Galaxy cluster are the titans of space, and when they collide extraordinary things can happen. A new study using NASA's Chandra X-ray Observatory examines the repercussions after two galaxy clusters clashed.
Galaxy clusters are the largest structures in the Universe held together by gravity, containing hundreds or even thousands of individual galaxies immersed in giant oceans of superheated gas. In galaxy clusters, the normal matter, like the atoms that make up the stars, planets, and everything on Earth, is primarily in the form of hot gas and stars. The mass of the hot gas between the galaxies is far greater than the mass of the stars in all of the galaxies. This normal matter is bound in the cluster by the gravity of an even greater mass of dark matter.
Because of the huge masses and speeds involved, collisions and mergers between galaxy clusters are among the most energetic events in the universe. The galaxy cluster Abell 1775 is located about 960 million light years from Earth, and a team of astronomers recently announced that they found a spiral-shaped pattern in Chandra's X-ray data of that system. These results imply a turbulent past for the cluster.
When two galaxy clusters of different sizes have a grazing collision, the smaller cluster will begin to plow through the larger one. (Because of its superior mass, the bigger cluster has the upper hand when it comes to gravitational pull.) As the smaller cluster moves through, its hot gas is stripped off due to friction. This leaves behind a wake, or tail, that trails behind the cluster. After the center of the smaller cluster passes by the center of the larger one, the gas in the tail starts to encounter less resistance and overshoots the center of its cluster. This can cause the tail to "slingshot" as it flies to the side, curving as it extends away from the cluster's center.
The newest Chandra data contains evidence — including the brightness of the X-rays and the temperatures they represent — for one of these curving "slingshot" tails. Previous studies of Abell 1775 with Chandra and other telescopes hinted, but did not confirm, that there was an ongoing collision in this system. There is also another competing idea that the Chandra data represent gas in the cluster that is being sloshed around, similar to how wine moves in a glass jerked sideways, after the collision.
Astronomers will likely need more observations and modeling of Abell 1775 to help decide between these two scenarios. Regardless, it will be an interesting system for Chandra and other telescopes to continue to study.
Revised: March 24, 2023