A large international team of over 80 astronomers using NASA’s Chandra X-ray Observatory, the South Pole Telescope, and eight other world-class observatories, has discovered an extraordinary galaxy cluster – one of the largest objects in the Universe – that is breaking several cosmic records.
A composite image of the Phoenix Cluster (X-ray: NASA / CXC / MIT / M. McDonald, UV: NASA / JPL-Caltech / M. McDonald, optical: AURA / NOAO / CTIO / MIT / M. McDonald)
Officially known as SPT-CLJ 2344-4243, the cluster has been dubbed the ‘Phoenix Cluster’ because it is located in the constellation of the Phoenix and because of its remarkable properties.
The Phoenix cluster, lying about 5.7 billion light years away, is the most powerful producer of X-rays of any known cluster and among the most massive. Stars are forming in this object at the highest rate ever observed for the middle of a galaxy cluster. The data also suggest the rate of hot gas cooling in the central regions of the cluster is the largest ever observed.
“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,” said Dr Michael McDonald of the Massachusetts Institute of Technology and lead author of a paper published today in Nature (arXiv.org version).
Like 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. The prevailing wisdom once had been that this hot gas should 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 suggest that 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.
With its black hole not producing powerful enough jets, the center of the Phoenix cluster is buzzing with stars that are forming about 20 times faster than in the famous 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 causing the galaxy and the black hole to add mass very quickly – an important phase the researchers predict will be relatively short-lived.
“The galaxy and its black hole are undergoing unsustainable growth,” said co-author Dr 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,” said Dr Martin Rees of Cambridge University, who was not involved with the study. “The cooling of hot gas might be a much more important source of stars than previously thought.”
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Bibliographic information: McDonald et al. 2012. A massive, cooling-flow-induced starburst in the core of a luminous cluster of galaxies. Nature 488, 349–352; doi: 10.1038/nature11379
