Phoenix Cluster

Lore & Background

The Phoenix Cluster was first reported in a 2010 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. It was detected at frequencies between 95, 150 and 220 GHz using the Sunyaev–Zeldovich effect. The cluster was noted for having the largest X-ray luminosity of any cluster described by the survey. A bright, type-2 Seyfert galaxy, 2MASX J23444387-4243124 (later named Phoenix A), lies 19 arcseconds from the apparent center.

Owing to its extreme properties, the Phoenix Cluster has been extensively studied. A multiwavelength study by M. McDonald and colleagues showed an extremely strong cooling flow rate of roughly 3,280 M☉ per year, described as a runaway cooling flow. This is one of the highest ever seen in the middle of a galaxy cluster. The very strong cooling flow, in contrast to other clusters, has been suggested to result from a feedback mechanism that may not yet be established in the Phoenix Cluster; the heating mechanism expected from the central black hole is inadequate to create feedback (unlike the Perseus and Virgo clusters). This is supported by the high starburst activity of Phoenix A, where stars form at 740 M☉ per year, and the central active galactic nucleus has not produced sufficient energy to ionize the galaxy's gas and prevent starburst activity.

The central elliptical cD galaxy Phoenix A hosts an active galactic nucleus sharing properties of both a quasar and a type 2 Seyfert galaxy, powered by a central supermassive black hole. The galaxy has an uncertain morphology. Its isophotal diameter is 110.48 kiloparsecs (360,300 light-years), making it one of the largest known galaxies. The central black hole's mass has been estimated on the order of 100 billion M☉, possibly exceeding this mass, though not yet measured through orbital mechanics. Such a high mass may place it into a proposed category of stupendously large black holes (SLABs).

Reader's Guide

The Phoenix Cluster's significance lies in its extreme properties, which challenge standard models of galaxy cluster evolution and feedback. Its runaway cooling flow and exceptionally high star formation rate in the central galaxy indicate that the usual feedback mechanism—where the central black hole heats the surrounding gas and prevents runaway cooling—has not yet become established. This makes the cluster a unique laboratory for studying the early stages of feedback and the interplay between cooling flows, starbursts, and active galactic nuclei. The estimated mass of its central black hole, potentially exceeding 100 billion M☉, places it among the most massive black holes known and raises questions about the upper limits of black hole growth. Observations by the James Webb Space Telescope in 2025 detected an intermediate-temperature cooling gas (~300,000 Kelvin) bridging the gap between hot and cool gas, a phenomenon unseen in other clusters, providing a key missing link in understanding the cluster's rapid star formation. The cluster's legacy is as a benchmark for extreme cooling flows and a testbed for theories of galaxy formation and black hole feedback.

Did You Know?

• The Phoenix Cluster was initially detected in 2010 during a 2,500-square-degree survey of the southern sky using the Sunyaev–Zeldovich effect by the South Pole Telescope.
• Its central galaxy, Phoenix A, has a star formation rate of 740 M☉ per year, compared to the Milky Way's 1 M☉ per year.
• The central black hole's estimated mass of 100 billion M☉ would have a Schwarzschild radius of 295.25 billion kilometers (0.031 light-years).
• In 2025, the James Webb Space Telescope detected an intermediate-temperature cooling gas (~300,000 Kelvin) in the cluster, a phenomenon unseen in other galaxy clusters.