Chandra grabs a slingshot in a crash

When the titans of space – the clusters of galaxies – collide, extraordinary things can happen. A new study using NASA’s Chandra X-ray Observatory examines the aftermath of two clusters of galaxies colliding.

Clusters of galaxies are the largest structures in the Universe held together by gravity, containing hundreds or even thousands of individual galaxies submerged in giant oceans of superheated gas. In galaxy clusters, normal matter, like the atoms that make up stars, planets, and everything on Earth, mostly comes in the form of hot gas and stars. The mass of hot gas between galaxies is much greater than the mass of stars in all galaxies. This normal matter is linked in the cluster by gravity to an even larger mass of dark matter.

Due to the enormous masses and speeds involved, collisions and mergers between clusters of galaxies are among the most energetic events in the Universe.

In a new study of the Abell 1775 galaxy cluster, located about 960 million light years from Earth, a team of astronomers led by Andrea Botteon from the University of Leiden in the Netherlands announced that they had found a spiral-shaped pattern in Chandra’s x-ray data. These results imply a turbulent past for the cluster.

When two clusters of galaxies of different sizes have a grazing collision, the smaller cluster will begin to cross the larger one. (Due to its greater mass, the larger cluster has the upper hand when it comes to gravitational pull.) As the smaller cluster moves, its hot gas is removed due to friction. This leaves behind a wake, or tail, which trails behind the cluster. Once the center of the smaller cluster passes through the center of the larger one, the gas in the tail begins to meet less resistance and passes the center of its cluster. This can cause the tail to “shoot a slingshot” as it flies sideways, curving away from the center of the cluster.

Chandra’s most recent data contains evidence – including the brightness of the x-rays and the temperatures they represent – for one of these curved “slingshot” tails. Previous studies by Abell 1775 with Chandra and other telescopes suggested, but did not confirm, that there was an ongoing collision in this system.

A new image of Abell 1775 contains x-rays from Chandra (blue), optical data from the Pan-STARRS telescope in Hawaii (blue, yellow and white) and radio data from the LOw Frequency ARray (LOFAR) in the Netherlands ( red). The tail is labeled in this image with a region of gas with a curved edge, called a “cold front,” which is denser and cooler than the gas it sinks into. The tail and cold front all curve in the same direction, creating a spiral appearance. A separate labeled image shows the field of view of the Chandra data.

Astronomers previously discovered that Abell 1775 contained a huge jet and a radio source, which is also visible in this new composite image. This jet is fed by a supermassive black hole in a large elliptical galaxy at the center of the cluster. New data from LOFAR and the Giant Metrewave Radio Telescope (GMRT) in India reveals that the radio jet is actually 2.6 million light years away. It’s about twice as long as astronomers previously thought and makes it one of the longest ever seen in a cluster of galaxies. The structure of the jet changes abruptly as it passes through the lower density gas in the upper part of the image, through the edge of the cold front, implying that the collision affected it.

According to the new study, gas movements inside the cluster could be responsible for other structures detected by observing Abell 1775 in radio waves, such as two filaments located near the origin of the jet (one from them is labeled). The LOFAR and Chandra data also allowed the researchers to study in detail the phenomena that contribute to the acceleration of electrons both in the jet of this galaxy and in the radio emission near the center of the largest cluster.

There is another explanation for the appearance of the cluster. As a small cluster approaches a larger one, the dense hot gas from the larger cluster will be attracted by gravity. Once the smaller group has passed the center of the other group, the direction of movement of the group gas is reversed and it returns to the center of the group. The cluster gas again travels through the center and “laps” back and forth, similar to the lapping of wine into a glass that has been shaken sideways. The lapping of gas ends in a spiral because the collision between the two clusters was off-center.

Team Botteon favors the slingshot tail scenario, but a separate group of astronomers led by Dan Hu from Shanghai Jiao Tong University in China favors the sloshing explanation based on Chandra and XMM data. Newton from ESA. The slingshot and sloshing scenarios involve a collision between two clusters of galaxies. Eventually, the two clusters will completely merge with each other to form a single, larger galaxy cluster.

Further observations and modeling of Abell 1775 are needed to help decide between these two scenarios.

An article describing the results of Botteon’s team has been published in the journal Astronomy and astrophysics and is available online. The separate work on the “sloshing” theory led by Dan Hu has been accepted for publication in The Astrophysics Journal and is also available online.