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Most Massive And Luminous Galaxy Cluster Wows Astronomers: Detection May Confirm Long-Held Theory
Now a multi-institution team led by MIT researchers has identified a galaxy cluster seven billion light-years away that dwarfs most known clusters, churning out a dazzling 740 new stars per year in the central galaxy. The galaxy cluster is among the most massive and most luminous in the universe. While scientists have formally catalogued the cluster by the name SPT-CLJ2344-4243, the MIT-led group has a more informal moniker: the Phoenix cluster, named after the constellation in which it resides.
An artist’s impression of a galaxy at the center of the Phoenix Cluster.
Image: NASA/CXC/M. Weiss
Michael McDonald, a Hubble Fellow in MIT’s Kavli Institute for Astrophysics and Space Research, says 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 massive starburst.
“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 says. “But the central galaxy in this cluster has somehow come to life, and is giving birth to prodigious numbers of new stars.”
McDonald and his colleagues publish their findings this week in Nature.
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 says, 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.
The image on the left shows the newly discovered Phoenix Cluster, located about 5.7 billion light years from Earth. This composite includes an X-ray image from NASA’s Chandra X-ray Observatory in purple, an optical image from the 4m Blanco telescope in red, green and blue, and an ultraviolet (UV) image from NASA’s Galaxy Evolution Explorer (GALEX) in blue. The Chandra data reveal hot gas in the cluster and the optical and UV images show galaxies in the cluster and in nearby parts of the sky.
“What’s interesting about the Phoenix cluster is that we see almost all the cooling that was predicted,” McDonald says. “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.”
Getting the complete view
The Phoenix cluster was first detected in 2010 by researchers using the South Pole Telescope, 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.
Image: UV — NASA/JPL-Caltech/M.McDonald; Optical — AURA/NOAO/CTIO/MIT/M.McDonald; Microwave — NSF/SPT
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 says. “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 explains 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, says the extreme starburst identified by the group may illustrate how the most massive primeval galaxies may have formed. He adds 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 says. “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 says. “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, McDonald guesses that perhaps the phenomenon is not as exceptional as it appears.
“It could be a timing thing, where 1 percent of the time you get this vigorous star formation and runaway cooling,” McDonald says. “It might be that every cluster we see goes through this phase, but it’s so short-lived that this is the only one we’ve found. And we were in the right place at the right time.”
“Phoenix Cluster”
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, as explained here and in our press release. Stars are forming in the Phoenix Cluster at the highest rate ever observed for the middle of a galaxy cluster. The object is also 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.
Like other galaxy clusters, Phoenix contains a vast reservoir of hot gas — containing more normal matter than all of the galaxies in the cluster combined — that can only be detected with X-ray telescopes like Chandra. This hot gas is giving off copious amounts of X-rays and cooling quickly over time, especially near the center of the cluster, causing gas to flow inwards and form huge numbers of stars.
These features of the central galaxy are shown in the artist’s illustration, with hot gas in red, cooler gas as blue, the gas flows shown by the ribbon-like features and the newly formed stars in blue. An animation [link to animation] shows the process of cooling and star formation in action. A close-up of the middle of the optical and UV image [link to optical/UV close-up] shows that the central galaxy has much bluer colors than the nearby galaxies in the cluster, showing the presence of large numbers of hot, massive stars forming.
These results are striking because 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. 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 from the black hole in the center of Perseus, in the form of powerful jets, created giant cavities and produced sound waves with an incredibly deep B-flat note 57 octaves below middle C. Shock waves, akin to sonic booms in Earth’s atmosphere, and the very deep sound waves release energy into the gas in Perseus, preventing most of it from cooling.
In the case of Phoenix, jets from the giant black hole in its central galaxy are not powerful enough to prevent the cluster gas from cooling. Correspondingly, any deep notes produced by the jets must be much weaker than needed to prevent cooling and star formation.
Based on the Chandra data and also observations at other wavelengths, the supermassive black hole in the central galaxy of Phoenix is growing very quickly, at a rate of about 60 times the mass of the Sun every year. This rate is unsustainable, because the black hole is already very massive, with a mass of about 20 billion times the mass of the Sun. Therefore, its growth spurt cannot last much longer than about a hundred million years or it would become much bigger than its counterparts in the nearby Universe. A similar argument applies to the growth of the central galaxy. Eventually powerful jets should be produced by the black hole in repeated outbursts, forming the deep notes seen in objects like Perseus and stopping the starburst.
The Phoenix Cluster was originally detected by the South Pole Telescope, using the Sunyaev-Zeldovich effect, as explained in more detail in a blog interview [link to blog article] with the first author of the paper, Michael McDonald. In a separate article we give more details about the Sunyaev-Zeldovich effect, including a historical perspective, in an interview [link to Chronicles article] with one of its co-discoverers, Rashid Sunyaev.
The full author list of the Nature paper by Michael McDonald is: M. McDonald, M. Bayliss, B. A. Benson, R. J. Foley, J. Ruel, P. Sullivan, S. Veilleux, K. A. Aird, M. L. N. Ashby, M. Bautz, G. Bazin, L. E. Bleem, M. Brodwin, J. E. Carlstrom, C. L. Chang, H. M. Cho, A. Clocchiatti, T. M. Crawford, A. T. Crites, T. de Haan, S. Desai, M. A. Dobbs, J. P. Dudley, E. Egami, W. R. Forman, G. P. Garmire, E. M. George, M. D. Gladders, A. H. Gonzalez, N. W. Halverson, N. L. Harrington, F. W. High, G. P. Holder, W. L. Holzapfel, S. Hoover, J. D. Hrubes, C. Jones, M. Joy, R. Keisler, L. Knox, A. T. Lee, E. M. Leitch, J. Liu, M. Lueker, D. Luong-Van, A. Mantz, D. P. Marrone, J. J. McMahon, J. Mehl, S. S. Meyer, E. D. Miller, L. Mocanu, J. J. Mohr, T. E. Montroy, S. S. Murray, T. Natoli, S. Padin, T. Plagge, C. Pryke, T. D. Rawle, C. L. Reichardt, A. Rest, M. Rex, J. E. Ruhl, B. R. Saliwanchik, A. Saro, J. T. Sayre, K. K. Schaffer, L. Shaw, E. Shirokoff, R. Simcoe, J. Song, H. G. Spieler, B. Stalder, Z. Staniszewski, A. A. Stark, K. Story, C.W. Stubbs, R. Suhada, A. van Engelen, K. Vanderlinde, J. D. Vieira, A. Vikhlinin, R.Williamson, O. Zahn, and A. Zenteno.
Contacts and sources:
Jennifer Chu, MIT News Office
NASA
2012-08-15 15:41:20
Source: http://nanopatentsandinnovations.blogspot.com/2012/08/most-massive-and-luminous-galaxy.html
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