Massive galaxy cluster spawns more than 700 stars a year

An artist's depiction of the center of the Phoenix cluster. Hot gas is red; cooler gas, blue. Newly formed stars are in the outer part of the galaxy.
Los Angeles Times

A newly discovered cluster of galaxies, more than 5 billion light years from Earth, may help astronomers understand a basic, but vexing, question about our universe: How do galaxies spawn their stars?

Cluster SPT-CLJ2344-4243 is among the most massive clusters of galaxies in the universe, and produces X-rays at a rate faster than any other known cluster.

It also creates new stars at an “unmatched” pace of more than 700 per year, said Michael McDonald, a Hubble fellow at MIT and lead author of a paper detailing the cluster’s properties, published online Wednesday in the journal Nature.

“This extreme rate of star formation was unexpected,” he said during a NASA news conference Wednesday, noting that the Milky Way forms just one or two stars a year.


McDonald and his colleagues nicknamed the massive cluster “Phoenix” because it is located in the Phoenix constellation and, he said, because although it is probably very old (on the order of 6 billion years), it produces a lot of stars -- making it look as if it is emerging anew.

The cluster, 7.3 million light years across, was initially spotted by the National Science Foundation’s South Pole telescope in 2010, according to an interview with McDonald posted by the Chandra X-ray Observatory in Cambridge, Mass.

At first, astronomers didn’t realize how unique the cluster was. But in the months that followed, McDonald and his co-authors analyzed a variety of signals from the cluster including X-rays, optical light, and infrared radiation it emitted — and realized they had a “remarkable” object on their hands.

Cooling of the hot gases in the centers of galaxies is what drives the formation of stars. The Phoenix cluster’s high X-ray output was an indication that the gases in its center were cooling very, very rapidly, the scientists said — at such a high rate that the cluster could potentially create thousands of stars a year.
That kind of rapid cooling and star formation are expected to take place in a large fraction of galaxy clusters, but the phenomenon had never been observed in the past, McDonald said — a conundrum known as “the cooling flow problem.”


News conference participant and University of Cambridge astrophysicist Martin Rees, who was not involved with the study, said that in most galaxies, black holes are thought to prevent the expected star formation, by emitting energy that heats gases so they cannot coalesce into stars.

“It’s clear that this is a cluster where the feedback from the black hole is inefficient,” he said. “It’s by studying these extreme phenomena that we understand the symbiosis between galaxies and black holes.”

Further study of SPT-CLJ2344-4243 and massive galaxy clusters like it could also help astronomers figure out how unique Phoenix actually is, added Bradford Benson, a University of Chicago astrophysicist and study co-author. He said studying more clusters could lead scientists to determine whether star-making phases like the one observed in this study tend to be relatively brief (say, a few million years), or extended (100 million years).

Click here to visit a webpage from the Chandra X-Ray Observatory detailing the Phoenix cluster discovery.