Object: Colossal galaxy cluster
Mass: 2.5 quadrillion solar masses
Star formation rate: 740 new stars per year
They are some of the largest objects in the universe, but most galaxy clusters are lazy bums.
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To account for all the stars we know exist, astronomers expect these great collections of galaxies, which can encompass trillions of times the mass of the sun, to centre around one busy-bee galaxy, capable of churning out new stars at a rate of thousands per year. But such a cluster had never been found. Worse, most of the clusters in our part of the cosmic neighbourhood seemed to churn out just one measly star a year.
Enter SPT-CLJ2344-4243, also known as the Phoenix cluster, situated 5.7 billion light years from Earth. Its central galaxy has finally been found to meet productivity standards, churning out an estimated 740 stars the size of the sun per year.
鈥淚t鈥檚 by far the most star-forming central galaxy in a cluster in the universe,鈥� says of the Massachusetts Institute of Technology. The previous record holder made between 100 and 150 stars per year 鈥� 鈥渟econd place is not even close鈥�. However, Phoenix may not hold this title forever.
Black hole hot-head
Astronomers had been puzzled as to why all the other clusters were such slackers. When galaxy clusters form, the space between the galaxies is full of hot gas, about 10 million degrees Celsius. But as they age, the gas should cool to the ambient temperature of space, getting denser and denser, until the hydrogen atoms fuse together and ignite infant stars. This cooling gas falls toward the centre of the cluster, so the central galaxy should end up churning out the most stars.
Astronomers had seen evidence for cooling in several galaxy clusters in the form of X-rays, which is like 鈥渢he steam it鈥檚 giving off鈥� due to cooling, McDonald says. 鈥淏ut we don鈥檛 see the results of that cooling. The results should be the gas cools all the way down near absolute zero and makes stars. But we don鈥檛 see those stars.鈥�
These clusters may be lethargic thanks to a hyperactive furnace in the form of a hot-headed black hole. Each galaxy has a super-massive black hole at its centre, which apparently emits just enough energy to keep the star factory uncomfortably warm, halting the production line.
鈥淭he central black hole seems to know about the cooling and it puts the same amount of heat back into the system, so you don鈥檛 get this runaway star formation,鈥� McDonald says.
Steamy cluster
The Phoenix cluster is different. When it was discovered in 2010, using the South Pole Telescope, astronomers immediately recognised it as among the top three most massive galaxy clusters ever seen 鈥� it weighs between 1.26 and 2.5 quadrillion times the mass of the sun.
Follow-up observations with ground-based X-ray telescopes showed that it was also the most X-ray luminous 鈥� or 鈥渟teamy鈥�, in McDonald鈥檚 terminology 鈥� galaxy cluster. If the sun鈥檚 energy were emitted only in X-rays, the Phoenix cluster would shine as bright as two trillion suns.
鈥淭hat鈥檚 when we really realised how exciting this was,鈥� McDonald says. More X-rays means more cooling, perhaps enough to overcome any heating from a black hole, which potentially means more star production.
Starburst nailed
Now, further observations with a range of different space and ground-based telescopes have helped nail down the prodigious star formation rate. Observations in X-rays, ultraviolet, infrared and optical wavelengths show that cold gas is streaming toward the cluster鈥檚 central galaxy, triggering bursts of star formation in and around it.
鈥淭his process is thought to be the way that a large fraction of all the stars in the universe are formed,鈥� says of the University of Maryland, who was not involved in the new work. Future observations with the could help clear up the exact locations of the starbursts, McDonald says.
But the Phoenix cluster鈥檚 work ethic might only last until its own central black hole turns up the heat. The black hole that lies at its centre is still relatively young, and should become more active in the next few million years.
Crazy time
鈥淭his galaxy is in a really lucky time,鈥� McDonald says. 鈥淚t has a lot of fuel, and no one鈥檚 telling it to stop. It鈥檚 allowed to just go crazy. In principle, within a few million years the black hole is going to catch on to what鈥檚 going on and stop it.鈥� That could explain why most clusters look so sluggish. 鈥淚f every galaxy went through this you鈥檇 only expect to catch it in action 1 per cent of the time,鈥� McDonald says.
Glimpsing Phoenix in this stage of its life could be crucial to understanding black holes鈥� effects on the universe more generally. 鈥淭he bottom line is, we don鈥檛 really understand how black holes influence their environment,鈥� Mushotzky says.
Finding more industrious clusters as far away or further than the Phoenix cluster is also important, he adds. 鈥淭he hope is that when more such objects are followed up, we鈥檒l get a picture of how this process works over a very large chunk of cosmic time.鈥�
Journal reference: , DOI: 10.1038/nature11379
