Supermassive black holes (SMBHs) are cosmic giants that reside at the centers of nearly all large galaxies, dictating the evolution of their host systems through powerful gravitational and energetic influence. The SMBH at the heart of the galaxy Phoenix A stands out as one of the most extreme examples yet discovered, pushing the theoretical limits of how large these objects can grow. Studying this object provides astronomers with unique insights into the physics of accretion, energy feedback, and the growth of the largest structures in the cosmos.
Locating the Cosmic Giant
The black hole known as Phoenix A is anchored deep within one of the universe’s most massive known structures, the Phoenix Cluster. To locate this immense system in the night sky, one must look toward the southern celestial hemisphere, specifically within the boundaries of the constellation Phoenix. This constellation, named after the mythical bird, gives the entire cluster and its central galaxy their designation.
The precise astronomical coordinates for the center of the cluster place it far south of the celestial equator, with a Right Ascension of 23 hours, 44 minutes, and 40.9 seconds, and a Declination of minus 42 degrees, 41 minutes, and 54 seconds. While the coordinates pinpoint its direction, the true measure of its location is its staggering distance from Earth. Light from the Phoenix Cluster has traveled for approximately 5.8 billion years to reach our telescopes. This means we are observing the black hole as it existed when the universe was only about two-thirds of its current age, making it a valuable target for understanding early cosmic evolution.
The Phoenix Cluster Environment
The Phoenix Cluster, officially cataloged as SPT-CL J2344-4243, is classified as one of the most massive galaxy clusters known, containing a total mass on the order of two thousand trillion times that of the Sun. This colossal structure acts as the immediate environment for the central black hole and is home to hundreds of individual galaxies bound together by gravity. The space between these galaxies is not empty but is filled with an enormous reservoir of superheated, X-ray emitting gas known as the intracluster medium (ICM).
The Phoenix Cluster’s ICM is exceptionally luminous in X-rays, producing more X-ray energy than any other known cluster. This intense X-ray emission is a direct result of the hot gas being dense and concentrated at the core. In typical clusters, this hot gas near the center should rapidly cool and fall inward, triggering a massive burst of star formation in the central galaxy. Most massive clusters show little star formation, suggesting that an energy source—usually the central black hole—prevents the gas from cooling.
The Phoenix Cluster presents an exception to this general rule. Despite containing a powerful central black hole, the central galaxy is currently forming stars at an astonishing rate, estimated to be up to 500 to 740 solar masses per year. This suggests that the black hole’s activity has not been fully successful in quenching the cooling flow, allowing cold gas to condense and fuel a massive stellar birth event. The cluster’s combination of record-breaking mass, high X-ray luminosity, and vigorous star formation makes it a laboratory for studying the complex interplay between black holes and their surrounding cosmic structures.
Record-Breaking Activity and Growth
The black hole at the center of the Phoenix A galaxy is an ultramassive object whose estimated properties challenge the known limits of black hole growth. Its mass is estimated to be around 100 billion times that of the Sun, placing it among the most massive black holes ever inferred. This colossal mass gives it an event horizon, the point of no return, with a diameter of roughly 590 billion kilometers—approximately 3,900 times the Earth-Sun distance.
This immense black hole is not dormant; it is currently undergoing an extremely high rate of gas accretion, which fuels its spectacular energy output. It is estimated to be growing in mass at a rate of approximately 60 solar masses per year. As matter spirals into the black hole, it forms an accretion disk where friction and intense gravity heat the material to extreme temperatures, releasing vast amounts of energy that power the galaxy’s active galactic nucleus.
This energy is primarily released in the form of powerful radio jets that erupt outward from the black hole’s poles, carrying highly energetic particles deep into the surrounding cluster gas. These jets inject mechanical energy into the intracluster medium, creating large cavities within the hot gas visible in X-ray images. The power output of these jets is comparable to the X-ray luminosity of the cluster core, demonstrating the black hole’s attempt to thermally balance and regulate the cooling gas. However, the Phoenix Cluster’s high star formation rate indicates that the energy injection is either temporarily insufficient or that the black hole is uniquely promoting the cooling of some gas phases while heating others.