An artist's impression of a galaxy at the center of the Phoenix galaxy cluster.
An artist's impression of a galaxy at the center of the Phoenix galaxy cluster.

UA Astronomers Help Identify Biggest, Brightest Galaxy Cluster

The discovery may confirm a long-held theory about the birth of stars at the core of galaxy clusters.
August 16, 2012
Extra Info: 

Watch how stars are born in the Phoenix galaxy cluster in this video animation on NASA's Chandra X-Ray Observatory website

University of Arizona astronomers have helped identify the brightest and most rapidly star-forming galaxy cluster to date, as part of a multi-institution team led by researchers at the Massachusetts Institute of Technology.

The cluster lies 7 billion light years away and dwarfs most known clusters, churning out a dazzling 740 new stars per year in its center. The Phoenix cluster, named after the constellation in which it resides, is among the most massive and most luminous in the universe. 

As vast as the Milky Way may seem, our sprawling galaxy is but a speck next to the largest structures in the universe: galaxy clusters – collections of hundreds to thousands of galaxies bound together by gravity. At the heart of most galaxy clusters sit massive old galaxies, within which only a few new stars are born each year. 
According to Dan Marrone, an assistant professor at the UA Steward Observatory who was part of the research team, thousands of galaxy clusters have been found throughout the universe. The Phoenix cluster measures roughly 10 million light years across, 100 times the diameter of the Milky Way galaxy and several thousand times more massive. 
“For more than 30 years, we have expected that the gas in some of these clusters should be cooling quickly and feeding the galaxy at the center, allowing for new stars to be born, but we haven’t seen this in any of the several thousand galaxy clusters we have observed,” Marrone said. “It is interesting to finally find one that actually matches our expectation.”
Michael McDonald, a Hubble Fellow in MIT’s Kavli Institute for Astrophysics and Space Research, said that aside from its mass and brightness, the Phoenix cluster bears another exceptional quality: While the cores of most galaxy clusters appear red, indicating that their stars are all very old, the galaxy in the core of the Phoenix cluster is bright blue – an indication that the surrounding gas is cooling at a rapid rate, generating ideal conditions for a high rate of star formation. 
“Central galaxies have typically been referred to as ‘red and dead’ – just a bunch of old stars orbiting a massive black hole, and there’s nothing new happening,” McDonald said. “But the central galaxy in this cluster has somehow come to life and is giving birth to prodigious numbers of new stars.”
Looking for a cool core
The new galaxy cluster may shed new light on a decades-old astrophysical conundrum termed the “cooling flow problem.” Gas at the core of a cluster, spewed from nearby galaxies and supernova explosions, should naturally cool over time, forming a flow cold enough to condense and form new stars. However, scientists have been unable to identify any galaxy cluster that does, in fact, cool at the rates predicted. 
One explanation, McDonald said, may be that a cluster’s natural cooling is somehow interrupted. He cites the Perseus cluster as an example: The black hole at the center of this cluster emits jets of particles that may act to reheat the core, preventing it from cooling completely. 
“What’s interesting about the Phoenix cluster is that we see almost all the cooling that was predicted,” McDonald said. “It could be that this is earlier in the evolution where there’s nothing stopping it, so it cools and becomes a starburst... in fact, there are few things forming stars in the universe faster than this galaxy.”
The Phoenix cluster was first detected in 2010 by researchers using the South Pole Telescope, or SPT, a 10-meter-wide telescope in Antarctica that scans huge patches of the sky for new galaxy clusters. McDonald and his colleagues recently used the space-based Chandra X-Ray Observatory to study the most massive clusters identified by the South Pole Telescope. Immediately, the Phoenix cluster stood out in the X-ray data as the brightest of the clusters – a finding that prompted McDonald to follow up with more observations of the cluster from more telescopes.
At the same time, a group around UA associate astronomer Eiichi Egami pointed the Herschel Space Observatory at the Phoenix cluster, but with a different research objective. 
“We were looking at these clusters of galaxies because we use them as powerful gravitational lenses that magnify much more distant galaxies behind the clusters and allow us to study galaxies far out in the universe," Egami said. “We have a large Herschel program here at the UA observing hundreds of these big clusters. When we realized we were looking at the same cluster, we provided some additional brightness measurements to the SPT team.”
Getting the complete view
The team ultimately acquired images of the Phoenix cluster from 10 different telescopes in space and on the ground around the world. Each telescope observed the cluster at different wavelengths, illuminating different features of it. 
“The central black hole is very bright in the X-ray, but the star formation is very bright in the optical and ultraviolet,” McDonald said. “So you need to work together with all these different telescopes to get a complete view.”
The team combined data from all 10 telescopes to determine the galaxy cluster’s mass and luminosity. To calculate the mass, the group first measured the cluster’s temperature, which was estimated by observing the cluster’s peak wavelength. McDonald explained that the wavelength at which an object peaks reveals information about its temperature – so the researchers identified the Phoenix cluster’s peak wavelength in the X-ray spectrum, then calculated its temperature. 
From the cluster’s temperature, the group calculated its mass: The hotter a ball of gas, the greater its overall mass. The researchers found the Phoenix cluster is easily among the most massive clusters in the universe. 
The group then looked for signs of star formation; new stars are particularly bright in the ultraviolet, and the researchers found that ultraviolet images taken of the cluster revealed hundreds of young stars in its core. The cluster’s extreme luminosity also indicated that it was cooling very rapidly, most likely providing the fuel for star formation. 
Brian McNamara, a professor of astrophysics at the University of Waterloo in Waterloo, Ontario, said the extreme starburst identified by the group might illustrate how the most massive primeval galaxies may have formed. He added that the Phoenix cluster’s exceptional behavior may result from a faulty mechanism at its core.
“It shows cooling and star formation during a phase when the supermassive black hole lurking in the galaxy's nucleus seems to be asleep at the switch,” McNamara said. “But once the black hole gets going and begins to push the hot atmosphere aside, perhaps in another 100 million years or so, it should shut down cooling and reduce the star formation rate in a feedback process that is active in most galaxy clusters.”
McDonald hopes to access the Hubble Space Telescope to continue studying this massive galaxy cluster. “You’d see these fantastic blue filaments where stars are forming out of cooling streams,” McDonald said. “It should look quite remarkable, instead of our ground-based images which show a blob of blue light.”
As for the cluster’s seemingly anomalous cooling, the scientists think that perhaps the phenomenon is not as exceptional as it appears.
Marrone said: “The fact that clusters like this one are so rare suggests that either it is not a common occurrence in the life of a galaxy cluster to go through an episode of intense star formation, or else such episodes have to be very short-lived.”
In addition to Egami and Marrone, the team of authors of the paper, which has been published in Nature, also includes UA astronomers Timothy Rawle and Marie Rex, research associates working in Egami's group.