A team of astronomers has discovered a galaxy cluster, and its heart is teeming with stellar activity. This astonished the researchers since the core of a galaxy cluster is supposed to be a quiet place, certainly not the main hub of star birth. But it seems that this unexpected transformation is fueled with stolen gas from other galaxies.

The galaxy cluster has the memorable name SpARCS1049+56. Although, since stars are being rapidly born, maybe "SpARCS" (sparks) is quite an apt name. SpARCS was discovered by the Spitzer Space Telescope and the Canada-France-Hawaii Telescope. These telescopes observe infrared light, which means that they can see regions of star formation. However, their resolution is too poor to pinpoint from where exactly the activity originates, so researchers took the above photo using the Hubble Space Telescope. In the photo, there are bright, central galaxy clusters that are undergoing extreme star birth surrounded by orbiting galaxies. 

This rapid growth is extremely unexpected because the center of a galaxy cluster is usually old and inactive. It seems that this cluttered region of dead stars can reanimate itself with a little bit of help. This cluster is the first ever to show that their cores can grow rapidly using gas stolen from other galaxies. 

“We think the giant galaxy at the center of this cluster is furiously making new stars after merging with a smaller galaxy,” commented Tracy Webb, lead author of the paper from McGill University. The results can be seen in the Astrophysical Journal.

This would explain the pace at which the core is forming new galaxies: 800 times faster than the Milky Way, which produces about one star per year. 

A galaxy cluster is a group of hundreds to thousands of galaxies all bound together with the force of gravity – much like how the planets of the Solar System orbit the Sun, but on a much bigger scale. “The galaxies at the centers of clusters, called Brightest Cluster Galaxies, are the most massive galaxies in the Universe," said Webb. "How they become so huge is not well understood.”

Some of the features in the image reveal the reason why the galaxy started bubbling away with stellar activity. “Hubble found a trainwreck of a merger at the center of this cluster. We detected features that looked like beads on a string,” said Adam Muzzin, a co-author on the paper from the University of Cambridge. 

The "beads on a string" that Muzzin refers to describes the galaxies as the beads and the trail of hydrogen gas as the string. This is a telltale sign that the cluster has recently undergone something called a wet merger. This is simply where the gas from two galaxies rich in gas collide. (A dry merger is a galaxy collision where there is a lack of gas). The gas is given energy from the friction generated in the collision, and lots of new stars are born. 

Wet mergers have been seen in the past. However, this is the first example of a wet merger in the center of a galaxy cluster that has then spat out a dazzle of stars. The next step the researchers want to explore is to find out how common this phenomenon is in the universe. 

^{The center of the galaxy cluster has undergone a wet merger. There is a tidal tail (outlined) that is produced from this interaction. NASA/STScI/ESA/JPL-Caltech/McGill.}