Phoenix Cluster
Most X-ray-luminous cluster known, with extreme cooling and starburst.
The Phoenix Cluster, cataloged as SPT-CL J2344-4243, is a massive Abell-class type I galaxy cluster found in the southern constellation of Phoenix. It was first spotted in 2010 as part of a 2,500-square-degree survey of the southern sky conducted by the South Pole Telescope collaboration, which used the Sunyaev–Zeldovich effect to detect it. Ranked among the most massive galaxy clusters known, it has a mass around 2×10^15 solar masses and holds the record for the most X-ray luminous cluster ever discovered, emitting more X-rays than any other known massive cluster. Its comoving distance from Earth is 8.61 billion light-years (2.64 gigaparsecs). The SIMBAD Astronomical Database lists about 42 identified member galaxies, though the actual count could reach up to 1,000.
The cluster was first reported in a paper by R. Williamson and colleagues, part of a South Pole Telescope survey that identified 26 galaxy clusters. Observations were made at frequencies of 95, 150, and 220 GHz. Of these clusters, 14 had been previously identified, while 12—including the Phoenix Cluster—were new discoveries. The survey noted that this cluster had "the largest X-ray luminosity of any cluster" described. A bright type-2 Seyfert galaxy, 2MASX J23444387-4243124 (later named Phoenix A), lies 19 arcseconds from the cluster’s apparent center and serves as its central galaxy.
Due to its extreme properties, the Phoenix Cluster is considered a key object of its kind. A multiwavelength study by M. McDonald and colleagues revealed an exceptionally strong cooling flow rate of about 3,280 solar masses per year, described as a runaway cooling flow—one of the highest ever measured in a cluster core. Unlike other clusters, this intense cooling flow may result from a feedback mechanism that has not yet stabilized; the central black hole’s heating appears insufficient to create the feedback seen in clusters like Perseus and Virgo. This is supported by the high starburst activity in Phoenix A, where stars form at 740 solar masses per year (compared to the Milky Way’s 1 solar mass per year). The central active galactic nucleus does not produce enough energy to ionize the galaxy’s gas and prevent this starburst.
The cluster’s central elliptical cD galaxy, Phoenix A (also known as RBS 2043 or 2MASX J23444387-4243124), hosts an active galactic nucleus that shares traits of both a quasar and a type 2
- mass
- ~2×10^15 M☉
- distance
- 8.61 billion light-years (2.64 gigaparsecs)
- X-ray luminosity
- highest of any known cluster
- member galaxies
- 42 identified in SIMBAD; possibly up to 1,000
- central galaxy
- Phoenix A (2MASX J23444387-4243124)
- cooling flow rate
- ~3,280 M☉ per year (runaway cooling flow)
- star formation rate in Phoenix A
- 740 M☉ per year
Lore & Background
The Phoenix Cluster was first reported in a paper by R. Williamson and colleagues during a survey by the South Pole Telescope in Antarctica, being one of 26 galaxy clusters identified by the survey. The detection was conducted at frequencies between 95, 150 and 220 GHz, with 14 of the clusters previously identified and 12—including the Phoenix Cluster—being new discoveries. The cluster was noted for having 'the largest X-ray luminosity of any cluster' described by the survey. A bright, type-2 Seyfert galaxy, later named Phoenix A, lies 19 arcseconds from the apparent center.
Reader's Guide
The Phoenix Cluster is significant for its extreme properties, which challenge standard models of galaxy cluster evolution. Its extremely strong cooling flow rate of roughly 3,280 M☉ per year is one of the highest ever measured in a cluster center, described as a runaway cooling flow. Unlike other clusters such as Perseus and Virgo, the feedback mechanism from the central black hole appears inadequate to prevent runaway cooling, leading to massive starburst activity in the central galaxy Phoenix A, where stars form at 740 M☉ per year. The central supermassive black hole may have a mass on the order of 100 billion M☉, potentially making it the most massive known. In 2025, the James Webb Space Telescope detected intermediate-temperature cooling gas (~300,000 Kelvin) bridging hot and cool gas phases, a phenomenon unseen in other clusters.
Did You Know?
- The Phoenix Cluster was one of 12 new galaxy clusters discovered in a 2010 South Pole Telescope survey.
- Its central galaxy, Phoenix A, has a star formation rate 740 times that of the Milky Way.
- The central black hole may have a mass of 100 billion M☉, potentially the most massive known.
- In 2025, JWST detected intermediate-temperature cooling gas (~300,000 K) in the cluster, a missing link in its rapid star formation.
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