In the southern constellation Phoenix, about 5.8 billion light-years away, lies one of the most extreme clusters of galaxies known. It was found in 2010 by the South Pole Telescope, which detected the way its hot gas distorts the cosmic microwave background passing through it. The Phoenix Cluster holds about 2 × 10¹⁵ solar masses, and at its centre is a giant galaxy whose black hole may be the heaviest ever estimated: Phoenix A.
How heavy?
Phoenix A’s mass has never been measured from the orbits of stars or gas around it, the way the masses of black holes such as M87* were. Instead, in 2016 Michael Brockamp and colleagues estimated it from a model that treats the black hole as a “calorimeter”: the energy it must have pumped into the surrounding gas over billions of years implies how much it has grown. Their figure is of the order of 100 billion Suns.
Other estimates come out about eight times lower. Even at the lower figure Phoenix A would be among the most massive black holes known, but the exact number is uncertain, and it should not be treated as a measurement.
A cluster that will not cool quietly
The Phoenix Cluster is remarkable for another reason. In most giant clusters, the central black hole keeps the surrounding hot gas from cooling and forming stars. In 2012 Michael McDonald and colleagues reported that in the Phoenix Cluster gas is cooling at a very high rate and the central galaxy is forming stars at about 740 times the mass of the Sun every year. The Milky Way manages about one.
In 2025 a team led by Michael Reefe used the James Webb Space Telescope to map gas at temperatures of a few hundred thousand degrees in the cluster’s core, catching the gas in the act of cooling for the first time on large scales. The black hole, it seems, cannot quite keep up with the flood of cooling gas around it.