Astronomers peering deep into the heart of a cluster of galaxies some 5.7 billion light-years away have found that the cluster’s central galaxy is producing stars at an extraordinary rate – up to hundreds of times more stars per year than any of its siblings in a handful of other well-studied clusters.
The largest, most productive galaxies in the universe are found at the centers of galaxy clusters, researchers say. And astronomers thought they had a pretty good handle on why these galaxies were at the top of the star-formation heap – acquisitions and mergers involving other galaxies.
The new find, however, in what the astronomers are calling the Phoenix Cluster, suggests an unexpected source of gas available to make new stars in these central galaxies – the gas between galaxies in the cluster.
Even more intriguing, it also could shed light on the interplay between supermassive black holes at the centers of galaxies and the galaxies themselves. That interplay is thought to limit the size each actor in this cosmic tug of war can reach.
As clusters go, the Phoenix Cluster is among the largest yet detected. The entire cluster spans some 8.5 million light-years. It contains hundreds of thousands of Milky Way-size galaxies, and sports an overall mass of 2,500 trillion suns. X-ray emissions from all of the hot gas it contains shine brighter than the X-ray emissions from any other known cluster.
The galaxy at the center of the Phoenix Cluster is "crushing the record" for star birth in galaxy clusters, says Michael McDonald, a researcher at the Kvali Institute for Astrophysics and Space Research at the Massachusetts Institute of Technology in Cambridge. Dr. McDonald is the lead author of the discovery's formal report, which appears in Thursday's issue of the journal Nature.
The cluster's formal title is SPT-CLJ2344-4243. It lies in the direction of the constellation Phoenix, a minor grouping of stars in the Milky Way and visible in the Southern Hemisphere.
Prior to 2010, no one knew this enormous collection of gas and galaxies existed.
That year, a microwave telescope conducting a sky survey from its frigid spot at the South Pole detected a "shadow" in the universe's microwave background – emissions that were discovered in 1964 as a ubiquitous radio hiss all across the sky. Theorists identified it as the echo of the Big Bang – the sudden release of energy that gave rise to the universe some 13.8 billion years ago.
That shadow was cast by the Phoenix Cluster, explained Bradford Benson, an astrophysicist at the University of Chicago, in a briefing for reporters on Wednesday. The cluster was blocking the microwave radiation as it traveled from its more distant source – the edge of the observable universe. The cluster's traits only became clear after researchers trained eight additional ground- and space-based telescopes on the object to view it at different wavelengths.
Using NASA's Chandra X-Ray Observatory, currently in orbit around Earth, the team saw that the cluster's X-ray emissions not only were the brightest of any known cluster, but that the brightest region appears toward the cluster's center. That increase in X-ray brightness is due to emissions the gas gives off as it cools during its migration from the cluster's periphery toward its center.
Based on that brightness, the team estimates that gas in this cluster is cooling faster than gas in any other cluster. It's this gas that ultimately chills enough to form stars – hot gas is too diffuse and energetic to collapse into the small, dense clumps that continue to contract under gravity until their cores ignite.
Meanwhile, observations at other wavelengths showed the central galaxy as a bright blue object – signaling the formation of large numbers of stars. Indeed, the team estimates that the central galaxy was adding as many as 740 newborns a year when the light the team recorded left the cluster 5.7 billion years ago. The previous record-holder, a cluster 3.3 billion light-years away and known as Zw 33146, pumped out a leisurely 79 new stars a year. The Milky Way, by contrast, is a sluggard, at 1 new star per year.
The difference in star-formation rates among clusters' central galaxies, and the unusually rapid cooling that gas in the Phoenix Cluster displays, suggests that intergalactic gas is a far-more important source of raw material for stars in these central galaxies than previously thought.
More broadly, the discovery could shed light on a vexing question: Why do nearby clusters show high levels of X-ray emissions, but very small flow of cooler gas toward the cluster's center? Based on the X-ray emissions, these clusters should be in star-forming overdrive. But analysis of their central galaxies show puny star-formation rates compared with the Phoenix Cluster.
Researchers have invoked supermassive black holes as the likely regulator.
In galaxies where star formation is prodigious, the black holes snag some of that in-falling gas and slowly ramp up the amount of radiation emitted as that material begins to fall into them. This radiation is emitted in jets of highly accelerated plasma that reach far above and below the galaxies, sending shock waves into the intergalactic gas and heating it.
At some point, a supermassive black hole's emissions grow so large and the intergalactic gas becomes so hot that the cooling process virtually halts and continued star formation occurs largely from supplies of gas already in the galaxy – or from gas stolen from other galaxies during close passes or collisions.
With the galaxy in the Phoenix Cluster, the rate at which gas is falling toward the cluster's center "is so huge that, vast as the black hole itself probably is, it's not able to hold this in-fall of gas at bay," says Martin Rees, an astrophysicist at the University of Cambridge in Britain, who is not a member of the research team but offered his perspective during the briefing.
One explanation for what the team sees is that such high star-formation rates occur only briefly, before the black hole is activated and exerts its influence on the inflow of cold gas.
That explanation "is the most likely one," says Avi Loeb, an astrophysicist who heads the astronomy department at Harvard University in Cambridge, Mass. If the feeding period is short, occurring early in a galaxy's history, "when you take snapshots of clusters, only a small fraction of those snapshots would show this phase."
Or the high rate of star formation that the team detected could be part of a cycle, since once the black hole shuts off the supply of gas, it's ultimately starving itself. Once its activity falls and the radiation jets disappear, the intergalactic gas has a chance to cool, and the process begins anew.