A galaxy cluster is a cosmic behemoth—a conglomeration of hundreds to thousands of galaxies bound together by gravity. It's also a cosmic mystery. Hot gas flows to the center of these clusters, cooling in the process—a situation that should create a fertile birthing ground for new stars. But astronomers have found this mostly not to be the case. Now, scientists have discovered a galaxy cluster 5.7 billion light-years away from Earth whose center is abuzz with rapid star formation. Astronomers say a likely reason this particular cluster is so productive is that that the cooling of gas at its center is not being countered by the emission of hot jets from a central black hole.
The cluster, named the Phoenix galaxy cluster, is one of the biggest in the universe. It was discovered in 2010 with the help of the South Pole Telescope. The telescope has helped researchers detect such clusters by exploiting a phenomenon known as the Sunyaev-Zel'dovich effect, which causes massive galaxy clusters to leave an impression on the cosmic microwave background: a faint, universe-spanning glow of light left over from the big bang. Like all galaxy clusters, the Phoenix cluster is made up of a mix of dark matter, stars, and gas, with a total mass 2500 trillion times that of the sun.
As soon as it was discovered, the cluster drew the attention of a team of astronomers led by Michael McDonald of the Massachusetts Institute of Technology’s Kavli Institute for Astrophysics and Space Research in Cambridge. McDonald and his colleagues studied the cluster at first using the space based Chandra X-Ray Observatory, and then followed up with observations taken by 10 space and ground-based telescopes imaging the object at different wavelengths of light. From the combined observations, the researchers concluded that the center of the cluster was forming stars at the massive rate of 750 stars per year. Their findings appear online today in Nature.
"This is an extraordinary burst of star formation," McDonald says, noting that it is at least five times the rate of star formation in the next most productive cluster. He and his colleagues also found evidence to suggest that the gas flowing into the center of the galaxy was cooling rapidly, just as they had expected.
It's no coincidence that "the most rapidly cooling galaxy cluster in the universe" happens to be the site of such frenetic star formation activity, McDonald says. "We think this cooling gas is providing an abundant supply of fuel for this starburst."
Why don't astronomers see a similar starburst in other clusters? Perhaps what’s different about them, McDonald says, is that the cooling of gas flowing into the center is slowed down by the heating effect of a black hole spewing out material from the center of the cluster. This is known to be the case for the Perseus cluster, for example.
The finding of Phoenix's high star forming rate is "significant," says Avi Loeb, an astrophysicist at Harvard University who was not connected with the study. It provides the first example of "a cluster where the gas is indeed cooling," he says "and a substantial fraction of the cold gas is turning into stars."
One possible interpretation of the discovery, Loeb says, is "that we are witnessing a short-lived phase in the evolution of clusters, just before the central massive black hole starts its feedback. If the phase is brief, we could only discover it in a small subset of all the snapshot images we have of clusters."*Correction 4:46 p.m., 21 August: The Phoenix cluster is 2500 trillion times more massive than the sun, not 25,000 trillion times more massive, as previously written.