天文学家发现超大星系团

Astronomers¹ have found an extraordinary galaxy² cluster, one of the largest objects in the universe, that is breaking several important cosmic records. Observations of the Phoenix³ cluster with NASA's Chandra X-ray Observatory⁴, the National Science Foundation's South Pole Telescope, and eight other world-class observatories⁵ may force astronomers to rethink how these colossal⁶（巨大的） structures and the galaxies⁷ that inhabit them evolve. Stars are forming in the Phoenix cluster at the highest rate ever observed for 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. The data also suggest the rate of hot gas cooling in the central regions of the cluster is the largest ever observed.

 

The Phoenix cluster is located about 5.7 billion light years from Earth. It is named not only for the constellation⁸（星座） in which it is located, but also for its remarkable⁹ properties.

 

"While galaxies at the center of most clusters may have been dormant¹⁰（休眠的） 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 Michael McDonald, a Hubble Fellow at the Massachusetts Institute of Technology and the lead author of a paper appearing in the Aug. 16 issue of the journal Nature. "The mythology¹¹ of the Phoenix, a bird rising from the dead, is a great way to describe this revived object."

 

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. This reservoir can be detected only with X-ray telescopes such as Chandra. The prevailing¹² 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 think 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.

 

The famous Perseus cluster is an example of a black hole bellowing¹³ 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 co-author Ryan Foley, a Clay Fellow at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. "The Phoenix cluster is showing us this is not the case -- or at least there are times the music essentially¹⁴ stops. Jets from the giant black hole at the center of a cluster are apparently¹⁵ not powerful enough to prevent the cluster gas from cooling."

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