caught by XMM while in the process of merging into one another.

The two giants are expected to eventually settle into a single, combined galaxy cluster. The discovery adds to existing evidence that galaxy clusters can collide faster than previously thought.

An incoming cluster has collided with the main cluster and the cores of the two clusters cores are deflected. The collision induces bulk velocities in the hot intracluster gas, which is detected through Doppler shifts of the X-ray spectral lines by XMM.

Doppler shift is an effect induced in waves where they are ‘shifted’ towards the lower-energy end of the spectrum if the source is moving away from the observer. During the merger, the cores of the two clusters have overlapped and from Earth’s line of sight, it is not easy to disentangle them.

Credits: University of Michigan (R. Dupke)