Chandra X-ray Image of Abell 521
This image shows the galaxy cluster Abell 521, located about 2.9 billion light years from Earth. Chandra X-ray data show hot gas in the cluster. The elongated shape of the X-ray emission suggests that the cluster has undergone a violent collision with another group or cluster of galaxies.
(Credit: NASA/CXC/INAF/G.Brunetti et al.)

GMRT Radio Image of Abell 521
This image of Abell 521 is from the Giant Meterwave Radio Telescope in India. The radio emission in the center of the cluster is bright at the relatively long wavelength of 125 cm (as shown), but is much fainter at shorter wavelengths. This variation in behavior agrees with theoretical predictions for acceleration of particles by turbulent waves generated by a violent collision. More observations with radio telescopes at these long wavelengths may result in the detection of hundreds or even thousands of colliding galaxy clusters.
(Credit: NRAO/NSF/INAF/G.Brunetti et al.)