The image on the left shows the newly discovered Phoenix Cluster, located about
5.7 billion light years from Earth. This composite includes an X-ray image from
NASA’s Chandra X-ray Observatory in purple, an optical image from the 4m Blanco
telescope in red, green and blue, and an ultraviolet (UV) image from NASA’s
Galaxy Evolution Explorer (GALEX) in blue. The Chandra data show hot gas in the
cluster and the optical and UV images show galaxies in the cluster and in nearby
parts of the sky.
The largest structures in the universe are galactic clusters, spanning millions of light years in diameter. Typically, the galaxies at the centre of these clusters are very old, with little new star formation taking place. But new observations by NASA’s Chandra X-ray Observatory, the National Science Foundation’s South Pole Telescope, and eight other world-class observatories have shown that this is not always the case. Galaxies at the centre of the Phoenix cluster, located about 5.7 billion light years away and with a diameter of 7.3 million light years, are spawning new stars at a record-breaking pace. The region is also pumping out X-rays more powerfully than any other cluster.
It has been known for some time that galactic clusters contain vast reservoirs of gas, holding more material than all the actual galaxies combined. This gas is so hot that it emits X-rays which can be seen by the Chandra orbital X-ray telescope, which is the only way it can be seen from such a great distance. Scientists had long expected that this gas should gradually sink to the centre of the cluster under gravity, and provide material to fuel star formation, but observations had always shown the central regions to instead be dormant, populated entirely with old stars. The hypothesis was that jets of expelled material from the supermassive black hole (SBH) at the core of the central galaxy add enough energy to the system to keep the cloud from collapsing. This has been observed happening in other galactic clusters, like the Perseus cluster.
But the Phoenix cluster shows that this mechanism does not necessarily work in all cases. Phoenix’s central galaxy has a relatively quiet SBH which is not jetting enegetically enough to stop the gas cloud from collapsing. As a result, the galaxy is producing new stars at a rate of about 740 per year — more than two brand new stars every single day!
This cannot continue for long though, as much of the in-falling gas will end up near the SBH. Some of this material will fall into the black hole, but the rest will be whipped around by tidal forces and ejected at extremely high speeds, causing the jets to flare up and restore the balance to the cluster. The question now is whether this is a normal cycle for galactic clusters, or if Phoenix is just having a temporary blip in the natural order.
The Phoenix cluster originally was detected by the National Science Foundation’s South Pole Telescope, and later was observed in optical light by the Gemini Observatory, the Blanco 4-meter telescope and Magellan telescope, all in Chile. The hot gas and its rate of cooling were estimated from Chandra data. To measure the star formation rate in the Phoenix cluster, several space-based telescopes were used, including NASA’s Wide-field Infrared Survey Explorer and Galaxy Evolution Explorer and ESA’s Herschel.