Massive Phoenix cluster forms stars at a record pace

Posted by TG Daily Staff



Astronomers have identified an "extraordinary" galaxy cluster which scientists describe as one of largest objects in the universe.



Interestingly, stars are forming in the Phoenix cluster at the highest rate ever observed for the middle of a galaxy cluster.

Phoenix also seems to be one of the most powerful producer of X-rays - of any known cluster - and among the most massive. Plus, data indicates that the rate of hot gas cooling in the central regions of the cluster is the largest ever observed.

The Phoenix cluster is located approximately 5.7 billion light years from Earth. It is named not only for the constellation in which it is located, but also for its unique properties.

"While galaxies at the center of most clusters may have been dormant for billions of years, the central galaxy in this cluster seems to have come back to life with a new burst of star formation," explained Michael McDonald of the Massachusetts Institute of Technology (MIT). 

"The mythology of the Phoenix, a bird rising from the dead, is a great way to describe this revived object."

Similar to other galaxy clusters, Phoenix contains a vast reservoir of hot gas, which itself holds more normal matter - not dark matter - than all of the galaxies in the cluster combined. 

It was once hypothesized that the hot gas would cool over time and sink to the galaxy at the center of the cluster, forming huge numbers of stars.

However, most galaxy clusters have formed very few stars during the last few billion years. 

Astronomers now believe the supermassive black hole in the central galaxy of a cluster pumps energy into the system, preventing cooling of gas from causing a burst of star formation.


According to Ryan Foley of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass, the famous Perseus cluster is an example of a black hole bellowing out energy and preventing the gas from cooling to form stars at a high rate. 

Repeated outbursts in the form of powerful jets from the black hole in the center of Perseus created giant cavities and produced sound waves with an incredibly deep B-flat note 57 octaves below middle C, which, in turn, keeps the gas hot.

"We thought that these very deep sounds might be found in galaxy clusters everywhere," said Foley."The Phoenix cluster is showing us this is not the case - or at least there are times the music essentially stops. Jets from the giant black hole at the center of a cluster are apparently not powerful enough to prevent the cluster gas from cooling."

With its black hole not producing powerful enough jets, the center of the Phoenix cluster is apparently buzzing with stars that are forming about 20 times faster than in the Perseus cluster. This rate is the highest seen in the center of a galaxy cluster but not the highest seen anywhere in the universe. However, other areas with the highest rates of star formation, located outside clusters, have rates only about twice as high.

The frenetic pace of star birth and cooling of gas in the Phoenix cluster are likely causing the galaxy and the black hole to add mass very quickly - an important phase the researchers estimate will be relatively short-lived.

"The galaxy and its black hole are undergoing unsustainable growth," noted  Bradford Benson of the University of Chicago. "This growth spurt can't last longer than about a hundred million years. Otherwise, the galaxy and black hole would become much bigger than their counterparts in the nearby universe."

Remarkably, the Phoenix cluster and its central galaxy and supermassive black hole are already among the most massive known objects of their type. Because of their tremendous size, galaxy clusters are crucial objects for studying cosmology and galaxy evolution, so finding one with such extreme properties like the Phoenix cluster is important.

"This spectacular star burst is a very significant discovery because it suggests we have to rethink how the massive galaxies in the centers of clusters grow," added Martin Rees of Cambridge University. 

"The cooling of hot gas might be a much more important source of stars than previously thought."