Astronomers have found an extraordinary galaxy cluster — one of the largest objects in the universe — that is breaking several important cosmic records. The discovery of this cluster, known as the Phoenix Cluster, made with the National Science Foundation's South Pole Telescope, may force astronomers to rethink how these colossal structures, and the galaxies that inhabit them, evolve.
Follow-up observations made in ultraviolet, optical and infrared wavelengths show that stars are forming in this object at the highest rate ever seen in the middle of a galaxy cluster. The object also is the most powerful producer of X-rays of any known cluster, and among the most massive of clusters. The data also suggest that the rate of hot gas cooling in the central regions of the cluster is the largest ever observed.
Officially known as SPT-CLJ2344-4243, this galaxy cluster has been dubbed the "Phoenix Cluster" because it is located in the constellation of the Phoenix, and because of its remarkable properties. Scientists at the University of Chicago's Kavli Institute for Cosmological Physics and their collaborators initially found the cluster, located about 5.7 billion light years from Earth, using the Sunyaev-Zel'dovich effect, the shadow that the cluster makes in fossil light leftover from the big bang.
Predicted in 1972, the effect was first demonstrated to find previously unknown clusters of galaxies by the South Pole Telescope collaboration in 2009. Observations of the effect have since opened a new window for astronomers to discover the most massive, distant clusters in the universe.
"The mythology of the Phoenix — a bird rising from the dead — is a perfect way to describe this revived object," said Michael McDonald, a Hubble Fellow at the Massachusetts Institute of Technology's Kavli Institute for Astrophysics and Space Research. McDonald is the lead author of a paper appearing in the Aug. 16 issue of the journal Nature, which presents these findings. "While galaxies at the center of most clusters have been dead for billions of years, the central galaxy in this cluster seems to have come back to life," McDonald said.
Like other galaxy clusters, Phoenix holds a vast reservoir of hot gas that contains more normal matter than all of the galaxies in the cluster combined. The reservoir of hot gas can be detected with X-ray telescopes like NASA's Chandra X-ray Observatory, and the shadow it makes in the light from the big bang can be detected with the South Pole Telescope. The prevailing wisdom had once been that this hot gas should cool over time and sink to the center of the cluster, forming huge numbers of stars.
However, most galaxy clusters have formed very few stars over the last few billion years. Astronomers think that the supermassive black hole in the central galaxy of clusters pumps energy into the system, preventing cooling of gas from causing a burst of star formation. 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.
With its black hole not producing powerful enough jets, the center of the Phoenix Cluster is buzzing with stars that are forming 20 times faster than in the Perseus Cluster. This rate is the highest seen in the center of a galaxy cluster and is comparable to the highest seen anywhere in the universe.
The frenetic pace of star birth and cooling of gas in Phoenix are causing both the galaxy and the black hole to add mass very quickly — an important phase that the researchers predict will be relatively short-lived.
"The galaxy and its black hole are undergoing unsustainable growth," said co-author Bradford Benson, a Kavli Institute Fellow at UChicago. "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 and so finding one with such extreme properties like the Phoenix Cluster is important.
"The beauty of the SZ effect for cosmology is that it is as easy to detect a cluster of galaxies in the distant reaches of the observable universe as it is for one nearby," said UChicago's John Carlstrom, the S. Chandrasekhar Distinguished Service Professor in Astronomy & Astrophysics. "The magnitude of the effect depends on the mass of the object and not its distance from Earth."
Galaxy clusters contain enough hot gas to create detectable "shadows" in the light left over from the big bang, which also is known as the cosmic microwave background radiation. This light has literally travelled for 14 billion years across the entire observable universe to get to Earth. If it passes through a massive cluster on its way, then a tiny fraction of the light gets scattered to higher energies — the Sunyaev-Zel'dovich effect.
The South Pole Telescope collaboration has now completed an SZ survey of a large region of the sky finding hundreds of distant, massive galaxy clusters. Further follow-up observations of the clusters at X-ray and other wavelengths may reveal the existence of additional Phoenix-like galaxy clusters.
Also contributing observations of the Phoenix Cluster were the Gemini Observatory and the Blanco 4-meter and Magellan telescopes, all in Chile, while several space-based telescopes were used to measure the cluster's star-formation rate.
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