Galaxy clusters producing stars has long been a theory. However, most galaxy clusters that we’ve observed are “red and dead”, and have produced all the stars that they can make. According to the theory, cluster formation should include a cooling phase which produces blue light from new stars.
Researchers have said they have found evidence that the Phoenix cluster, formally known as SPT-CLJ2344-4243, is making 740 new stars a year. By comparison, our galaxy, the Milky Way, produces only one or two stars a year.
“In the cores of some clusters of galaxies the hot intracluster plasma is dense enough that it should cool radiatively in the cluster’s lifetime leading to continuous ‘cooling flows’ of gas sinking towards the cluster centre, yet no such cooling flow has been observed,” the researchers wrote in science journal Nature.
“These observations reveal an exceptionally luminous galaxy cluster that hosts an extremely strong cooling flow (around 3,820 solar masses a year). Further, the central galaxy in this cluster appears to be experiencing a massive starburst (formation of around 740 solar masses a year), which suggests that the feedback source responsible for preventing runaway cooling in nearby cool-core clusters may not yet be fully established in SPT-CLJ2344-4243. This large star-formation rate implies that a significant fraction of the stars in the central galaxy of this cluster may form through accretion of the intracluster medium, rather than (as is currently thought) assembling entirely via mergers.”
The Phoenix cluster, which is roughly seven billion light years away, has a mass equivalent of roughly two and a half million billion suns. It is thought that the clusters are formed after colossal galactic mergers.
The theory is that a cluster will have a supermassive black hole at the centre which is surrounded by a massive ton of gas stripped from nearby galaxies and supernovae. Eventually, this gas will cool enough to start to come together and form stars.
Up until the discovery of Phoenix, there had been little evidence to support the cooling flow theory. After being spotted by the South Pole Telescope in the Antarctic, the Massachusetts Institute of Technology led a team that analysed Phoenix from 10 different telescopes globally.
The cluster gave out an extremely bright ultraviolet emission, which indicates the presence of hundreds of new stars and led researchers to conclude that Phoenix was producing around 740 a year.
“Not only is it the most X-ray luminous cluster in the Universe, but the central, most massive galaxy is forming stars at an unmatched rate,” said MIT researcher Michael McDonald.
The find also gives us more information on supermassive black holes, which are thought to be at the centre of every galaxy. The black holes are surrounded by gas which their gravity pulls in, but is also shoved away by huge jets of energy that the black holes release.
"It's not able to hold that infall of gas at bay, and this gas is falling in and forming stars,” said Astronomer Royal Sir Martin Rees.
"That's a very extreme phenomenon, that's what's so special about this system. This is a fascinating step toward putting this picture together of the tussle."